Crydom Solid State Relays Introduction

Introduction

To

Definition:

A SSR (solid state relay) can perform many tasks that
(electromechanical
no

moving mechanical parts within
device that
of

semiconductors,

Isolation and

Over the last ten years many standards have been set regarding

SSR

packages, most notably the rectangular package introduced

by us

in

the early
standard for power switching using
from 1 to 125

relay)

relies

on

the electrical, magnetic and optical properties

relay

switching function,

1970s

A

can

perform, The

and electrical

which has

SSR

differs

it.

It

is

essentially

components

now

become

SSRs, with models ranging

an

in

that

an

electronic

to

achieve its

an industry

EMR

it

has

Applications:

Since its

acceptance

domain of the
come from Industrial Process Control applications, particularly
heat/cool
transformers, The list of applications for the

introduction

in

many areas, which had previously been the sole

EMR

temperature control, motors, lamps, solenoids, valves,

the SSR, as a technology, has gained

or the Contactor, The major growth

SSR

areas

is

almost limitless,

have

Solid

State

The

Advantages

• Zero voltage turn-on, low EMil

• Random turn-on, proportional control

•

Long

life

• No contacts - handles high inrush current loads
No acoustical noise

•

• Microprocessor compatible
Design flexibility

•
Fast

response

•

• No moving parts

•

No

contact bounce

In

terms of internal design, the

similar

in

controls a load,

that
the

SSR

photocoupling and transformer coupling, and

by
means of a magnetic

Relays

of

SSRs

(reliability) > 109 operations

SSR

that each

and

EMR,

has

an

input electrically isolated from the output

Fig,

1 shows the basic configurations of both

In

the case of the

coupling,

RFI

and the

SSR,

EMR

are

fundamentally

the isolation

in

is

the

achieved

EMR

by

The

following
manufacturing equipment, food equipment, security systems,
industrial lighting,
production equipment, on-board
instrumentation systems, vending machines, test systems, office
machines,
metrology

The

Advantages

When

utilized

SSR

provides many of the characteristics that

the

EMR;
reduced electromagnetic interference, fast response and high
vibration resistance

In

today's environment
expect, improved performance from the components that
The

SSR

significant

enhanced by the use of Surface Mount
advantages
lifetime,
contacts to deteriorate, which
within

an

SSRs

has
no moving
dUI'ing

operation, makes the

in

unfriendly environments,

are

typical

fire

and security systems, dispensing machines,

medical equipment, display lighting, elevator control,

equipment entertainment lighting,

of

the

in

the correct manner for the intended application, the

a high degree of reliability, long service

are

offers Designers, Engineers and Maintenance Engineers

advantages

are

namely consistency of operation and longer usable

The SSR has no moving parts

EMR,

The long

become

well

pmts to become fractured, detached, or

examples

Solid

State

significant benefits from SSRs,
we

have

all

over alternative

are

term

reliability of components used within

established throughout industry, and with

SSR

of

SSR

applications:

power

control, traffic control,

Relay:

are

often elusive

life,

significantly

come to demand, rather than to

we

technologies,

Solid State circuitry, These

to

wear out

often the primary cause of failure

solution more robust when used

or

to

I'esonate

further

arcing

use,

PHOTODETECTOR

OPTICAL

COUPLING

(A)

AC solid state relay

state

relay

it

takes for

COIL

(8) Electromagnetic relay (EMR)

and

electromagnetic

SSR

Its

mechanical mass to react to the application

in

200

mW

Fig I Solid

Compming the two technologies, the input control circuit of the
SSR

is

functionally equivalent to the coil of the
output device of the
EMR

contacts, The operating speed of the
the time
and removal of a magnetic field, Operating speed of the

primarily determined by the switching speed of the output device,

typically much faster - microseconds for

TRIGGER

(SSR),

~RMATuR:J

MAGNETIC
COUPLING '

configLJIations,

peJiorms the switching function of the

EMR

is

DC

SSRs compared

0:

W

m

m

::J

z

Ul

o

I

MECHANICAL'

CONTACTS '

[',

-~-6

EMR,

while the

dependent upon

SSR

is

to

milliseconds for

to

phase angle and frequency

the

zero
longed.
both the

In

EMR

EMRs.

voltage/current

the case of AC input control, the operating speeds of

and

SSR

are

and filtering considerations.

In

most AC

of

types,

SSRs,

response time

the line, and

may

be

deliberately

in

the case

is

similarly extended due to phase angle

related

of

pro-

DC

Switches

The

output of a

DC

SSR

can

be

a bipolar power transistor, with the
emitter and collector connected to the output terminals, or a power
MOSFET.

Fig.

2 illustrates the schematic

and

structure of the two
bipolar transistors types, PNP and NPN, the choice of which
primarily a matter of economics, since relay isolation makes it
impossible
transistors

to

tell the difference externally. Current flow

is

described by the expression:

in

the

is

Output

The

switching capability

Switching

AC

or

DC

designation of

as

ments, which can also be

EMITIER

SASE

~

COLLECTOR

B

Devices

an

SSR generally describes its output

opposed to its input control voltage require-

AC

or

DC.

RL

E

r

+

(A)

PNP

-

schematic

l'~

~

IC

RL

(ALT)

E

P

N

P

C

(B)

PNP

structure

Fig.

3,

Referring to

a family of curves

tionship between base current

increases
pOints
the load resistance.
is

traversed very quickly (typically less than 10 microseconds),

as

base current increases along a load line between

"A" and

"8"

defined

as

In

switching devices such

the drive current from the preceding stage

in

state, or

excess of

186

for the

is

shown indicating the

18

and collector Ic' Collector current

rela-

the active region and determined by

as

SSRs,

this region

as

is

on

state.

either at

The

180

transition

for the off

is

usually
hastened by built-in positive feedback or hysteresis, which also
prevents

(active)

"hang up" and possible destruction

region caused by the slow transition of

SATURATION

REGION

I

ON

_

STATE

5,}

____

Afo,>;

~

0:

"/~

0:

::J

o

,'--------1''''<;).0--

0:

______________

~

o

o

---------

'l-( I

()AfO(~-

-----------------~.J

_____________

______________

_____ -----------

<.<'

------------------

IS7

________

in

the high dissipation

an

input signal.

BASE

CURRENTI

B

IB6

B5

IB4

IS3

---

B2

I

B1

Fig. 2 PNP

COLLECTOR

BASE

and

NPN

IB

EMITIER

S

transistor

RL

~

IC

+

J~

E

r

RL

(ALT)

C

N

P

N

types.

(C)

(D)

NPN

NPN

schematic

structure

II~------------------~,B--------ISO

COLLECTOR

Fig. 3 Transistor

voltage-current

VOLTAGE

The ratio of base current

VCE

characteristic

to

collector current

--'----~~CUT

1

OFF

STATE

curves.

is

the gain or amplifica-

OFF

REGION

tion factor of the transistor:

In

DC

SSRs the degree of amplification

small

available photocoupler current.
put current rating, the more stages of gain
polarity

is

observed, the load can be switched

of the

relay

output terminals,

as

is

the

is

directly related

As

a result, the higher the out-

are

case

for

required.

in

AC

As

series with either

SSRs.

This

to

long

is

the

as

true

for any two terminal isolated switching device. However, there are

three terminal DC output configurations where the load side of the

supply

is

power
shown

in

connected to a separate terminal

Fig.

4.

The purpose of the third terminal may be to provide

entry for additional internal power, or

output

saturate the
(0.2 volt). The load
output, while the other

output

The

transistor type, shown
common-emitter configuration,
PNP

for a ground referenced load
reference load
used

in

the common-collector (emitter-follower) mode, but would

transistor and achieve a lower voltage drop

is

then dedicated to one terminal of the relay

is

common to both drive and load circuits.

also becomes a consideration -

(Fig.

(Fig.

48). The transistor types could

full base drive

in

what is described as the

4A),

and

on

NPN

for a positive

be

reversed and

the

in

SSR,

as

order to

defeat the purpose of achieving the lower (saturating) voltage drop.

(A)

PNP

with

ground

(-)

referenced

load

To

maximize signal gain with two-terminal outputs, the
transistor and its driver are usually wired
complementary gain compounding configuration

is

amplification factor
In

either case, the output forward voltage drop

volt

DC,

which

most

applications. Since any number

approximately the product of the

is

similar to

AC

SSRs

and considered acceptable for

in

a Darlington

(Fig.

5)

is

in

the region of 1

of

alternating PNP/NPN
stages can be added to increase gain with no increase
drop, the complementary output of
lower

voltage

drop

is required,
previously described three-terminal outputs of
adding

an

external transistor and driving
This technique can
switching capability

also be used to increase current
in

applications where no suitable SSR exist.
The external transistor can, of course,
the two-terminal gain compounding mode; however,
the existing 1.2 volt

In

summation, the more common two-terminal

DC

drop of the

higher voltage drop of approximately 1.2 volts, but
load flexibility of a true
hand, even with
respect
advantage of

to

the

lower voltage drop (0.2 volt) and,

relay.

input/output

common

power supply terminal, but it has the

Fig.

58

is

preferred. Where the

the

only

alternatives

Fig.

4,

or by similarly

it

in

the saturating mode.

be

added for current gain

it

SSR

by about another 0.6 volt.

output

it

The three-terminal output

isolation, polarizes the load with

in

some cases,

lower off state leakage current.

output

or

where the

two

stages.

.2

in

voltage

are

the

or

voltage

in

will

augment

has the
provides the
on

the other

a

Fig. 4 Three-terminal,

DC

output,

common

"(0.

+

emitier

J

(8)

NPN

with

positive

configurations.

