I am trying to identify a fried component in a fan controller. I believe the fan is a PWM fan, and my guess is that this component might be a transistor as part of the PWM circuit because that function is not working when I take readings with a multimeter (but this is a guess, and not a very educated one as I don't have the schematic for this board). It's a friend's fan controller and he fried it by accidentally sticking the DC power adapter plug into the fan molex connector instead of the power socket.

The component is Sot-23 (as per my measurements with calipers), and it has what I believe is "G1" marked on it (not 100% sure it's G1 because the component is fried, but I feel like I'm pretty confident).

Close up of fried component

Backed out a little

I looked up the G1 components on some of the SMD component marking catalog sites, and I believe there are only a few options of what it could be... but I am not sure how to figure it out absolutely. I highlighted the ones on the sites that fit the bill in the image below, and while I'd appreciate if someone could help identify the part, I'm equally or maybe more interested in what my next steps should have been to figure it out on my own (I'm a hobbyist and still learning... and enjoy the learning). I am not sure what I should be looking for when I read the data sheets. If it wasn't fried, I would assume that I could measure the voltage at each of the pins and that could give me some clues, but given it's fried, what would be the best course of action here?

Chart with SMDs with G1 Markings

In any case, thanks in advance for your time, and apologies for any newb questions.


  • 3
    \$\begingroup\$ The board looks simple enough. You should be able to trace out a schematic for that section. Additionally you could look at all the datasheets and get a transistor similar enough in specs. Hfe/gain, current capacity/bandwidth. \$\endgroup\$
    – Passerby
    Commented Dec 2, 2021 at 6:01
  • \$\begingroup\$ Perhaps : G1 MMBT5551 Motorola SOT-23 NPN transistor HV, 180V, 200mA, 350mW, B=80..250, >300MHz from smd.yooneed.one/code4731.html . Verify "topology" of pins (pinout). \$\endgroup\$
    – Antonio51
    Commented Dec 2, 2021 at 6:13
  • \$\begingroup\$ Curious resistor values. Might be some voltage dividers on the collector, which is a bit weird. \$\endgroup\$
    – Lundin
    Commented Dec 2, 2021 at 7:36

1 Answer 1


In your board you also have a "J3Y" marking SOT-23. It is a S8250 from Galaxy Microelectronics: S8050 datasheet

So your "G1" SOT-23 is probably a MMBT5551 from Galaxy Microelectronics as well. MMBT5551 datasheet

Picture from LCSC

enter image description here


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