SSR doesn't work, how to debug further?

[n4te]

I have an AQV252GA relay (datasheet) but it doesn’t work.

Schematic:


Wiring should be type "C" (in brackets):


Electrical characteristics:


I get no voltage with a DMM between pin 5 and GND-5V (tried with and without a load).

I get 0.95V across pins 1 and 2. That seems a little low (should be 1.14V?). I swapped the resistor for 200ohm and still get 0.95V. That seems a little weird, I’d expect voltage drop to change.

I have 2 of these relays on my board, and both don’t work in the same way.

How can I debug further?

EDIT: I see this chart:


At ~25C dropout voltage is ~1.15V. I measure a 0.95V drop across pins 1 and 2 -- if that's right maybe it's not enough? But why didn't it change when I tried a 200ohm resistor? I don't know what to do from here.

[PCB.Wiz]

Maybe measure how much current is going thru the diode ?

[n4te]

Good idea, thanks. I'll have to disconnect one side of the current limiting resistor so I can put my DMM in series. I'll do that now and report back.

One more data point: using the DMM's diode mode on an unsoldered AQV252GA, I see a 1.058V forward voltage drop. I tried a few and all read the same.

[langwadt]

Good idea, thanks. I'll have to disconnect one side of the current limiting resistor so I can put my DMM in series. I'll do that now and report back.

One more data point: using the DMM's diode mode on an unsoldered AQV252GA, I see a 1.058V forward voltage drop. I tried a few and all read the same.

if you are certain of the resistor value just measure the voltage over the resistor

[n4te]

if you are certain of the resistor value just measure the voltage over the resistor

The resistors measure to within 1ohm or so. Voltage measured over R9: 29.53mV

I rotated the resistor 45 degrees to break the circuit. Inserting the DMM between R9 and GND, I see 70.1 microamps. Why would that be so low?

I tried 3 different MCUs, same results on all.

[n4te]

If I disconnect the MCU/3.3V and use the DMM diode mode on the relay input, the relay turns on and my 5V load on the other side gets powered! Something must be wrong on the input side of the relay, but what?

[n4te]

If I measure between GND and pin 1, I see 3.3V which comes from an ESP32 output pin, but the relay doesn't turn on.

If I jumper 3.3V from the ESP32 supply to pin 1, then the relay turns on.

Why doesn't the relay work with the output pin? In code I set the pin to output and set it high. I do measure 3.3V coming out of it, but the relay doesn't turn on. Is it because the pin can't source enough current? It should be ~5.6mA using 402ohm. Could it be the ESP32 output pin is broken? My circuit has 2 relays and I've tried 3 ESP32s, so they'd all need to be broken, which sounds crazy.

[PCB.Wiz]

Inserting the DMM between R9 and GND, I see 70.1 microamps. Why would that be so low?
If I disconnect the MCU/3.3V and use the DMM diode mode on the relay input, the relay turns on and my 5V load on the other side gets powered!

Why doesn't the relay work with the output pin?
In code I set the pin to output and set it high. I do measure 3.3V coming out of it, but the relay doesn't turn on.
Is it because the pin can't source enough current? It should be ~5.6mA using 402ohm.

That all sounds like you have only light pullups enabled.
Measure the MCU pin, when it is driving the relay.

Diode test mode is usually ~ 1mA, and that confirms SSR is all ok.
MCU's can drive LOW better than they can drive high, but 5mA should be ok in both directions, with port configured in full CMOS drive mode.

[n4te]

Measure the MCU pin, when it is driving the relay.

Sure, it's 0.98V. I don't understand why though? How does that clue help?

That all sounds like you have only light pullups enabled.

Impossible, I would never enabled pullups... SON OF A BITCH! What's this other code, doing other things including setting pullups, with a list of pins that includes the wrong pins? The pin list was for a previous revision of the board. Man... it just happened to include BOTH relay output pins, to maximize my confusion and misery.

You are a genius and a scholar, PCB.Wiz, thank you!