± -

(A)

NPN Darlington output

(6) CO:llplementmy output

(+)

referenced

load

AC

Switches

The most commonly used output devices
Controlled
thyristors
thyratrons of

Rectifiers (SCRs) and Triacs, generally

(so

named because of their similarity to the gas discharge
the vacuum tube

era).

in

Thyristors

AC

are

conductor switches whose bistable state depends

PNPN

feedback within a basic four-layer

ness for

SSR

use

lies

in

their ability to switch high power loads, with
practical values up to 120 amperes
480 volts
can withstand one-cycle

RMS,

with

less

than 50

peak-curmnt surges

structure. Their attractive-

and

high

AC

line

mA

of gate drive.

in

excess of ten times

their steady-state ratings.

ANODF

GATE

Cf,THOOE

SSRs

are

Silicon

known

a family of semi-

on

regenerative

voltages, up to

In

addition, they

as

Fig. 5 Two-iem7l1)a/,

gain

compounding

DC

output

conflguralions.

Fig. 6 7vvG-tmnsistor

The

SCR

in

both directions

in

its on state, thus a "controlled"

analogy

of

SCR

IS

a three-terminal unidimctional device that blocks current

in

its off state,

operation

and

rectifiel-.

performs much like a mctifier

The

SCR

is

best illustrated

by the two-transistor analogy shown

as

an

can be used

On/Off switch, it

in

Fig

6.

While the transistor

is

essentially a continuously
variable current device where the collector-emitter current flow
controlled by a small, but proportional, amount of base-emitter
current. The
Once it
cannot be turned off by its gate.
anode to cathode voltage and current below a critical

SCR

The regenerative

high current and surge capability, but it
thyristor's sensitivity to sharply
acteristic known
inadvertent turn-on, without the benefit of a gate
shown
which a

resulting
network, which limits the rate of
controls this effect. The
specified
volts per microsecond, typically
schematic symbol for the
in

Figs.

"center" gate fired device commonly

SCR,

on

the other hand, has only two states,

is

triggered on by a small briefly applied gate signal, it

Only with a reversal or reduction of

revert

to

its blocking off state.

(latching)

as

characteristic of the thyristor provides its

is

also responsible for the

rising

voltages, a

dv/dt, or rate effect.

less

desirable char-

This

phenomenon causes

signal.

in

A of

Fig.

rising

"anode" voltage

in a dv/dt

in

the catalogue

6 represents the internal

can

inject a tum-on

turn-on.

In

a SSR, the built-in snubber

rise

rate

above which tum-on

as

minimum dv/dt,

SCR

capacitance through

signal

of the applied voltage, largely

can

is

expressed

500 volts per microsecond. The

SCR

7 A

and a typical

and

B.

The structure represents a conventional "edge" or

ANODE

used

(A)

SCR

structure

in

SSRs.

Schematic symbol

on

level

The

capacitor

into the

occur,

in

are

or

off.

will the

gate,

(RC)

usually

terms of

shown

The triac
in

is

direction
schematic symbol implies
structure

is

a three-terminal bidirectional device that blocks current

its off state; but, unlike

when

triggered

(Fig.

8B)

is

essentially that of

an

on

(Fig.

SSR,

the triac conducts

by

a single gate signal. As

8A)

the triac

is

a true

an

inverse parallel pair of

AC

switch. Its

in

either

the

PNPN switches integrated into one device. Though the power

terminals appear symmetrical, they

measurement purposes. The triac gate
terminal similar to the gate-cathode relationship of the

are

designated

is

associated with the

MT1

and

SCR.

MT2

Apart

for

MT1

from the uniqueness of a single gate controlling oppositely polarized
switches with a common signal, the switching characteristics can
be likened to those of a pair of SCRs,
voltage current characteristic of Fig. 8C. Even though the

switches

are

combined into one device, they

as

can

be seen from the

still

two

exhibit individual

characteristics, such as different breakdown voltages, holding

currents, and trigger

Triacs do have a limitation compared with a pair of

commutating dv/dt
be

as

low

as

dv/dt capability at turn-off

levels.

SCRs

in

that the

(the

dv/dt applied to the switch

at

turn-off) can

5V/[tS. For a switch consisting of a pair of SCRs,

is

the critical dv/dt rating, 500V/[tS,

so

a

100 times improvement over a triac.

MAIN

ERMINAL2

(A)

Jf

GATE

MAIN

TERMINAL 1

Schematic symbol

CATHODE

P

N

REVERSE

BLOCKING / /

VOLTAGE

~)====~\~~

~B~~~~~~R

VOLTAGE

(C)

Voltage-current characteristic

(B)

PNPN

structure

ON

/STATE

HOLDING BREAKOVER

CURRENT

GATE

TRIGGERED

ON

VOLTAGE

V

~

(BLOCKING)

OFF

STATE

BREAKLR-

VOLTAGE

Fig. 8 Bidirectional

N

OFF

STATE

(BLOCKING)

(8)

Parallel

PNPN

structures

~

- - -

-~~~~~71

CURRENT

ON/

STATE

(C)

Voltage-current characteristic

thyristor

(triac).

OFF

STATE

(BLOCKING)

BREAKOVER

TAGE

7

Fig. 7 Undirectional

thyristor

(SGR).

SSR

Operation

In a bid
description
ing

themselves

Most
DC
at their input

DC

current through the photocoupler

to increase the understanding of

is

included.

of

the

intemal circuitry of

a prerequisite to

SSRs

in

the higher current

control options. Indeed many

in

It

order to provide a practical operating voltage

Inputs

Figs.

9A

and

B illustrate two typical

range

is

tailored to provide the minimum input current required to
operate the
(typically
dissipation

CONTROL

+

Q----.!I----f--,

DC DIODE

CONTROL (SERIES)

SSR, at the specified

3 volts

+Q---~0A-------.

DC

DC).

in

the current limiting component (typically 32

PROTECTIVE

DIODE

(PARALLEL)

~

\

PROTECTIVE

has

to

be

an

SSR

the

use

ranges

The high end of the range

SSRs,

an

SSR

said

that

an

in-depth understand-

and

how

it

functions

of

SSR

in

many

applications.

are

offered with either AC or

have

some form of current limiting

DC

input circuits for controlling

LED.

The low end of the input

turn-on

(must on) voltage

is

(A)

Dropping resistor

-

(8) Constant-current

-

Operational

are

not

in

range.

dictated by

Vdc).

circuit

favored
components
circuit of
short across the incoming

as

being more reliable and fail safe, since

would

have

to

fail

Fig.

AC

CONTROL

to create

10A,

a single diode breakdown would place a dead

line,

thus creating a possible heat hazard.

an

unsafe situation.

two

(A)

Two-diode input

or

more

In

the

-

AC

CONTROL

Fig.

10

Typical

AC

input

circuits.

Either of the AC input circuits

ing from a DC source and, therefore, might be considered as
AC-DC; however,
The circuit of

similar to that

circuit of

resistors, since they

In

from a

Fig.

both cases, the

DC

SSR

Fig.

10B should operate with a

of

the AC

i0A

might

would

SSR

signal of either

in

Fig.

inputs

are

(RMS)

have

dissipation problems with the input

no

longer operate at a 50% duty cycle.

would have the uniqueness of operating

polarity.

i0A

rarely

source.

-

-

(B)

Bridge input

is

also capable of operat-

characterized

On

the other hand, the

in

DC

control range

that

way.

Fig. 9 Typical

As
inverse
protection prevents damage to the
the constant-current device. The series diode permits reversal up
to the
With

by
of a higher magnitude

the

noise immunity by a
approx.)

AC

AC

voltages, with a typical operating
ohm
capacitive
B.

DC

input

circuits.

a precaution against inadvertent voltage reversal, a series or

parallel diode

PIV

rating of the diode with negligible reverse current flow.

an

inverse parallel diode, the reverse protection

dissipation

series

diode

is

usually included

in

the dropping resistor, so brief voltage transients

will

not damage the diode or

is

favored because

value

equal to its forward voltage drop (0.6 V

in

the input circuit. This

photocoupler

it

also raises the

LED

and possibly

LED.
level

is

However,

of voltage

limited

Inputs

inputs models

input impedance.

filtering and dropping resistors,

While both circuits work equally well, the circuit

are

usually suitable for both 120 and 240

range

of 90 to 280

Full

wave rectification

is

used, followed by

as

shown

in

Vac

Figs.

in

Fig.

Vac

line

and 60 K

10A and

10B

Well

designed AC input-output
power sources operating
are

both within the specified limits of voltage, frequency and isola-

tion.

Line

frequency for both input

as

47

to 63 hertz, the upper limit of which

control power since the input

upper frequency

triac, which has definite frequency

commutate off.

higher frequencies. However, because of circuit time constraints
the drive circuitry, other SSR parameters become the limiting
factors

(e.g.

tum-on

The

DC

an

Optical coupling

is

delayed each half cycle with eventual lock-on or lockout).

Coupler

voltage

less

of the type.

oscillator, which

input-output

limit for

An

SCR

output pair

the zero switching window may

is

generally used to drive the coupling system regard-

Even

with transformer coupling,

in

tum converts the

is

by far the most common means of achieving

isolation. With this method, the input element is

SSRs

can operate from separate

at

different frequencies,

and

output

is

is

rectified and filtered. However, the

an

output

is

less

limitations, related to its ability to

is

capable of operating at much

DC

to

as

is

typically specified

not critical for the input

flexible, especially for a

be

extended and/or

DC

AC.

long

as

is

used to drive

they

in

generally a light emitting diode
control power into infrared light
phototransistor or photo-SCR
and converted back into electrical

The forward voltage drop of the
volts at normal input currents of 2 ­breakdown
protected

voltage

is

typically less than 3 volts and

by a series or (inverse) parallel standard diode,

(LED)

which converts the input

energy.

This light

on

the other side of the isolation gap

is

energy.

LED

is

in

the region of 1.2 to 1.8

20 mA. The LED reverse

previously described.

