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
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