240A25 View Datasheet(PDF) - Unspecified

Part Name

Description

MFG CO.

240A25

MP Model, 60 VDC, 3 Amp, DC Control Solid State Relay (SSR)

Unspecified 

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 happening?

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.

DATA SHEET

Form 0859-180904

PAGE 22

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