Hysteresis

Due to the wide variation
voltage to guarantee

level,

immunity

is

typically 1 volt. This threshold can be higher where

is

used

in

diode

series with the
"off" and the maximum operate voltage
and not largely influenced by hysteresis
pickup and dropout of

rapidly

in

either direction, on or off, over a very narrow band,

probably less than

in

photocoupler sensitivities, the minimum

"off", which

well

below the forward bias threshold of the

an

0.1

volt, unless hysteresis

is

also considered the

LED.

The 2 volt range between the

EMR.

The transition

is

an

indeterminate state

as

in

the case for the

is

is

deliberately built

collected by a

is

usually

as

SSR

noise

LED,

an

additional

generally made

in.

DCSSR

The circuit of

transistor
DC

or rectified and filtered
protect the
voltage protection. With no input applied, the phototransistor
optocoupler
is

permitted to saturate.

and

no

When a DC
to the optocoupler, the phototransistor turns
This allows

load. Should the turn-on signal be applied
fashion,
which will enhance the turn-off command at its base. This will

speed up the turn-on process and thereby hasten the
transistor

Unlike
to flow

signal

SSR,

0.8 volt to 1.6 volts, which gives

dissipation; therefore, heat sinking requirements

Fig.

11

is

an

example of a high current

SSR

incorporating hysteresis. The input control can be

AC.

R1

is

a current limiting resistor to

LED

in

power

the photocoupler, and

is

in

its off or high impedance state, and transistor 01

In

this condition,

is

applied to the load.

CR1

02

through

input above the threshold voltage of the

on,

02

through

05

to turn

on,

and

power

in

03

will apply a feedback voltage to the emitter of

05

through its high dissipation

an

AC

SSR

which has a latching function, current continues

in

the drive circuit of a

is

removed. The

on

DC

state voltage

SSR,

region.

holding

is

similar to that of

rise

to most of the package

are

DC

bipolar

provides reverse

in

05

are

LED

is

applied

biasing off

is

applied to the

01.

a slowly ramped

01,

output

it

on

until the input

an

also

similar.

the

off,

AC

Hysteresis occurs where the input voltage required to sustain the

output

on

state

is

reduced once the transition

is

made, lowering the
turn-off voltage accordingly. Likewise, once the output returns to
the off state, the input turn-on voltage

level. The effect

is

to

speed up the transition and separate the

"pickup" and "dropout" control points.

threshold effects caused by a slowly ramped

is

raised back to its initial

In

doing so, any adverse

on

control signal

are minimized.
The hysteresis characteristic

applications where the thyristors
regenerative action of their own, and the control signals

is

not generally required

in

AC

relays have

in

an

are

most

inherent

derived

SSR

from logic with clearly defined states, and rapid transition times,
such

as

TTL.

It

would be of value, however,

transistor, DC SSRs, where hesitation

in

high current, bipolar

in

the high dissipation,
lransitional region might be catastrophic to the output transistors,
and

the

resultant

"snap

action"

would

reduce

or

eliminate

this possibility.

OUTPUI

cc:nnOL

Fig

11

Optically

isolated

DC

SSR

with

hysteresis.

The turn-off process
turn-off signal
voltage from

01.

This will speed up the transition to

03

from hesitating

ACSSR

TERMINALS

OUTPUT

DC

CONTROL

VOLTAGE

Fig.

12

Control

and

Zero

Switching

Zero

voltage turn-on

used

in

some
high inrush currents during initial turn-on. Without
load voltage

is

is

the reverse of the turn-on

is

slowly ramped down, tile

removal

(Fig.

11).

of the feedback

will enhance the turn-on command at the base of

off,

again preventing

in

the high dissipation region.

TURN-ON

SIGNAL

J

terminal

voltages

(or

zero crossing),

to reduce electromagnetic interference and

output

AC

OFF

SSRs

applied randomly

ACTUAL

TURN-ON

for

to

the load

ZBm

ON

voltage

as

TURN-OFF

SIGNAL

tum-on

illustrated

zero

at

any

,~

__

ACTUAL

TURN-OFF

L~_

relay

in

Fig.

crossing, the

point

in

the

If the

05

12,

line
voltage cycle. With the zero crossing feature, the line voltage
switched to the load only when it
with a maximum value of
change

in

power results, and proportionally lower

is

close to

± 15 volts peak. Thus, a

zero,

typically specified

EMI

vel-y

levels

small

are
generated. After zero crossing. the "Zero" switching voltage, which
defines the switching window limits, may also be expressed
terms

of

phase angle, or time. converted

as

follows:

is

is

ill

Voltage to phase angle

cj>

or

Phase angle

to

time

(5°):

(15

=

sin-

=sin-

T =

volts):

Zsw.

1

-------

Line V RMS

1

----

120 x

}2

cyc. ms

}2

cyc. deg x

15

1.41

max

(v2)

cj>

When a DC

input greater than the threshold voltage
applied to the optocoupler, the phototransistor turns
of

R2

and

R3

are

such that 01

line

voltage

is

above zero, thus holding the

zero crossing. When the

will

line voltage

remain

is

close to zero

on

if the instantaneous

SSR

positive or negative direction, the phototransistor
saturation

triac. The triac

the input
zero. The
except for a
by the
is
the

long enough for the pilot

will

control

result

small

delay before turn-on.

used to reduce the

SSR.

remain

on,

is

removed

is

a continuous sine wave applied to the load,

discontinuity at

dv/dt

SCR

to trigger, turning

being retriggered each half cycle, until

and

the

AC

line

current goes through

each

zero

line

The

snubber network of

applied at the output terminals

of

the

LED

on.

The values

off until the next

in

either a

holds 01 out of

on

the

crossing, caused

R7

and

C1

of

is

8.3

= 180 x 5

0.23

ms

=

Zero current turn-off
used

in

AC

SSRs,

triggered, the thyristor stays

until

switching load current drops below its "holding"

turns

off.

For a resistive load, this point

as

shown

in

Fig.

energy

in

the load

which

in

this case

eliminated.

This

is

an

whether

inherent characteristic

zero

voltage

is

on

for the balance of the half cycle,

of

the thyristors

employed or not. Once

level,

is

also close to zero voltage,

12. With

is

probably the most desirable feature of the

an

inductive load, the amount of stored

is

a function of the current flowing through

is

so small that inductive kickback

where

is

virtually

SSR,
when compared to the destructive effects of "arcing" contacts
when switching inductive

loads with

an

EMR.

ACSSR

The

schematic of

SSR

circuit, which includes the

by

the inhibit action of 01
control to the
current

limiting resistor used to protect the
optocoupler, and
input applied, the phototransistor
high

impedance state
this condition, the pilot
off

and

no power

R1

CONTROL

o---+CR.-'

_ I

Fig.

13

Optically

SSR

OPTICAL

COUPLER

isolated

Fig.

13

illustrates a simplified optically coupled

as

can

be

DC

CR1

provides

and

transistor 01

SCR

is

is

applied to the load.

AC

SSR

R4

with

R2

zero

turn-on feature, implemented

described

or rectified and filtered

reverse

in

the following. The input

LED

voltage protection. With

in

the optocoupler

is

permitted to saturate.

AC.

portion

is

in

R1

of

its off or

prevented from firing, thus the triac

lero

SCR

crossing

R5

detector.

R7

C1

AC

is

the

no

OUTPUT

The

minimum delay for turn-on after

on

individual circuit design, while the

delay.

The

the maximum
within these
the

"notch". Subsequent turn-on points are generally lower and

fairly consistent

allowable limits, referred

in

initial

amplitude, with circuit gain being the primary

zero

turn-on

crossing depends largely

zero

detector circuit dictates

pOint

can occur anywhere

to

as the

"window"

or

controlling factor.

it

it,

Once the output thyristor turns
power by the

current ceases to

lower forward voltage drop

flow.

This
of the package dissipation,
a function of the current through

This

is

why

the

paralleling

on,

the drive circuit

of

the thyristor, and

voltage, which

varies

from device to device and also

it,

ranging from 0.8 volt to 1.6 volt.

of

two

is

responsible for most

or more SSRs is difficult,

is

deprived of

as

necessitating the use of balancing resistors, etc. to preclude the

In
is

possibility of current

Solid

a

Crydom manufactures
various package styles, mounting options, terminal types and

State

switching capability.
the correct

Selecting

In a bid

SSR

the

to specify the exact
consider the input drive requirements, output
current,
be

isolation and installation requirement

used and how
power will dictate whether the
mounted.

In
necessary to remove heat from the
designs incude
characteristics that

General

The
SSR,
the

Parameters

following parameters relate to isolation between parts of the

namely input to output of the

SSR,

and the output to the outer case of the

"hogging".

Relay

for the correct application

Ideal

SSR

it

should

loads greater

We

Characteristics

an

extensive

have

SSR

be

than

range

of Solid State

endeavoured to make the selection of

as

easy

as

possible.

for

an

mounted.

SSR

application,

In

many instances the load

is

PCB, panel,

voltage,

i.e.

where

it

is

load

5 to 7 amps, a heat sink becomes

SSR

body. Certain

Relays

important to

is

the

or

integral heat sinks, while others have dissipation

are

inherently within the product.

SSR,

input to the outer case of

SSR.

in

or output

SSR

to

DIN

rail

Dielectric

as a
of
minimum

Insulation

measured at

the

Maximum

tive

Ambient

limits, usually given for both operating and storage conditions.
The
consideration by the thermal dictates of heat dissipation and the

possible requirement

Strength

voltage (RMS)

the

SSR can

value.

Resistance

SSR.

Also

withstand

500

volts

at

The minimum resistive value

Capacitance

coupling between control

Temperature

maximum

operating

of a heat

referred

to

as

"isolation voltage". Expressed

50/60

hertz,

that

the isolated sections

without

DC

Input

and

Range

temperature

sink.

breakdown. Considered a

between the isolated sections

to

Output

Maximum

power output terminals.

The surrounding air temperature

may

(ohms)

value

require

usually

of

of capaci-

close

Minimum

SSR
maximum

Transient

applied voltage that
malfunction while maintaining its off state. Transients
this
tions
can be

internal bias networks or by capacitor ratings.

