What is this SOT23 Transistor?

[cube]

Hi all.

I'm having trouble working out what's wrong with the PWM circuit that controls the heated insoles for my ski boots  .  And I'm new to SMD components.

There are a couple of SOT23 transistors marked 4Gt 64 which I think are BC860C and do the PWM, the collector of one of these is connected to the base of the transistor that does the actual switching for the heater element - this switching transistor is also a SOT23 package and marked A6DA5. I just can't work out what this is.  I think it's where the fault lies because it measures 4 ohms from base to emitter.  It has to switch about 400ma.  When I look up A6D for SOT23 package I find MMUN2114 but that is only rated to 100ma collector current unless I read it wrong.

I've attached a picture of it. Anyone got any suggestions on what this SOT23 A6DA5 could be?

Thanks.

[thm_w]

Can you tell what the the bottom left and right side pads go to? Might help to determine the part.
The top left is obviously base or gate.

It doesn't seem to match the silkscreen of a MMUN2114.

Have you measured the heater wire resistance and verified that part is still OK?

[cube]

Thanks for replying.  Yes, checked the heater element on another battery pack and it's ok.

The +ve from the 4.8v battery back goes straight to one side of the element.
The other side of the element goes to the middle-right of A6DA5 - guessing this is the collector.
The bottom-left of A6DA5 goes to -ve of the 4.8v battery pack, so A6DA5 switching the connection from the element to -ve.

Hope that helps, I can post some more images or diagrams if needed.

Thanks!

[thm_w]

OK so low side switched.

Should be alright to put in a suitably rated N channel FET (2A+ 20V+), and see if it works as expected? Assuming you've already removed the old part and the 4 ohm is on the part itself.

[fzabkar]

IRLML2402TR, International Rectifier / Infineon, n-channel MOSFET, 20V, 1.2A, 0.25 ohm RDSon, SOT23, marking Aywlc:
https://www.infineon.com/dgdl/irlml2402pbf.pdf?fileId=5546d462533600a401535664e5ef25fa

https://www.aliexpress.com/w/wholesale-IRLML2402TR.html (photos)

http://markingcodes.com/search/c/a6

[cube]

Sorry for the delayed reply and thanks very much for your help @thm_w and @fzabkar.  I was really busy just before Christmas then away.

I found some A2SHB SOT23 N MOSFETs on ebay rated at 20v 3A.  I bought some of these and soldered one into each PCB to replace the faulty one, and was delighted it fixed the problem so I had nice warm feet while away skiing!

So a big thank you for your help, I've not done much SMD work or diagnosing faults like this, great result.