Strange issue with SSR

**Scampy** · - 18th September 2018, 11:10

Guys, I'm building a thermostat project that uses G3MB-202P solid state relays, and have a strange issue with one of these outputs.

Basically 5v comes from the micro (in this case its an arduino) to the low voltage + pin of the SSR, with the minus pin to ground. In parallel with each SSR is a 560 ohm resistor an LED to indicate when the relay is being switched on by the controller. This is replicated four times for each "channel".

Channels 2,3 and 4 all work fine and drive the heaters correctly, but channel one is not switching the mains supply when the LED is lit to indicate it should be on. I verify that there is no power by testing with a DVM set to 500v ac, and it reads zero volts at any of the three connection points between the PCB and the actual heater. However having placed the DVM across the terminals for a second time I then get voltage and the heater gets driven for a short period and then I get the same issue again.

My guess is that it's a faulty SSR... but wondered if any of you guys had any comments to confirm that

**AvionicsMaster1** · - 18th September 2018, 14:02

Swap a few leads or lines of code to switch SSR and retest.

Do you have the same heaters on all channels? It doesn't appear the SSR has internal protection but channel 1 may be cycling if overcurrenting. You could try swapping heaters around also.

**amgen** · - 18th September 2018, 14:49

just put led in series with ssr + and no resister

**Scampy** · - 19th September 2018, 09:12

Thanks for the suggestions. I ended up swapping the SSR for another and the unit is working as it should.

All loads for channels 1 -3 are the same (100w), with channel 4 being 150w, so not excessive for the relay, they all run cold.

**Alberto** · - 21st September 2018, 12:46

Scampy, these solid state switch come already equipped with a current limiting resistor already integrated inside the switch.

If you place a led with its resistor in parrallel to the ssr there is the possibility that the current circulating in the ssr branch is not enough to trigger the switch. (Mind these switches work from 5 Vdc to 32 Vdc) since you are using 5 volts (from the arduino output pin), you are in the low current region, so anything that drop a little more the current to the ssr will have the effect to un-trigger your switch.

Follow the amgen advice, connect the arduino pin to your led (+terminal) and the led (-teminal) to the ssr (+terminal) while ssr (-teminal) to ground.

Alberto