Maximum

momentary current flow for a specified time duration, typically one
line cycle (16.6 milliseconds) for
value and provided with current versus time curves.
may

Load

Current

to

perform

load current and given

Overvoltage

value may turn

are

met. The transient period, while not generally specified,

in

the order of several seconds, limited by dissipation

Surge

be

lost during,

The minimum load current required

as

specified. Sometimes combined with the

as

the "operating current range".

The maximum allowable excursion of the

an

SSR

can withstand without damage or

on

the

SSR

nondestructively if current condi-

Current

(non-repetitive)

AC.

Usually specified

and

immediately following, the

The

maximum allowable

surge.

in

excess of

as

Relay

by

the

a peak

control

in

Input

Parameters

Control

across the input terminals,

the output

Voltage

"Max Reverse Voltage".

Maximum

above which the output
known

as

Minimum

below which the output
known

as

the

SSR

Maximum

source,

usually specified

range

(the
closed").
be

given

Minimum

age which defines input power requirements,
in

addition to, input current.

Maximum

tion

of

a turn-on control signal and the transition of the output

device to its

Maximum

of the turn-on control
to its

blocking

Output

Parameters

Operating

output, over which
otherwise perform
stated

separately

Maximum

capability of
dictates of heat sink and ambient temperature conditions.

Range

The

range

of voltages which, when applied

will

maintain

terminals. A negative voltage

Turn-On

"must operate" or "pickup".

Turn-Off

"must release" or "dropout",

"noise immunity"

Input

output

This

in

terms of input impedance at a

Input

Turn-On

Turn-Off

Voltage

Load

Voltage

Voltage

Current

is

assumed to

defines the input power requirements, which can also

Impedance

TIme

fully conducting

Time

(off)

state.

an

as

(AG).

Current

an

SSR,

The

is

guaranteed to

The voltage applied to the input at or

is

guaranteed to

level.
The

maximum current drain

at

a nominal point within the control voltage

be

Minimum impedance at a given volt-

The

maximum time between the applica-

(on)

The

maximum time between the removal

signal

and

Range

The range

SSR

will

specified.

which

Line

The

maximum steady-state load current

may

an

"on" condition across

is

stated separately as

voltage applied to the input at or

be

in

the

on

be

in

the off state. Also,

it

is

often considered to be

on

in

the

on

state unless "normally

given

voltage.

as

an

alternative to, or

state.

the transition of the output device

of

voltage applied

continuously block or switch and

frequency

be further restricted

is

either included or

by

state. Also

the driving

to

the

the thermal

Maximum

typically expressed

Maximum

that appears across the

current. Not to
Repetitive Turn-On", or used to calculate power

Maximum

the
seconds"

8.3

Thermal

"degrees celsius per watt"
erature gradient between the output semiconductor junction
and the
necessary for calculating heat sink values and allowable current
and

Power

power dissipation (watts)

voltage drop (power

provided

Maximum

voltage that appears across the output terminals immediately prior
to

initial
Also referred to
permissible turn-on window.

Maximum

off

immediately prior to turn-on at

the initial half cycle, with a turn-on control signal applied. This

parameter
on" feature.

Maximum

state
turn-on
voltage over the operating temperature

Overcurrent

On

State

be

I~

Maximum non-repetitive pulse-current capability of

SSR;

used for fuse selection. Expressed
(A2s)

with a stated pulse width, typically between 1 and

milliseconds.

Resistance,

SSR

case

temperature limits.

Dissipation

in

the form of curves over the current

Zero

turn-on, following the application of a turn-on control

Peak

state

voltage

applies equally to

Off

State

leakage current conducted through output terminals, with no

control

Signal

(non-repetitive)

as a RMS

confused with "Zero Voltage Turn-On" or "Peak

value for one-second duration.

Voltage

Drop

SSR

output terminals at full rate load

Junction

(DC/W)

(Tel

for any given power dissipation.

(at

Rated

Current)

resulting primarily from the effective

loss)

in

the output semiconductor. Sometimes

Voltage

as

Repetitive

Turn-On

the "notch" which defines the limits of the

Turn-On

that

appears

each

SSRs

Leakage

applied.

Current

Usually

Similar to the above, but

The maximum

to

Case

, this value defines the temp-

The maximum average

The maximum

Voltage

across

the

subsequent half cycle following

with or without

The maximum

specified at maximum rated
range.

(peak)

diSSipation.

as

"ampere squared

)

(R

Expressed as

SJc

range.

(peak)

The

maximum

output

the

voltage

RSJC

off state

terminals

"zero turn-

(RMS)

(TJ)

Signal.

(peak)

is

off

Minimum

age

without turning on

Usually

Off

State

dv/dt

(Static)

The rate of rise of applied volt-

across the output terminals that the

in

the absence of a turn-on control signal.

expressed

as

a minimum value at maximum rated voltage

terms of "volts per microsecond" (V/[ts).

SSR

(AC)

can withstand

+3.5 V MIN

in

Mechanical
Weight

Encapsulation
Product

Characteristics

Is

given

in

oz.

and

Specifies the material used

Dimensions

Are contained within the lines included

in

gramms.

as

the

SSR

encapsulant.

each product section.

Driving

To

activate
maximum turn-on
state occurs when zero or
applied
would
SSR
conditions
accepted, but undesignated, standard for the

DC
polarity, and

Due

trolling the input to

two input

the

SSR

an

SSR

output, a voltage greater than that specified for

is

applied to the input

(3

volt

DC

typical). The off

less than the minimum turn-off voltage

(1

volt

DC

typical).

be

90

volts RMS for

deSignated

as

would be reversed. Generally, normally open

For

an

AC

input type, the typical values

on,

and 10 volts

normally closed or form

RMS

for off. For

B,

the previous on-off

is

SSR.

is

considered as being a steady-state

AC

is

a reasonably

well

shaped sinusoidal waveform.

DC

voltage of one

to consideration of input to output isolation, the switch con-

an

SSR

can be placed

terminals, assuming polarity

in

series with either of the

is

observed

(DC).

The same

an

the

flexibility applies to the output side, where the load may also be
placed

in

series with either output terminal. There are a few spe­cialized types, usually with more than two input or output terminals,
that

have

dedicated functions

(i.e.

V cc logic input and common).

(A)

PNP transistor

Of-

+

::J

"-

SSR

f-

::J

+3.5 V MIN

+

SSR

0°

o

0°

f-

::J

"-

to

in

is

(8) NPN transistor

NPN

~

~

+4.5 V MIN

Of-

+

SINK

MODE

SSR

0°

::J

"-

f-

::J

Ie)

TIL

gate

The

activating signal may be derived from mechanical contacts or

solid state devices such

supply voltage through these contacts may be equal

turn-on voltage

(3

volts

as

those shown

in

Fig.

14.

The minimum

to

DC

typical), whereas the positively or neg-

the

SSR

atively referenced transistors require a minimum supply voltage a few
tenths of a
DC.
voltage drop when driven

TTL

A standard
capability,

volt above the specified turn-on threshold,

This

is

because of their approximate 0.2-0.4 volt

in

the grounded emitter (saturating) mode.

Drive

Methods

TIL

gate can drive most SSRs with its 16 mA sink

Fig.

14C. However, very few SSRs

say

can

be driven reliably

3.5 volts
on

state

with the gates' available source current of only 400 microamperes.
Also,

the SSR minimum voltage threshold I'8quirements

in

the source mode

The

relationship of the

cally

in

Fig.

gate V cc

should

limits of say 5 volts ± 1
referenced

SSR

(i.e.

gate output

TIL

15.

In

this configuration the

be

common and comply with the

and

the gate at logical

in

the positive leg of the

gate to

an

SSR
SSR

OSlo.

It

can be seen that with a positively

(0),

Q2

is

illustrated schemati-

supply voltage and the

is

are

not met

SSR).

TIL

specified

operating much like

Fig.

Fig.

14

15

SSR

cirive

Typical

met/locis.

circuit

of a TTL

WRONG

gate

ciriving

an

SSf"

SSR.

a discrete
In

this mode the gate can sink up to 16 mA with a maximum 0.4
volt
a minimum of
which is sufficient
voltage tolerances,

With a negatively referenced

conducts, but does not saturate, since it
follower (common collector).
400 microamps; however, the accumulated voltage drops

NPN

transistor

drop. Subtracting 0.4 volt from the worst case Vcc of 4.5 volts,

4.1

volts will appear across the SSR input terminals,

in

the grounded-emitter saturated state.

to

turn on

the values would be adjusted accordingly.

most

SSRs. For different supply

SSR

and the gate at logical

is

operating

In

this mode the gate can source up to

as

an

are:

(1),

01

emitter

Integrated circuits with open collector outputs
used

to

drive SSRs,
output transistor without
pull-up, and generally
Open collector
that the
Furthermore, the
as

and

SSR

the

IC

V cc' provided that one side

SSR

maximum voltages and currents

as

in

Fig.

17. The open collector

an

active (transistor) or passive (resistor)

has

enough power to drive

outputs can also be logically ORed

may be controlled

SSR

supply voltage does not have to be the same

by

anyone

is

common, and the transistor

are

are

also commonly

IC

has

an

an

SSR

directly.

like

discretes,

of the many outputs.

not exceeded.

so

R1(IRDROP)

The sum of these values subtracted from the worst case V cc results
in

a minimum output voltage specified

below

the

SSR

turn-on

Although some SSRs may operate satisfactorily

not recommended that this be done. Both the available current and

the minimum

optically isolated SSR.
It should

relates only to a negatively referenced load.
voltage source to a positively referenced load
appear to
15,

02

essentially

V

-------T-------------,

DD

voltage are considered inadequate for the typical

be noted that the 2.4 volt gate output

be

greater than the off state voltage. Referring

would be off and

an

open circuit with virtually

CMOS

BUFFERED

GATE

i

+ 01

threshold (assuming a 3 volt turn-on).

