I am trying to drive an SSR, picture posted below. I will be using a basic transistor circuit to prevent overcurrent on the gpio, such as the one below. But I know I need to make two connections the SSR control side. What should I connect to the input DC negative port? I am assuming just make a ground connection here?
NO:
5V to SSR+, SSR- to pin 3 (collector), pin 2 (emitter) to GND.
That's correct but most SSRs can be directly driven by the Arduino, as long as they are rated to be driven from 5V. The specs I found say 12mA for that relay and the Arduino can put-out up to 40mA (20mA max recommended).
That's not the case with most electo-mechanical relays... They usually need a driver circuit.
Some SSRs require an external current limiting resistor (not this one), and then they can operate from almost any voltage with the right resistor.
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@JCA34F does bring up an interesting point, which took me a moment to realize. I am using an npn transistor not a pnp so the transistor should be on the low side of the circuit. Not the high side as I have drawn. Is this conclusion correct?
@DVDdoug I actually didn't know it would take 12mA as I hadn't found the spec. So awesome that you were able to, should mention I am driving from esp32.
If GPIO voltage is 3.3, I would drop base resistor to 680.
If the driving current is only 12mA and GPIO is 3.3V. Then a 1k resistor with Vsat=0.5V will give: (3.3-0.5V)/1000 will give 2.8mA. With a conservative gain estimation of 10 this should be more than enough current needed.
But would you mind confirming the NPN comment I made above, just wanted to make sure we were on the same page with this?
Correct, your drawing is an emitter follower, emitter voltage will not be higher than input voltage - base / emitter diode drop (about 0.65V), so emitter V will be 3.3 - 0.65 = 2.65V.
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That should work fine, to be sure, with circuit in operation, put your volt meter from emitter to collector, V should be < 0.3.
Interesting how do you know this?
30+ years in industrial automation.
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Haha fair enough I suppose.
Or just replace Q503 with a small signal logic level mosfet and call it a day. Heck, even a 2n7002 would do fine there in this particular case, although it's not logic level.
Good idea. The transistor model was chosen for me by my inventory otherwise I'd explore other options. Also wanted to mention, I think this circuit is better than the one I posted earlier. The 10k resistor will help ensure continuity.
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What on earth does that mean? 
Actually, it serves no function. It would if Q503 were a MOSFET.
OK, so I said that slightly differently! 
And i was waiting for a reply to mention the FET.
Here's your cue 
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