Solid-State Relays

Opto 22 • 43044 Business Park Drive • Temecula, CA 92590-3614 • www.opto22.com

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requires a small amount of power to operate, derived from

whatever voltage source is placed on the load terminals. A

typical multimeter will not supply sufficient voltage to cause

the relay to change state. Second, AC SSRs contain zero-

voltage turn-on and zero-current turn-off circuits. The SSR will

not be able to turn on unless there is AC voltage connected to

the output terminals. Most test equipment will supply a DC

voltage to the relay, so it will never see the zero-voltage

transition it requires to turn on. To test an SSR, it is best to

operate it at the actual line voltage it will be used at, driving a

load such as a large light bulb.

Q : I have an SSR driving a load. The load turns on

okay, but never seems to turn off, unless I remove

power from the relay entirely. What might be hap-

pening?

A: This is normally a problem when using an SSR with a high-

impedance load, such as a neon lamp or a small solenoid.

Loads like these often have relatively large initial currents, but

relatively small “hold in” currents. The result is that the off-state

leakage current through the relay (see previous section) is

insufficient to cause the load to turn on to start with, but

sufficient to keep it on, once started. The solution is to place a

power resistor, sized for 8–10 times the rated maximum

leakage current for the SSR in parallel with the load. Make sure

that this resistor has a high enough power rating for the

application. For example, for a 5 mA leakage current at 120

VAC, a resistor drawing 50 mA would be desirable. Using

Ohm’s Law, the resistor value becomes 2,400 ohms. This

resistor will dissipate 6 watts, so a 7.5 or 10-watt size power

resistor should be used.

Q : I have a new AC SSR driving a solenoid. It turns on

okay once, but will not turn on again. What is going

on?

A: Some solenoids, some types of halogen lights, and some

types of strobe lights incorporate a diode in series with the coil

or filament. This causes the light to behave as a half-wave

rectifier. Opto 22 SSRs have a built-in R-C snubber circuit in

parallel with the output. The capacitor in this circuit charges

up but cannot discharge through the series diode, causing a

voltage to appear across the SSR terminals. Because the SSR

must detect the AC waveform cross through zero volts on the

load terminals, it will not be able to turn on again. The solution

here would be to put a high-value resistor (several tens of

Kohms) across the terminals of the relay, to allow the capacitor

to drain its charge.