Has anyone had problems getting a solid state relay to work?

EDIT: see Spehro Pefhany's answer. Lessons learned:

  1. don't accidentally take out current limiting resistor

  2. solid state relay rated for AC load will probably just turn on small DC load.

  3. It makes a LOT of sense that since there's an LED inside you need a resistor, that's a Basic concept.

I'm using a SSR to control a AC load. To test it I'm using an LED load. The problem is, I can't get the relay to turn on. I'm using the S202S02 from Sharp (digi-key part 425-2403-5-ND.

First a was using a uP (Atmel XMEGA128A1) but that wasn't working so I switched to just straight up using Vcc. (Note: I didn't bother with a transistor, just connected it right to uP)

The SSR has 4 pins: the output pins (indicated by the AC tilde) and two input pins: + and -. The problem is that I apply vcc (3.3/5V) to plus, ground - and then connect one output pin to vcc and the other pin to an led which is connected to a resistor and then ground.

ssr link if image doesn't embed

I know the led works because if I connect vcc to it it turns on.

What am I doing wrong? I'm pretty sure SSRs can switch DC loads.

I've used two different relays in my circuit so I don't think I have a bum relay

  • \$\begingroup\$ If the SSR is intended to switch 120 VAC, I wouldn't count on it to switch 3.3 V DC. \$\endgroup\$ Dec 7, 2014 at 2:19

1 Answer 1


I have some bad news for you. Those SSRs have a "naked" infrared LED at the input. If you apply 3.3V or 5V on the input terminals without a current-limiting resistor, you'll more-than-likely destroy the SSR almost instantly. You should connect a series resistor to limit the current to something reasonable given the Vf of the LED. Something like 15mA or 20mA.

enter image description here

Also, they're unsuitable for switching low voltage DC- they're designed to switch high voltage AC at the zero crossings. Probably they won't switch off once they've switched 'on' (depending on whether your LED current happens to exceed the holding current of the triac), or in the case of your circuit they will switch 'on' once only per SSR.

  • \$\begingroup\$ That makes a lot of sense. Originally I used a current limiting LED but all the relay did was turn on, in rewiring the circuit I forgot to put it back in. Darn. Thanks! \$\endgroup\$ Dec 7, 2014 at 5:43

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