CR1

I

SINK

(0)

+

CR1

vBE

is

reverse biased, thus presenting

vF

as

2.4 volts, which

in

in

the logical 1 state

It

does not represent a

(SSR)

zero

potential across the

:----------/---------

3.2mA

MAX.

--

--

(1)

(2.5 V

1

I

(0)

SOURCE

0.4 V

(1)

is

0.6 volt

this mode, it

, where

it

again

1500

OHM

MINIMUM

INPUT

IMPEDENCE

AT5V

SOURCE)

SINK

would

to

Fig.

SSR.

·1

Fig.

16

Buffered

CMOS

gate

driving a high

input

impedance

SSR.

VCC

+=-~----~----,

--+-~--~-~---~_T--_+~

is

Fig.

17

Open

collector

IC

outputs

driving

SSR

in

logically

SSRs do not generally require pull-up or shunt resistors for noise
reduction or any other
assigned
the output (unless otherwise designated). Input
be
any significance
SSR

Some
the

an

the

appearing
out of the circuit, thus permitting similar devices to be paralleled
and enabled,
example,

be
applied enable Signal. Only
would

to

a particular logic

extremely long and through noisy environments before noise of

to

change state.

IC

devices have "three state" (tristate) outputs. These have

normal high and low states

additional high impedance state activated by

high

impedance

as

an

as

in

this configuration a number of ORed driver stages can

individually polled

activate the

functional reason.

level,

produces

would appear at the input terminals to cause the

as

described for standard

state, no

open input to a driven

desired, without interacting with each other. For

as

SSR.

source

to

their logic states by a sequentially

the drivers with outputs at logical 0

.-------------------

i

~

+ 0

:

~

'

_____ H ____________

ORed

configuration.

An

open input, if not

an

open or off state

lines

an

or

sink

SSR.

The

~

SSR

a.

oB

_

would

have

TTL,

enable signal.

current

IC

flows,

is

essentially

+

in

to

plus

In

Ie

and

Other

Drive

Sources

Most CMOS and NMOS logic families
SSRs, except for a

buffered gate can reliably drive
requirements
sink mode the same
or a

4050 (non-inverting) CMOS hex buffer driving such
with a common 5 volt supply. CMOS can, of course, operate at
higher

voltages, but care must be taken not to overstress the gate

with excessive dissipation.

few

specially designed types. However, a CMOS

(Le.

> 1500 ohms at 5 volts) and

as

TTL.

Fig.

will

an

SSR

16 shows

not directly interface with

that has low input power

is

also driven

1/6

of 4049 (inverting)

an

in

SSR

the

Leakage

The off state leakage current

in
could not possibly turn
leakage current of any packaged solid state driving device (e.g.
temperature
with the SSR. One method
current (amps) by the maximum input impedance (ohms) of the
SSR.
turn-off voltage. If it
may be required.

from

the

Figs. 14

to

17

controller, etc.) should first

This should result

Drive

Source

in

the driving semiconductors shown

is

significant, just a few microamperes, which

on

the

SSRs.

However,

is

to

multiply the maximum leakage

in

a voltage that

is

not, a resistive shunt across the

the off state (output)

be

checked for compatibility

is

less than the specified

SSR

input

Thermal

One

of the

major
stressed too
the

SSR
employ a
excess

OUTPUT

SEMICONDUCTOR

(JUNCTION

TEMPERATURE)

Fig.

18

strongly,

package must

heat

of 1 watt

A

simplified

Considerations

considerations

is

that
be

sink.

SSRs

per

amp.

-------10-

when

using a SSR,

an

effective

employed.

The

have a relatively

NO HEAT SINK

HEAT FLOW

HEAT SINK

WITH

which cannot

method of

removing
most common method
high

"contact" dissipation,

(AIR

8-YV\r@-YV\r(0-YV\r0)

R"JC

.~

R"c,

t~

R""

.~~

CASE'

TEMPERATURE

thermal

model.

'

HEAT SINK

TEMPERATURE

heat

from

is

AMBIENT

TEMPERATURE)

be

to

say

12

watts for a 10 amp

SSR,

assuming a 1.2 volt effective (not

actual) voltage drop across the output semiconductor. The power

(P

watts)

is

dissipation

voltage drop

(E

DROP

Assuming a thermal resistance from junction to case

in

1.3°CIW and inserting the above typical values into the equation,
solutions can be found for unknown parameters, such

determined by multiplying the effective

) by the load current

(I

LOAD

)'

(R

OJc

as

maximum

)

of,

say,

load current, maximum operating temperature, and the appropriate
heat sink
known, the third can

(a)

thermal resistance. Where two of these parameters

be

found

as

shown

in

the following examples:

To

determine the maximum allowable ambient temperature,

for

1°CIW heat sink

maximum

allowable

and

10 amp load

T3

of 100°C:

=

12

(1.3 +

0.1

(12

watts) with a

+ 1.0)

are

= 28.8

hence,

=TJ-28.8

T

A

With loads of

air

current around the

will

become necessary to make sure the radiating surface
contact with a heat sink.
baseplate of the
good thermal transfer between the

less

than 5 amps, cooling by

SSR

is

usually sufficient. At higher currents

free

flowing

Essentially this involves mounting the

SSR

onto a good heat conductor, usually aluminum;

SSR

and the heat sink can

air

or forced

is

in

good

be
achieved with thermal grease or heat sink compound. Using this
technique, the
reduced to a negligible value of O.1°CIW (celsius per watt) or
This

is

usually presumed
plified thermal model
considered
the user are the case to heat sink interface
mentioned,

Thermal

SSR

case to heat sink thermal resistance

and

included

in

Fig.

18 indicates the basic elements to

in

the thermal design. The values that

and

the

heat sink to ambient interface

Calculations

in

the thermal data.

are

(R

ocs
(ROSA)'

(Rocs)

The

determinable

)'

as previously

less.

sim-

be

Fig. 18 illustrates the thermal relationships between the output
semiconductor junction and the surrounding ambient.
temperature gradient or drop from junction

to

T,I -TA

ambient, which

is

the

is

the
sum of the thermal resistances multiplied by the junction power
dissipation

(P

watts). Hence:

by

= 100 - 28.8

it

(b)

To

determine required heat sink thermal resistance,

for

71.2°C maximum ambient temperature and a

10 amp load

(12

=

watts):

71.2°C

is

100 - 71.2

12

=

1°CIW

(c)

To

determine maximum load current, for 1°CIW heat sink

and

71.2DC

ambient temperature:

P=--------

-

(1.3 + 0.1)

-

T

TA

J

+

Rues + RUSA

100 71.2

1.3+0.1+1.0

Where

T

,J

Junction temperature,
Ambient temperature,
Power dissipation X

c'C

DC

watts
Thermal resistance, junction to case °C/W
Thermal resistance. case to sink, °C/W
Thermal resistance, sink to ambient, °C/W

To

use the equation, the maximum junction tempemture must be

known,

typically 125"C, together' with the actual power dissipation,

hence,

=

12

watts

P

12

1.2

= 10 amperes

Regardless of whether the
cooled by other means, it
conditions by making a direct base plate temperature measurement
when certain parameters
used except
ambient
temperature gradient now becomes TJ -

resistance
(P

watts). Hence:

Parameter relationships
maximum allowable case temperature, maximum load current, and
required junction to case
two parameters are known, the third can be found
following examples (using previous values):

(d)

To
for

that

temperature

(RsJcl multiplied by the

determine maximum allowable case temperature,

RSJC

= 1 .3°C/W and 10 amp load

SSR

is

used

on

is

possible to confirm proper operating

are

known. The same basic equation

base plate temperature

) and

(T

are

Rscs

A

similar

in

(RsJcl

thermal resistance. Again, where

TJ - Tc = P

and

junction

that solutions can be found for

(12

(R

SJc

a heat sink or the case

(Tc)

is

substituted for

RSSA

are deleted. The

Tc

that

is

the thermal

power

dissipation

as

shown

in

watts):

)

the

is

is

(n

To

determine required thermal resistance

case temperature and 10 amp load

100 - 84.4

12

In

examples
mined
Similarly,
air

without
radiating characteristics of the package
given and when
stated

(a)

through

as

they relate to ambient

conditions can be determined for

a heat sink, provided that a value

it

as

(R

)'

The equation would appear

SJA

(c)

SSR operating conditions are deter-

air

temperature using a heat sink.

is,

it

is

more commonly combined with

(12

watts):

an

(R

SCA

(RsJcl,

SSR

)'

This value

as

follows:

for 84.4°C

operating

is

given for the

is

(RsJcl

in

free

rarely

and

hence,

(e)

To

determine maximum load current, for

84.4°C case temperature:

P=

=

hence,

ILOAD=-E--

= 12 x 1.3
= 15.6

Tc

= TJ -15.6

=100-15.6

100 - 84.4

1.3

12

watts

DROP

P

RSJC

= 1 .3°C/W and

Or

Where

RSCA

= Thermal resistance, case to ambient, °C/W

RSJA

= Thermal resistance, junction to ambient, °C/W

The equation can be used to calculate maximum load current and

as

maximum ambient temperature
values

are

inclined to be

affect the case

stacking, air movement, etc).
Generally, free air performance

SSRs

of 5 amps or

The question

measured. There
made more difficult when the SSRs are closely stacked, each
creating a false environment for its neighbour.
approach
horizontal plane approximately 1 inch away from the subject
This technique

is

is

to place a temperature probe or thermocouple

is

less

to

air relationship (i.e., positioning, mounting,

less,

which

often raised

is

no clear-cut answer for this. Measurement

reasonably accurate and permits repeatability.

before. However, the resultant

precise due to the many variables that

is

associated with

have

no metallic base to measure.

as

to where the air temperature

PCB

or plug-in

One suggested

in

the

SSR.

is
is

12

1.2

= 10 amperes

Ratings

The free air performance of lower powered

in

the catalogue by means of a single derating curve, current versus
ambient temperature based
adequate for most situations.

on

the foregoing formulas, which

SSRs

is

usually defined

is

-~--j/-c--!---i

LOAD

-----,----;-i--

10 15

CURRENT

r-+-+--r--+----i-_----"~~=__L--1'1O

25/0 10 20 30 40 50

(ARMS)

20

MAX

AMBIENT

TEMPERATURE

60

rC)

70 80

100

105

As

a general rule, a heat sink with the proportions of the 2 inch

length of extrusion

amps, while the 4 inch length (curve

(curve(a))

is

suitable for SSRs rated up

(b))

will

serve SSRs rated up to

to

10

20 amps. For power SSRs with ratings greater than 20 amps, a
heavy

duty

heat sink of

the

type

shown

in

Fig. 22

becomes
necessary. The performance of a 5.5 inch length of this extrusion
would approximate the characteristics shown

in

Fig.

