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

© 2006–2014 Opto 22. All rights reserved. Dimensions and specifications are subject to change. Brand or product names used herein are trademarks or registered trademarks of their respective companies or organizations.

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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

crimp-on terminal lugs or spades; make sure such

terminals are tightly crimped, and even drip some solder

into the joint to ensure good electrical contact and

protection from corrosion.

Q : How can I test my SSR?

A: It is not possible to test an SSR by the same methods used

to test mechanical relays; a typical SSR will always show an

infinite impedance to a resistance meter placed across the

output terminals. There are a few reasons for this. First, the SSR