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:

SOT23 R4

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:

Transitor test schematics

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 ---

PCB trace:

CB86 PCB trace

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.

  • \$\begingroup\$ It could very well also be a PNP transistor. You can find out by measuring the current on the anticipated gate/base pin. What is the load? It looks like 2 of the R4 components are connected together and to a A1 component. Would be good to understand that as well. \$\endgroup\$
    – datenheim
    Commented Feb 8, 2023 at 21:16
  • \$\begingroup\$ @datenheim thank you for a suggestion. I added more info to question. \$\endgroup\$ Commented Feb 9, 2023 at 8:18

1 Answer 1


The SI3404 N-Ch 30V 5.8A FET is a SOT23 which is marked R4. From your description you are charging the gate when you touch it and an enhancement mode MOSFET will remain on with a charged gate and go off if you short it back to Ground. If the Diode test on your Multimeter Is high enough, it may be enough to charge the gate enough to go on. The minimum gate threshold for the SI3404 is 1V but to make sure,3V will definitely turn it on to conduct.

  • \$\begingroup\$ I managed to get it working, assuming that it is NPN. I used BC817- NPN SOT23, and it seems like working. But you pointed directly at item which was in the question. Thank you. I'll mark your answer as accepted. \$\endgroup\$ Commented Sep 29, 2023 at 6:58

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