23.

-,--

Fig.

19

Thermal

derating

curves

(25

A

SSR).

Heat

Sinking

Under worst case conditions the

exceed the maximum allowable shown

SSR

case temperature should not

in

the right hand vertical

scales of Fig.19.
A typical finned section of extruded aluminum heat sink material

shown

in

outline form

would approximate the same thermal characteristics

Fig.

21, likewise, a 4 inch length would approximate curve

is

assuming the heat sink

an

plane, with

Fig.

20

unimpeded air flow.

TypicalligtJt

duly

aluminum

in

Fig

20. A 2 inch length of this material

is

positioned with the fins

0.15

INCH

heat

sink

extrusion

(end

view).

as

curve

in

the vertical

(a)

(b).

This

2.62

INCHES

4.75----

INCHES

Fig.

22

An

end

view

of a typical

heavy

duty

aluminum

heat

sink

extrusion.

is

100

in

~

~

80

m

:2

<{

~

0

60

~

ill

W

5'

~

40

~

'"

20

'"

20

Fig.

23

Typical

free-moving

temperature

Not

all

heat sink manufacturers show their characteristics

degrees

C per watt
above ambient,
found by dividing power dissipation

(DC).

For

example, taking the 50 watt point
the

free

air

curve would indicate a 40 degree

40

rise

as

60

air

characteristics

versus

power

(OC/W);

shown

80

100

POWER DISSIPATED

(WAnS)

of a heavy

dissipated.

some show them

in

Fig.

23.

In

this

(watts)

120

140 160

duty

heat

sink,

as

a temperature

case,

a value for

into the temperature

on

the dissipation

rise.

Hence:

in

terms of

RUSA

rise

is

rise

scale,

Fig.

21

~

\-'

"',

\2

'S

w

0

Z

~

iii

a:

<i

:2

a:

ill
I

r-

Typk;al

3.0

2.5

2.0

1.5

1.0

'-----~-~-----~----'--~

heat

sink

10

characteristics.

15

DISSIPATION [WADS)

20

40
50

= 0.56°CNV

In

many applications, the

wllich may

30 35

25

flatness, using thermal compound. and removing paint to maximize
effectiveness, a base plate

also

be more than adequate

maximum ambient may
operation

as

previously mentioned.

SSR

be

is

mounted to a panel or base plate,

as

a heat sink.

(SSR)

temperature measurement at

all

that

is

necessary to confirm proper

By

ensuring

If

When a load rating is close

an

SSR installation does not provide
selection
types that are available. Each configuration has its own unique
thermal
manufacturers' performance curves and applications data.

Surge
Current

is

made from the wide variety of commercial heat sink

characteristics

Ratings

and are usually well

and

High

Loads

an

adequate heat sink, a

documented

Inrush

with

It

is

generally derived from the peak surge

rating

as

follows:

Where

Ipk peak surge current - (sinusoidal)

duration of surge (normally 8.3
(.0083 seconds

For example, for a 25 amp
rating, the value would be

in

SSR

with a 250 amp one-cycle surge

260 amp-squared seconds.

(one

the formula)

cycle)

output thyristor

I-tS)

After

improper
common causes of
seriously impair the

it

would be wise

the load.

are

There

such as heating elements and incandescent lamps, can prove
problematic. Capacitive loads can also prove equally problematic
because of their initial appearance
currents can occur while charging, limited only by circuit resistance.
Inductive loads, on the other hand, tend to impede high inrush
currents;
express purpose of
filters, chokes, etc.). However, inductive loads can give rise to high

inrush currents.
Inductive loads have traditionally created more problems

rather than turn-on due to stored energy and
inherent zero current turn-off characteristics of thyristors used
SSRs

is

Surge

Ratings

The highest surge current rating of
steady-state
nonrepetitive peak current for one
a surge of this magnitude

SSR

lifetime. The preceding cautionary notes would tend to reduce

the attractiveness of the high surge capability

SSR;

however, they apply only to the extreme limits where the
should not be designed to operate anyway. When a reasonable
surge safety margin

Generally,
since the output transistors (nonregenerative) are usually rated for
continuous operation
for

the

DC
excessive
destroy the relay if the surge
capacity
fuse protection, the
current capability).

heat

sinking, surge current is

SSR failure. Overstress of this type can also

life of the

to

carefully examine the surge characteristics of

very few completely surgeless SSR loads. Resistive loads,

in

fact, inductance

limiting high fast rising peak currents

most beneficial

RMS value, and

is

DC

SSRs do not have

SSR

to

cut off

current

is

However, the resultant over-dissipation may

required,

SSR.

Therefore,

as

short circuits. High surge

is

often inserted into a circuit for the

in

this regard.

an

SSR

it

is

usually given as the maximum

line

cycle.

is

allowable only 100 times during the

used, conditions rapidly improve.

an

overcurrent surge capability,

at

their maximum capacity. The tendency

(culTent

as

SSR

could be over specified (have a

limit), thus impeding the flow of

IS

prolonged.

may

be

the

one

of

the more

in

a new application

(e.g.

on

"back EMF". The

is

typically 10 times the

It

should be noted that

(100%) of the AC

If

overcurrent

case

when

designin[1

turn-off

in

can'yin[1

hi[1her

EMI

AC

SSR

Inductive

High inrush lamp and capacitive loads sometimes include a series
inductor such

initial inrush current, but the combination

the SSR

include some inductance, its effect
negligible.

function, such

significant influence

The majority of SSRs

as
current loads relative to their rating. GG4?WHFKGGRF?FUGPGVDGDCrydom relays
for operation

to the minimum current rating of the

become
leakage may
small solenoids that
continue to
means of a shunt or parallel impedance, thus reducing this voltage

below

A saturating inductive load
the

normal conditions. However, when saturation occurs the

tance falls to a very low value, resulting

to that of the Copper resistance of the coil winding.
several cycles of surge currents
state value, which

Transformer

Extremely high current surges
formers, especially those with a penchant for saturation.
voltage
possibility and might require that special precautions

is

At the instant turn-on, transformer current

the
the
core. When the
dictated by power factor, a maximum back
will tend to counter the magnetizing current, thereby reducing or
eliminating the surge.

loads

as

a choke or transformer. This

as

an

inductive load. While most

with resistive loads

Only those loads that utilize magnetics to perform their

as

transformers

on

SSR

will

low

as 0.3, especially if they are switching medium to high

at

0.5

pf.

When a load

significant

run.

the drop out or off threshold of the load.

SSR.

The

when

have a deleterious effect on certain loads such

fail

The solution

AC

impedance of such a load

may

seriously affect the lifetime.

and

chokes,

operation.

operate inductive loads with power factors

is

so light

SSR,

compared

to drop out, or motors that buzz or even

is

to reduce the load impedance by

can

also cause switching problems with

in

excess of 30 times the steady-

will

tend to limit the

will

primarily be seen by

SSR

loads, even lamps,

is

usually

are

likely to have any

that its rating

the off state leakage may

to

the load current. The

is

relatively high under

in a fall

in

impedance close

This

is

induc-

can

SWitching

are

commonly associated with trans-

The

turn-on

highest peak usually occurring within a half cycle, depending

line

phase angle, load power factor, and magnetic state of the

feature

SSR

of

standard

is

energized at the ideal phase angle,

SSRs can increase

is

EMF

be

essentially

is

generated that

taken.

zero,

are

close

as

cause

zero

this

with

on

as

To

aid

in

the proper design of
given. This parameter expressed
useful

since

it

can relate directly to the published fuse charactei'istics.

SSR

fusing,

an

I:'t

rating is usually

in

ampere-squared seconds

However, when switched on

is

EMF is reduced, allowing
that can be further enhanced by residual magnetism

at,

or

neal',

zelo voltage, the back

an

increase in magnetizing current

in

the core,

which almost always exists since ferromagnetic core material has a
natural tendency to

If

a random turn-on

likelihood of transformer core saturation

remain

magnetized at turn-off.

SSR

is

used to switch transformer loads, the

is

greatly reduced.

While soft start spreads the inrush current over many cycles, thus
reducing stress, it
saturating currents.
a brief burst of

EMI

also prevents the occurrence

Due

to its phase control nature, it can produce

noise during the ramp up period; possibly a

of

enormous

small sacrifice for the added benefits.

Switching

Dynamic loads, such
special problems for SSRs, in addition to those discussed for
passive inductors. High
stationary impedance
initial

surge, a solenoid core

steady-state current, possibly by dropping to

motors, the change
possibly dropping to

As

a motor rotor rotates, it develops a back
flow of current.
line voltage

and create "overvoltage" conditions during turn-off.
Mechanical loads with a high starting torque or
fans and flywheels, will,

period, which

SSR.

driving

When

case with a power
One of the best surge reducing techniques

with a

typical waveform on
system, once the
internal circuitry that advances the turn-on phase angle over several
half cycles.

