Connect a Solid State Relay to a PLC


Thread Starter

Rodolfo Contreras

The problem experienced is that when applied the AC voltaje to the SSR it produces an OFF current about 800microA which is enough to trigger the PLC
input so always the PLC is having a high signal.
What I need to know is what to agg to the SSR to avoid the 800 microA current so the input works properly.
Rodolfo Contreras
I'm assuming you have 110 V inputs on your AC input PLC.

Place an 8.2K, 3 watt resistor between the input terminal and neutral. The triac will turn off (no leakage) as soon as it sees the 8.2K load.

I've had to do this hundreds of times, and it works great.

Best regards ...

James Johnson

You need to add a 'pull down' resistor parrallel to your input. You did not indicate the ACV level you are working with but using ohms law (R=E/I), you should be able to calculate the proper resistor for your voltage. You need several pieces of information to determine this:

1. Min on level for your input module.(E)

2. Actual leakage current of Solid state device into 0 ohms. (I)

Using these parameters will give you the turn off voltage, however you will want to drop to a voltage about half of the min off voltage for consistant operation, so half your resistance.

Make sure you consider the wattage of the resistor. When the Solid state device is on, you will have full voltage across the device. Again, use ohms law to determine current at full voltage I=E/R, then EI for power consummed. Size up from there.::

Good luck.
James Johnson
You may also try Metal Oxide Varistors (MOV). Crydom can give you the specs on the correct size.
I typically use a 2500 Ohm 1/2 watt resistor to add a load to prevent this problem. Long ago I found out that this value or something close works very well for this and have never gone back to recalculate, but I never had a need to.