Crydom Solid State Relays Application Note

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August2011

Output Switching Functions of
Solid-State Relays

Arguably, the technical aspect of a
solid state relay we discuss most
frequently with design engineers and
technicians is the manner in which AC
output SSRs switch power to a load. In
many cases we find that the subject is
something most engineers have never
even considered, especially those with a
background in electromechanical relays. In
this case they are simply accustomed to
energising a coil and waiting for the arcing
& clanking to occur shortly thereafter. The
more subtle nature of a solid-state relay is
still something of a mystery to them,
especially terms such as “zero-cross”,
“random”, and “proportional control” and
the benefits therein. Fortunately these
switching modes and how they apply to
different types of loads are fairly easy to
explain.

crossing solid-state relay. At point (1) the
control voltage is applied to the input of the
relay. However, the relay does not allow
load current to flow through the output until
point (2), which is the next zero-crossing
point of the AC sine-wave. At point (3) the
control voltage is removed from the input
of the SSR, which then stops conducting
load current at point (4).

Zero-Crossing Solid-State Relays

Also known as a “synchronous” solid­state relay, this is the most common type
of SSR found in the market today. As the
name implies, the switching of the relay
from a non-conducting to a conducting
state occurs when the AC mains voltage
reaches the zero-crossing point of the
sine-wave. This minimises the surge
current through the load during the first
conduction cycle and helps reduce the
level of conducted emissions placed on
the AC mains.

Figure 1 gives a simplified diagram of
the input and output signals on a zero-

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Crydom SSR Ltd.

Arena Business Centre, Holyrood Close, Poole, Dorset, BH17 7FJ
Tel: [phone] Fax: [phone]

www.crydom.com

Registered in England: No. 05602150
Registered Office: Arena Business Centre, Holyrood Close, Poole, Dorset, BH17 7FJ
VAT Re

: 876 2756 77

Figure 1; Simplified Zero-Cross Waveform

Note that the output of the SSR does
not stop conducting until the load current
reaches the next zero-crossing point of the
AC sine-wave. However, this is not related
to the “zero-crossing” function of the SSR.
It is due to the fact that the SCRs in the
output circuit cannot turn off until the load
current falls below their specified holding
current (typically less than 100mA). This is
a characteristic of all AC output solid-state
relays, regardless of the switching type.

Zero-crossing relays are ideally
suited for most commercial and industrial
loads, such as resistive heating elements,
lamps and ballasts, and any other load

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with low initial impedance or capacitive
characteristics.

Random Turn-On Solid-State Relays

Also known as “asynchronous” or
“instantaneous” solid-state relays, these
relays turn on immediately after the
application of the control signal. In most
cases the output is fully conducting load
current in less than 100µS.

Figure 2 gives a simplified diagram of
the input and output signals on a random
turn-on solid-state relay. At point (1) the
control voltage is applied to the input of the
relay. The output of the relay immediately
begins to conduct load current (point (2))
as opposed to a zero-crossing relay, which
will wait until the next zero-cross point of
the AC sine-wave. At point (3) the control
voltage is removed from the input of the
SSR, which then stops conducting load
current at point (4).

Figure 2; Simplified Random Turn-On Waveform



Crydom SSR Ltd.

Arena Business Centre, Holyrood Close, Poole, Dorset, BH17 7FJ
Tel: [phone] Fax: [phone]

www.crydom.com

Registered in England: No. 05602150
Registered Office: Arena Business Centre, Holyrood Close, Poole, Dorset, BH17 7FJ
VAT Re

: 876 2756 77

Random turn-on solid state relays
are commonly used in applications where
precise control of power to the load is
required (phase-control applications). They
are also commonly used with inductive
loads, where the phase shift between
voltage and current can cause problems
with zero-crossing relays.

Proportional Control Solid-State Relays

The most common types of
proportional solid-state relays on the
market today are phase-angle controllers
and burst-fire relays. These solid-state
relays provide proportional power to the
load (from 0% to 100% in most cases)
based upon the value of an analogue
signal applied to the input. This can be a
0-5V, 0-10V, 4-20mA, resistive value, or
other varying signal that translates into a
desired load-power level. These relays are
often found in heating applications
requiring extremely precise temperature
levels, and lighting applications requiring
the gradual increase and decrease in the
brightness of a room or area.

Figure 3 gives a simplified diagram
of the output waveform on a phase-angle
controller / SSR. In this example we have
a 5V analogue signal applied to the input
of a 0-10V SSR (50%). The corresponding
output waveform shows the SSR turning
on at the peak of each AC half-cycle,
effectively applying 50% power to the load.
If we gradually increased the analogue
input from 5V to 10V, then we would see
the shaded areas in the waveform diagram
slowly disappear until we reached 100%
power to the load.