The slow transition to
the problems associated with zero, random, peak, and integral
cycle turn-on.
and

could

be

applied

as

motors and solenoids, etc., can create

initial surge current

is

usually very low. For example, after the

in

current from stall to

less

than 20%, depending

This

same back

of

will

pull

EMF

course, prolong the start-up surge

is

drawn because their

in

and "seal" at a much lower

less

than 25%. With

run

can be

even

greater,

on

the type.

EMF

that reduces the

can also add to the applied

high

inertia, such

as

should be taken into account when selecting the

the mechanical load

tool, worst case conditions should

an

expanded scale

control signal

This

is

also beneficial for lamps
in

is

most general applications.

is

unknown,

is

the soft start shown

in

Fig.

applied, the

full

line

SSR

voltage virtually eliminates

and

as

may

be

the

apply.

24.

With this

is

ramped

on

by

capacitive loads

The inrush current characteristic of tungsten

lamps is
thyristors used
The

parameters from a

somewhat

similar

to

the surge characteristic

in

AC

SSR

outputs, making them a good match.

typical ten times steady-state ratings which apply to both

cold start allow many
current ratings close to their own steady-state ratings.
have

even

higher instantaneous inrush currents. This

in

practice,

since line and

source

inductance become significant at higher currents,

limit the peak current. Generally the ten times steady-state rating

to

is

considered a safe number for lamps.

Protective

APPLIED

VOLTAGE

seR

VOLTAGE

Measures

DRIVE

CIRCUIT

~,

i:=-::-:-":c='c==~~~

~O-N-~~I

\,

,:'

/

---O~N--~'~~~

filament (incandescent)

SSRs

to switch lamps with

Some lamps

is

rarely seen

impedances

COM~~~~TING

--0

and

all

of which tend

(A)

SCR as

output

conducting

to

filament

switch

forward blocking)

of

(dv/dt

the

from

Fig.

24

0-100rns

100-200ms

200-300ms

Typical

ramped

circuit

on

over

configuration

14

cycles.

(B)

and

(A)

Circuit

Voltage

voltage

wavefolTIl

waveform

SSR

OUTPUT

10UTPUT

TRIGGER

PULSE

Fig.

25

Tum-off

conditions

for

SeRs

in

full-wave

bridge

circuits

inductive

loads.

of

soft

start,

with

phase

angle

switching

Noise

Susceptibility

Noise,

or

more properly defined

does

not

(EMI),
Some
and

of

drive

generally cause SSRs

the

techniques

circuits

are also
caused by voltage transients
added, for example, the response time which

as

Electromagnetic Interference

to

fail catastrophically.

used

to

reduce noise

effective

on

the input lines. When a capacitor

against

in

false

is

not critical for AC

the

coupler

triggering

SSRs may be lengthened, possibly from a few microseconds
tenths of milliseconds.
or

bursts

of

shorter

Due

to the induced

duration

are

delay,

rejected,

voltage transients

thus

improving

noise immunity.
Most AC

SSRs use thyristors

in

their drive and output circuits which,

due to their regenerative nature, can latch on for a whole half cycle

when triggered by a brief voltage transient, thus acting

stretcher.

In

addition to responding to the amplitude of the transient,

as

a pulse

a thyristor can also mistrigger when the rate of rise (dv/dt) of a
transient
suppressors
improves the tolerance of

dv/dt

or

applied voltage

are

(Rate

Effect)

exceeds

certain

limits. Transient

effective against the former, and the

an

SSR

to the latter.

RC

snubber

The expression dv/dt defines a rising voltage versus time expressed

in

volts per microsecond (VhlS). When applied

as "static" or "off state" dv/dt,

minimum dv/dt withstand capability of the

it

is

a parameter that defines the

SSR

or,

to

in

an

AC SSR

other words,

the maximum allowable rate of rise of voltage across the output

will

terminals that

not turn

on

the

SSR

(typically 500

V/rJ.8).

is

to

1N4005 l

28 Vdc 28 Vdc

1N4005 i

1N4753

~

1'1N4753

115 Vac

1'1N4753

Snubber

The internal
factor
with

two
slow
and sensitive drive circuits, but
can

rise.

While the typical internal snubber value and the typical
specification are adequate for
prevent what is commonly referred
problem
initially applied to the
mechanical switch, the resultant
the SSR and possibly "let through" a half cycle pulse. Fortunately,
most loads

RC

network (snubber) used

in

transient voltage

and

dV/dt suppression.

in

AC SSRs

is

It

deals effectively

facets of a voltage transient. Not only does the network

down

the rate

of

rise as seen

it

by

the

output

thyristors

also limits the amplitude to which

most

applications, they may not
to

as the "blip" or "bleep"

which occurs during start-up. That is, when power is

are

not troubled

SSR/load combination usually

fast

rising transient may Illistrigger

by

this pulse.

by

Ineans of a

a major

dv/dt

Fig.

26

Transient

suppression

techniques.

it

Suppressors

When overvoltage tmnsients occur, another form of sUPPl'8ssion
may

be required beyond the capabilities of the snubber. One
popular technique
terminals
predetermined

Devices, such

determined

load.

If

it

is

unacceptable for the load to receive

Ule

only solutions

SSR

with a blocking capability higher than the transient.

Fig.

26

illustrates typical
across the SSR
transients at the source. which call be the load itself for
inductive type loads.

is

to add a clamping device across the SSR

that

will

absorb

the

transient

energy

level.

as

zeners

and

MOVs,

will

conduct oilly

level

and above, thereby sharing the transient with the

any

transient

may

be

suppression of the transient source, or

output

methods

"contacts",

of

suppressing

as well as suppression of

above

at

the pre-

energy,

an

ti'ansients

DC

a

Diodes

and

Zeners

The diode shown across the load

in

A of

Fig.

26

is

the most effec­tive way of suppressing the possibly hundreds of volts of back EMF
that can be generated by the
this

method

are the SSR

is

sources, and the dropout time

coil at turn-off. The disadvantages

not

protected

of

the load may be extended by

from

other

of

transient

several milliseconds.

PEAK

PULSE

CURRENT

I

+

CLAMPING

VOLTAGE

The zener diode
SSRs

(less
the forward current mode (reverse for the
typically clamps

is

the ideal choice for protecting low voltage DC

than 100 volts

DC)

used

in

parallel with the output.

SSR),

as

a single diode would at approximately one volt,

the zener diode

thereby providing added reverse-voltage protection. When
zeners

are

used back-to-back
ages, they can be used to protect
switching AC

or DC,

loads. At higher voltages (greater than 100 volts) AC

economics

versus

transient protective device such

(in

series) with equal standoff volt-

SSR

outputs bidirectionally when

performance

as

may

suggest

the MOV (metal oxide varistor)

another

being the most popular.

In

two

FORWARD

CURRENT

CLAMPING

f

VOLTAGE

IA) Z81l8r

diode

V-I

Fig.

27

Comparison

The general

characteristic

of

zener

diode

and

MOV

characteristics.

rule

in

the selection of protective diodes and zeners

(B)

that their peak nonrepetitive (pulse) current ratings

be

equal

to,

or

greater

than,

the

minimum

MOV

I

\}-I

characteristic

LEAKAGE

CURRENT

(Fig.

load

27) should

current.
Conservative steady-state power ratings for these devices may be
ascertained from the following equation:

where

IL

VZ~

= load current

L

= load inductance

t,

= on/off repetition rate

in

DC

in

amperes
henrys

in

seconds

MOVs

For more hostile environments, the MOV can be used as follows:
across the incoming
can enter the system; across the
transients;

or,
transient sources.
mounted to the same

is

the impedance of the
30

joule unit

enough to be supported by its own leads.

If

a MOV

is
limiting impedance will only be that of the power generating source
plus the wiring.
transients from such a low impedance source, the larger panel

mount

(300 - 600 joule) variety of MOV may be required. The

greater

expense

suppression across the
Individual MOV specifications should be consulted

information regarding energy absorption, clamping properties and
physical

size,

from one manufacturer to the next.

line

to suppress external transients before they

load to suppress load generated

more frequently, across the

In

the latter case, the MOV can be conveniently

SSR output terminals

load

in

series with the MOV to limit current, a

is

usually adequate for brief spikes and also small

connected directly across the power

In

order to absorb the possibility of high energy

of

such a device

line

is

required

SSR

to protect

as

the load wiring. With

might

be

in

one place

line,

the current

justified

only.

for

it

from

in

precise

since the relationships of these parameters will vary

all

line

that

Example: A load with a resistance

of 0.0025 henry
switched

is

driven from a

on

and off 5 times a second:

1=---

L 4 ohms

= 7 amperes

t = - = 0.2 second

r 5

p=

= 0.613 watt

28 volts

1

2

7

A protective diode or zener with a

of

4 ohms and

28

volt

DC

x .0025

0.2

3/4

watt rating would suffice.

an

supply wllile being

inductance

Fuses

Semiconductor fuses are usually used
and

are

specialist fuses designed to protect while operating at close

to their full ratings. They are sometim8s ref8rr8d to as current

limiting fuses, providing extremely fast opening, while restricting let

through current far below the available fault current

destroy the

SSR.

Although these fuses
provide a means of protecting SSRs against high current overloads
where survival of tile

The

following

SSR

is

of prime importance.

are

the main parameters used
semiconductor fuse:

• Fuse voltage rating

• Fuse current rating

• Available system fault cLirrent

• Fuse peak let through cLirrent

• Fuse total clearing
withstanci capability of the

• Surge

(or

let througll)

SSR

in

conjunction with SSRs

that

are

not low cost, they

in

the selection

could

do

of

a

SSR

Applications

The diagrams

typical
steer the user
ideas.
protection

in

this section

SSR

applications. They

in

the right direction and to stimulate further design

are

conceptual illustrations of just a few

are

intended

as

design guides to

Some of the diagrams provide problem solving or circuit

and

others enhance

relay

operation.

