Hardy HI 2151/30WC Single-Scale Controller - Manual (Page 152)

HI 2151/30WC MANUAL
8-32
Using Solid State
Relays with Light
Loads (Optional Set
Relays)
About Solid State
Relays With Light
Loads There have been installations where solid state relays have been used
and failed to shut off a solenoid or relay when de-energized. The actual
problem comes from the internal snubbing network in parallel with the
Silicon Controlled Rectifier (SCR) which does the actual switching.
This network presents an impedance of 30K ohms, which means with
120 volts across, it will pass 4mA of AC current.
FIG. 8-2 SCR SWITCHING LOAD CIRCUIT
1. The SCR itself presents no leakage current. Some solid state relay
manufactures specify 20mA minimum load. This is based on the
presumption a relay or solenoid will drop out with only 4mA
through it, which is not always true. That may not be true. When
switching a light load with a solid state relay across the line, you
must look at the rated drop-out current of the load, and if it is less
than 4mA it may not turn off. The solution is to put a loading resis-
tor in parallel with the light load, to be sure leakage current is suffi-
ciently shunted away from the coil.
2. Assume a load like a relay with a coil of 15,000 ohms and of 5% of
nominal drop-out. When the solid state relay is off, there will still
be 1/3 of the line voltages across the relay, so it will not drop out.
For the relay to have 5% of the line across it, it and a parallel shunt
resistor must be 20 times less resistance than the 30K snubbing net-
work, or 1.5K ohms. Use less than a 1.67K ohm parallel resistor
and now total load is below 1.5K ohm or 80mA.