I have a board with a few SMD SOT-23 transistors and some of them are dead. I need to identify them.
They have marking "R4" and nothing else:
I've desoldered one of them and connected it on the bench - after a few tries I found the correct connection to make it work:
It turns on and off (and stays in the last state) if I touch PIN 2 when it is left with no connection.
As I understand it, it is a P-channel MOSFET, but every datasheet that I can find of P-channel MOSFETs shows that PIN 1 is drain, but as I can see from my test, PIN 1 on my MOSFET is source. The same is for PNP (if it is not a MOSFET) - PIN 1 is always collector, but I need it to be emitter.
How can I find a replacement for it? I believe it works in switch-mode, and it is part of the voltage and current measuring circuit.
Or maybe it is not a transistor at all?
--- EDIT ---
I tried to measure current on gate/base pin as suggested in comment: there is no measurable current. And I'm pretty sure now, that it is MOSFET as it shows diode measure only in one way - drain to source. As I understand bipolar transistor would show diode in two measurements.