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Question
The SSR does not turn OFF. What is causing this and what can be done about it?
Answer
There may be separate causes for the input side and the output side. The following provides separate explanations.
Input Side:
1.
Leakage current of the drive circuit on the input side may be causing a reset failure. Insert a bleeder resistor as a countermeasure.
Connect a bleeder resistor as in the figure above and set the bleeder resistance value so that the SSR input voltage is 0.5 V max. when the SSR is OFF.
2.
Inductive noise may be inducing voltage on the input circuit. As a countermeasure, separate the power lines and the signal lines. Also use twisted wire or shielded wire to reduce the inductive voltage to below the SSR reset voltage.
Output Side:
1. Reset Failure due to SSR Leakage Current
A few milliamps of leakage current IL flows on the output (load) side of the SSR even when there is no input signal. Therefore, this leakage current may cause reset failures if it is larger than load reset current. As a leakage current countermeasure, connect a bleeder resistor R in parallel with the load to bypass the SSR leakage current.
2.
Reset Failure Using Load with Low Power Factor
If the power factor of the load is low (guideline: cosΦ=0.4 max.), the delay in the load current phase will increase relative to the load power supply voltage phase, and a large transient voltage (dv/dt) will be applied to the SSR when it is about to turn OFF (i.e., the load current is near zero), resulting in the possibility of the SSR not being able to turn OFF (commutation failure).
The SSR has a built-in CR snubber circuit to limit the rate of change in the transient voltage, but leakage current will increase if the C value is increased, and reset failure in item 1 above may occur, so the C value is set to the greatest common factor.
Therefore, if reset failure occurs because the load power factor is low, the rate of change in the transient voltage can be limited to prevent reset failure by connecting a capacitor and resistor in parallel with the SSR load terminals.
The capacitor and resistor must be checked to match the load, but previous experience shows that a resistor of 100 Ω/1 W and a capacitor of 0.1 μF/250 VAC will prevent reset failure.
Also, as mentioned above, the leakage current will increase, so check that the reset failure in item 1 does not occur.
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