FAQ No. FAQ02155
The SSR does not turn OFF. What is causing
this and what can be done about it?
There may be separate causes for the input side and the output side. The
following provides separate explanations.
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
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.
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.
Note: Microloads can be supported by using a
bleeder resistor as described above. MOS FETs, however, can be used to
directly switch microloads without a bleeder resistor.
2. Reset Failure Using Load with Low
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.
Other Solid-state Relay
Can the output sides of SSRs be connected in series?
How are the internal SSR circuits configured?
What are the characteristics of SSRs compared to those of contact Relays?