Solid-State Relays
Opto 22 • 43044 Business Park Drive • Temecula, CA 92590-3614 • www.opto22.com
SALES 800-321-6786 • 951-695-3000 • FAX 951-695-3095 • sales@opto22.com • SUPPORT 800-835-6786 • 951-695-3080 • FAX 951-695-3017 • support@opto22.com
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Solid-State Relays DATA SHEET
Form 0859-170111
PAGE
21
Q: Which SSRs should I use with a printed circuit
board (PCB)?
A: If you are mounting SSRs to a PCB, use the MP or P series
SSRs , which are designed for that purpose.
We do not recommend mounting the terminal side of a Power
Series or Z series SSR to a flat PCB (or to any other flat surface),
because there may be some variation in terminal height or
alignment from one terminal to another and from one SSR to
another.
Q : Do you make multi-pole or multi-throw SSRs?
A: Opto 22 manufactures only single-pole, single-throw SSRs.
If multi-phase operation is required, just use a relay on each
phase. Because of the limitations on semiconductor devices of
the type used in SSRs, it is not practical to build single-device
multi-throw SSRs. However, an alternative to multi-throw
operation may be accomplished with multiple relays.
Q : Can I hook up SSRs in parallel to achieve a higher
current rating?
A: No. There is no way to guarantee that two or more relays
will turn on simultaneously when operated in parallel. Each
relay requires a minimum voltage across the output terminals
to function; because of the optical isolation feature, the
“contact” part of the SSR is actually powered by the line it
switches. One relay turning on before the other will cause the
second relay to lose its turn-on voltage, and it won’t ever turn
on, or at least not until the first relay fails from carrying too
much current.
Q : What does a “zero-crossing” turn-on circuit refer
to?
A: An AC sine wave will be positive for the first half of each
cycle and negative for the second half of each cycle. The
voltage will cross through zero when the sine wave changes
from the positive half-cycle to the negative half-cycle, and vice
versa. So the voltage crosses through zero twice with each full
AC sine wave cycle. “Zero-crossing” turn-on means that the
SSR will only turn on when the AC sine wave passes through
zero voltage. The actual turn-on will occur at or near zero
voltage. All Opto 22 AC output solid-state relays are designed
with a zero-crossing turn-on circuit. Zero-voltage turn-on has
the benefit of minimizing electrical noise. All Opto 22 AC
output solid-state relays use a zero-current turn-off circuit as
well.
Q : Can I use an AC SSR to switch DC?
A: No. Because of the zero-crossing circuit described above,
the relay will most likely never turn on, and even if it is on, it
will most likely not be able to be turned off.
Q : Can I use a DC SSR to switch AC?
A: No. The semiconductor device used in Opto 22’s DC SSRs is
polarized. It may break down and conduct for the portion of
the waveform that is reversed in polarity.
Q : Can a DC SSR be used to switch an analog signal?
A: This is not recommended at all. First, the voltage drop across
the relay will cause signal loss. Second, the conduction
characteristics of the SSR are very non-linear at low operating
voltages and currents. Use a mechanical relay; it will work
much better.
Q : What agency approvals do your SSRs carry?
A: In general, Opto 22 relays carry UL, CSA, and CE approval.
See http://support.opto22.com. Additionally, some SSRs
contain VDE-approved optocouplers; contact Opto 22 for
more information.
FAQ: SSR Troubleshooting
Q : My SSR does not function anymore. What may
have happened?
A: There is no “normal” mode of failure for SSRs. They just stop
working, by refusing to turn on or off. An improper installation
is often to blame for an SSR failure, as these are very simple,
reliable devices. If you have a failed SSR, it is important to look
at the normal operating parameters of that relay within the
larger system to make sure that the relay being used is
appropriate to the application, and that the relay is being
properly installed in the system. The three most common
causes of SSR failure are as follows:
• SSR improperly matched to load. The relay was
destroyed by overheating from carrying too much
current too long.
• SSR insufficiently protected. Remember, a
semiconductor is less tough than a simple metal contact.
Reverse voltages exceeding the PRV rating of the relay
will cause damage. Voltage spikes on the switched line,
perhaps from inductive kickback, may have destroyed
one or more of the internal switching devices.
Remember to use snubbers, transorbs, MOVs, and/or
commutating diodes on highly inductive loads.
• SSR improperly installed. The SSR was not mounted to
a large enough heat sink, or no thermal compound was
used, causing the relay to overheat. Also, insufficient
tightening of the load terminals can cause arcing and
ohmic heating of the relay. Opto 22 recommends 18
inch-pounds of torque on the load screw terminals.
Similar failures have also been attributed to the use of