Solid state relays (SSRs) are simple, reliable devices. If one stops working, typically it just refuses to turn on or off.

How do you test it to see if it has failed?

If you're used to testing mechanical relays, you'll find an SSR requires a different testing method: it requires a load.

A typical SSR always shows an infinite impedance to a resistance meter placed across the output terminals. That's because:

• An SSR requires a small amount of power to operate, derived from whatever voltage source is placed on the load terminals. A typical multimeter does not supply sufficient voltage to cause the relay to change state.
• AC SSRs contain a zero-crossing circuit (more info below). This circuit does not allow the SSR to turn on unless the field terminals see a transition through zero volts. Since most test equipment supplies a DC voltage to the relay, the relay will never see the zero it requires to turn on.

To test an AC solid state relay, then, you'll need some sort of AC load. It's best to operate it at the actual line voltage at which it's used, driving a load such as a standard incandescent light bulb.

Causes of failure for SSRs

If you have a failed SSR, take a close look at the normal operating parameters of that relay within the larger system. Make sure that the relay you're using is appropriate to the application and that it is properly installed.

The three most common reasons for an SSR to fail are:

SSR improperly matched to load. The relay can be damaged by overheating from carrying too much current, too long.

SSR insufficiently protected. Remember that a semiconductor isn't as tough as 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, can damage one or more of the internal switching devices. On highly inductive loads, make sure you use snubbers, transorbs, MOVs, and/or commutating diodes.

SSR improperly installed. If the SSR was not mounted to a large enough heat sink, or no thermal compound was used, the relay will overheat.

Insufficient tightening of the load terminals can cause arcing and resistive heating of the relay, too. Opto 22 recommends 15 to 16 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 these terminals are tightly crimped, and even drip some solder into the joint to ensure good electrical contact and protection from corrosion.

More about zero-crossing circuits

"Zero-crossing" turn-on and turn-off refer to the point on the AC waveform when the voltage or current is zero. It is at this point that an AC SSR will turn on or off:

• Zero-voltage turn ON occurs when the AC voltage waveform passes through zero.
• Zero-current turn OFF occurs when the AC current waveform passes through zero. 

Opto 22 AC relays are designed with a zero-crossing turn-on and turn-off circuit. When the AC waveform crosses zero, transients are minimized. This makes it much easier and safer for the semiconductor device in the relay to be turned on or off. It also generates much less electrical EMI/RFI noise.

By the way, this zero-crossing circuit means that you can't use an AC SSR to switch DC, because the relay will probably never turn on. And if it is on, it will probably not be possible to turn it off, as DC voltage typically never drops to zero.

Find the SSR you need

Opto 22 started business 40+ years ago with a new manufacturing method for SSRs that made them more reliable. Our SSRs are still known worldwide for their reliability. Find the SSR you need on our website.