Need help identify this transistor

My Ecobee thermostat is not generating stable 24V control signal, so I took it apart and it appears Q19 is damaged. I would like to replace the part but could not find any information on it. It’s a 2x2mm package, and potentially it’s a MOSFET as a transmission gate to connect an output (Y-wire) to the 24VAC source.

Any help or suggestion is greatly appreciated!


Hello Fischer!

Welcome to the forum!

I did a search on the marking information on the part itself and wasn’t able to find much, but do you know what model the ecobee thermostat is? It would be worth a search to try and locate a schematic and in turn a part number from there. Without any other information though, I’m not able to provide a solid list of options.

Hi Nate,

It is the latest ecobee thermostat (5th gen). I was hoping to find a schematic of any generation of ecobee but had no luck. I wonder if there are measurements to at least identify the functionality so can narrow down the list? Thanks!

Well we would need to know a few different things, most importantly we’d need to know whether it is N or P channel.

As far as I can tell right now, these are the options I can find: Discrete Semiconductor Products | Transistors - FETs, MOSFETs - Single | DigiKey

If I knew what the package was called I’d be able to narrow it down a lot more as well. I know a lot of these can be ruled out on the package size alone. It’s a bit hard to tell, but does it have only 3 terminations to the board or 6?

The package should be DFN-6 2x2mm. I am not sure if all the leads are wired, but looks like the top two left are shorted internally as they share similar burnt marks. The damage is most likely caused by over current, so I would assume those two leads are source/drain. Is it possible it’s a P + N as a CMOS transmission gate?

It’s hard to know for sure without good documentation… Its even trickier with 6 pins.

Here is an updated link with different variations of the DFN-6 package size. Different manufacturers call them different things so I kept it a bit broad.

Discrete Semiconductor Products | Transistors - FETs, MOSFETs - Single | DigiKey

Thanks. I’ll probably do some measurements to see if I can identify something more.

Hi Fisher,

Marking 1958 and 2x2mm package suggests that it is TI’s MOSFET.

Heke, AsamaLab


Thanks! I have ordered a few to confirm.

1 Like