AC

POWER

RUN

WINDING

Latching

SSR

Momentary push-button control allows the SSR to self-latch for
on-off, stop-start operations.

in/DC out type
Resistor

altemate

AC

POWER

(120-240

V)

SSRs.

R1

(10,000 ohms)

(N

0)

switch

-i

b

S~A~T

rv

is

R,

It

may be similarly configured for

is

required to prevent line short only if

used.

AC

OUTPUT

SSR

AC

19

INPUT 9

J I

10kQ

DC

1

NO

Lc

I] STOP

-i

Fig.

28

Latching

SSR

circuit.

Latching

Push-button control
to
time), thus preserving

SSR

with

Short-Circuit

as

in

Protection

the previous example, but

limit the load shorting current to

SSR

while the control signal

Latching characteristic permits lock-out

STOP

(ALTERNATE)

-i~

SSR

surge rating (for turn-off

until

the circuit

R2

is

is

removed.

is

reset.

tailored

Fig.

30

Motor

starting

switch.

Functional

Two
system. A third
the Y load were grounded,

SSRs

Three-Phase

may

be

SSR

Switch

for

used to control a Y or a delta load

would

be

required

as

in

a four-wire system.

Three-Wire

in

phase C

System

in

a three-wire

if

the center of

SSR

voltage
rating must be greater than line to line voltage for three-wire
systems and line to ground voltage for four-wire systems (with

neutral ground).

SSRs are most commonly used

control motors, where their current ratings depend
locked motor current

as

they do

heat sinking. Where a motor rating

in

three-phase applications to

as

on

normal

run

current

and

is

not given, a minimum

much

proper

on

SSR

current value can be estimated from the device surge curves,

rule

using the general

of six times the motor

second. This value must also

be

commensurate with thermal

run

current for one

and

lifetime requirements.

PHASE

A

0------0

"lu---I-o

AC

10

AC

POWER

(120-240

V)

Fig.

29

Latching

SSR

with

short-circuit

Motor

Starter

Switch

Initial locked rotor current flowing through
when rectified and filtered, turns
the start winding.
R1

is

reduced until the start winding

SSR should have a voltage rating approximately twice that

The

the

applied line to withstand overvoltage generated by the

of
current

LC.

As

the motor comes to speed, the voltage across

protection.

on

the

is

R1

SSR,

de-energized.

kQ

creates a voltage that,

which

in

turn activates

PHASE 8

PHASE C

Fig.

0----0

0---0

31

Functional

";0--++0

:

MECHANICAL

SAFETY

DISCONNECT

SERIES

OR

PARALLEL

CONTROL

three-phase

switch

for

three-wire

system.

Phase-Controlled

Dimming

A 555 timer and a photocoupler may be used with a non-zero

SSR

switching (instant-on)

IC

is

The

operating

from the output of the

interval begins and the

timing
on

the time constant of

SSR

turns on for the balance of the half cycle. Simultaneously,
discharged through a transistor
repeats every

The

phase angle firing point
However,
on),

the

half cycle.

at

the higher

IC

may over-dissipate due

and possibly require a

to provide isolated lamp dimming.

as

a one shot, triggered by a negative pulse

zero

detector circuit

SSR

R1

C1,

IC

in

is

independent of

DC

voltages and shorter firing angles

small

heat sink.

(Q1).

Once triggered, the

is

off.

Upon time out, dependent

output

(pin

3)

goes low and the

the

IC

(pin

7),

and

the process

DC

control voltage.

to

the repetitive discharge of

C1

(full

C1,

CONTROL

SWITCH

is

Fig.

32

P/Jase-contro/led

dimmllJg.

Three-Phase

Four

AC
motor, using the drive
before enabling the drive,
break which
(nos.

1 and

to

dv/dt or high voltage transients; therefore, resistors

R4

are
two resistors
current to

PHASE A

)~-----

Motor

Reversal

SSRs

can

provide a reversing function for a three-phase

logic suggested. The half cycle time delay

in

either direction, prevents make before

would result

4,

or 2 and

in

a line to

3)

could still mistrigger simultaneously due

line

short.

Two

inserted to limit the resultant surge current. The

plus the source impedance should limit the shorting

less than the peak one cycle surge rating of each

F1

+5

Vdc

DIRECTION

LOGICAL
LOGICAL

1 = FWD

0 = REV

opposing SSRs

R1

through

sum

of any

relay.

Semiconductor type fuses should be chosen to permit such a
condition for one
SSRs should
line

to

generated

have

line

voltage to withstand the combined

at

the moment of reversal.

cycle and open

a transient (blocking) rating equal to twice the

R2

as

soon

as

possible thereafter.

line

and back

EMF

ENABLE

PHASE B

0

PHASE C

0

Fig.

33

J,~r8e-phase

8.3

ms

TIME

DELAY

motor

6-\

O\.y

reversal.

F2

F3

Reversing

In

this configuration, four

a single power

Motor

supply.

Drive

for

DC

Motors

DC

SSRs

are

used for motor

The time delay before enabling the drive

reversal

from

either direction must be greater than the SSR turn-off time
preclude

the

possibility

of a hazardous

make

before

break

condition.
Internal

reverse

diodes or zeners

transients across the

low impedance of the power supply.

in

the

SSRs

will

suppress inductive

If

no

internal suppressors exist, a reverse diode should be installed

each

SSR

output

or,

across

alternatively,

at least twice that of the supply voltage may

be

should
of

current limited or fused to protect the wiring

a short circuit.

SSRs

with blocking ratings
be

used. The circuit

in

the event

in

to

INPUT

CONTROL

(SERIES

PARALLEL)

~V1-

OUT v

SSR1

(40 A MAX)

~V2-

(40 A MAX)

-0

IN

OUT

SSA2

IN

OR

L--

X

11

<;>

I

Rx

12

<;>

LOAD

72A

I

ACORDC

POWER

SUPPLY

I

Fig.

35

Paralleling

SSRs.

For zero voltage turn-on thyristor types, either one of the SSRs

must

be

capable of handling the

of a possible

half cycle mismatch. Thyristor

tum-on problems that can prevent

initial

full

paralleling.

load surge alone because

SSRs

have

additional

I

FWD

tr

(.)

MOTOR

DIRECTION f--"'r:=:r-=:::;=-1

OFF

FWD

REV

Fig.

34

Reversing

motor

drive

for

DC

motors.

Paralleling

SSRs with

SSRs

MOSFET

outputs

are

self balancing

and

easily

paralleled,
whereas most others with bipolar or thyristor outputs require special
attention.

the forward voltage drops should

be

matched to

Ideally,

achieve thermal balance and lowest dissipation; alternatively,

(Rx)

are

balancing resistors
For example, with
carry

32

amps.

Assuming

Vi

= 1.3 volts and

used to force current sharing

40 amps allowed through SSR1, SSR2 must

V2

= 1.5 volts (worst

/).V

R

=--

x

/).1

case).

as

shown.

Transformer

If

+

a momentary interruption
operate
current surge from a shorted winding. Two times
winding resistance must
the one cycle surge rating of the

As

an
should exceed the main winding voltage
voltage.

DC

controlled
Figs.
SSR1
requirement of
ted

120
be

on

Tap

Switching

in

power

is

acceptable, a time delay

is

suggested to prevent overlap and the resulting high

Rx

be

sufficient to limit the surge current to

SSRs.

additional precaution, the

SSR

blocking (breakdown) voltage

plus the highest tap

For

mUlti-tap switching the

AC

output types without special requirements.

368

and

C,

they

are

240

and 240 Vac input for SSR2

SSR2

is

that

Vac

line swing,

say

it

150

SSRs

Vac

output with 120

must

be

Vac.

are

generally logic driven

Vac

in

each case.

An

off below the highest expec-

When

SSR2

is

off,

and vice versa, thus activating the appropriate winding.

on

plus the

For

input for

important

SSR1

will

or

V2

-Vi

11

-12

= 0.025 ohm

Thus producing a total voltage drop

1.5 - 1.3
40-32

of

2.3 volts.

PRI

:l

PO:]

ACO
CON

INP

SEC

~
~

~
~

e.

~
~

~

~

;

~

~

;
;

>=

RDC

r-

TROL 2

UTS

COM

3

SSR1

'?

(A)

Multitap switching

SSR2

SSR3

'(

R

RX

RX

o~

X

AC

POWER

120/240

(8) Autotap selection (secondary)

Fig.

36

Transformer

Testing

tap

the

switching.

SSR

Many of the tests required to verify

SEC

SSR

performance

SSR1

INPUT

SSR2

INPUT

AC

AC

RX

RX

are

inherently

120

RL

Vac

hazardous and caution should be excercized, using adequate
safeguards for the

Possibly

the

determine proper function of

personnel conducting such tests.

simplest

of

all field

an

AC

tests

SSR

that

can be made

is

by means of a 3 volt

to

battery, a light bulb, and a piece of insulated wire. This simple
go/no-go test

Fig.

37

,-----+0+

Simple

is

illustrated by

INPUT

go/no-go

SSR

test.

Fig.

SSR

UNDER

TEST

37.

LAMP (40

W)

120

Vac

SSR2

AC

INPUT

AC

POWER

120/240

(C)

Autotap selection (primary)

SSR1

AC

INPUT

RX

CT

RL

A more complete performance check might include operating the
SSR

in

position with its actual load, while exercising the system

installation functionally through

all

of its specified environmental and

power combinations.
When connecting test equipment

SSR

output, protective fusing would
remember that with some equipment such
case must

some

transformer can

of

an

be

"floated" (ungrounded) and may

test

circuits

SSR

should not ever

an isolated

be

used to avoid this hazard.

mode, since the minimum
for

proper

SSR

operation

directly to the power circuit of

be

a wise precaution. Also,

as

an

oscilloscope, the

be

at

line

be

checked

current

probe

The

by

a multimeter

or

an

output functions

voltages and bias currents necessary

are

not

present,

thus

erroneous readings.

an

potential.

isolation

in

the ohms

producing

In