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I've reviewed many questions on the list, but still having difficulty driving my Fotek SSR-25 DA Solid State Relay (Spec Sheet - http://www.fotek.com.hk/solid/SSR-1.htm) from my Raspberry Pi.

Here's my circuit: 

GPIO 26 -------1kR -----+
                        |               +------------+
                        |               |            | 
                         -- +        +--+             = Fan
                               SSR                   |  
                         -- -        ---+             = 120v
                        |               |            |  
                        |               +------------+  
GND --------------------+

Here's what I know:

  1. Circuit works fine when input cycled from +5v to 0v with another controller (Labjack).
  2. Circuit causes fan motor to buzz when hooked up as shown (never turns on).
  3. Tested input switched between Pi's power rails +3.3v to 0v. Same behavior.

Here's what I suspect:

  1. +3.3v is not enough to drive my specific SSR (although specs would indicate it should), or,
  2. Pi not supplying enough current by GPIO26 to fully trip SSR.

Questions:

  1. Any tests recommended?
  2. How do you adjust current output on a GPIO pin (I'm using Python and it's associated libraries)?
  3. Would reversing the logic provide better switching (e.g. Wire SSR+ to 3.3v and switching SSR- between 0.0v and 3.3v)?
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8
  • If you can't trigger the relay using the 3V3 rail you won't be able to trigger the relay using a GPIO. The GPIO can supply far less current than the 3V3 rail.
    – joan
    Commented Jul 30, 2016 at 16:10
  • Thanks Joan, That's what I thought, and why I performed this test. I replaced the SSR with another, and it's even worse! Doesn't even flicker. I'm ordering a different SSR.
    – Coach443
    Commented Jul 31, 2016 at 15:24
  • What's wrong with an ordinary electromagnetic relay?
    – joan
    Commented Jul 31, 2016 at 15:36
  • Regarding question 1) what happens if you try to switch an ordinary bulb with the SSR?
    – Christoph
    Commented Jul 31, 2016 at 18:26
  • Regarding question 3) this might improve the situation, because you get the current directly from the 3.3V supply and not from the internal pin circuitry. However, the data I found about minimum high level output and maximum low level output voltage suggests that you don't get close enough to a guaranteed 3V across the SSR's LED either way, so your best bet might be using a transistor to drive the LED.
    – Christoph
    Commented Jul 31, 2016 at 18:51

2 Answers 2

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Your SSR has integrated resistors so you might wanna try it without the 1k resistor. The datasheet states that it draws 7,5mA at 12V, so the impedance should already be around 1,6k (you could test this). If you add another 1k the current @3.3V is limited to less than 2mA which might be a bit too low to drive the LED.

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2
  • Thank you Domme, I removed the resistor, the same result.
    – Coach443
    Commented Jul 31, 2016 at 15:26
  • @Coach443 Have you checked the voltage of 3.3V between GPIO26 and GND?
    – Domme
    Commented Jul 31, 2016 at 20:20
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Answer:

Although the device specs says otherwise, 3.3v is NOT enough to control my Solid State Relay (SSR).

I purchase a 4-relay SAINSMART board:
http://www.sainsmart.com/4-channel-5v-relay-module-for-pic-arm-avr-dsp-arduino-msp430-ttl-logic.html

I connected Vcc to 5v on my Pi.
I connected In1 to GPIO 26 on my Pi.
I connected GND to GND.

Then I connected my 120v to the normally open side of the relay and it works!

Power flows when my logic is LOW.
Power does NOT flow when my logic is HIGH.

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