Electrical – the type of this surface mount transistor

pnpsurface-mounttransistors

I have a no frost freezer which stopped freezing efficiently and after some research and trials I found the problem to be related to control board. And as of now I am pretty sure the problem is with this obviously burnt transistor that controls the compressor's relay.

The burnt transistor

Using a multimeter I figured it's a pnp type (Negative probe to base gave me a reading in diode mode) however the check was done on this other one that looks exactly the same and so I confirmed the type.

The other transistor

The burnt one did not give me overload when I touched either probes to base however it gave me a much lower reading for (probably B-C, I couldn't confirm C-E pins) than it read for (B-E). 24 , 420 respectively.

Now if my logic is correct, I want to know how can I figure it's precise model so I can change it? Marking reads A4t 95, as you can see, but a search on google was futile and I couldn't reach anything so figured asking here may return something.

Thanks in advance and sorry for my english.

Best Answer

Googling "SOT-23 transistor A4t" yields http://www.s-manuals.com/smd/a4 which suggests it might be a BAV70 dual diode, and http://www.s-manuals.com/pdf/datasheet/b/a/bav70_series_nxp.pdf datasheet, see section 4 on page 3 : Marking which says "BAV70 - A4" and "t: made in Malaysia".

I was thinking it may be diode logic to power a relay under two different conditions, but it could indeed be just flyback diodes. As such, I believe it's probably adequate ( 150mA continuous, 4A for 1 us! ) but if it failed short-circuit (as diodes do tend to do) it would prevent the relay operating.