3
\$\begingroup\$

enter image description hereenter image description here

I’m trying to repair a powered recliner touch sensor, and I’ve isolated the part that I think is the problem. The issue is, I don’t know what it is! In the first image, you can see that it has three pins, and the text “1AME”. In the second image, I tried to follow the PCB traces and draw up a schematic of the part.

I think that this is the part causing problems because the 10k resistor was measuring about 2.3k. On another, identical recliner (they are in a couch), I was reading the full 10k, and that recliner worked!

I know through a continuity meter that the “goes somewhere” part of the schematic is tied to +24V, but I don’t know if it is a direct path.

\$\endgroup\$
  • \$\begingroup\$ NFET... but if the gate oxide were actually blown the resistance would be even less. NPN bipolar transistor could be possible, but then one would expect a smaller series resistor on the base rather than as a pulldown on the gate. \$\endgroup\$ – Chris Stratton Mar 16 at 2:01
  • \$\begingroup\$ Seconding Chris. An n-channel MOSFET seems most likely here, driving that relay. Source is tied to ground, drain is connected to the relay, and gate to the input signal, with a pulldown resistor as well. You can probably replace it with any similar NFET, just check the current the relay coil draws (maybe measure on the good one, if you can't find a datasheet?) and look for a similarly rated one. This is a generic enough usage that you don't need an exact replacement. \$\endgroup\$ – Hearth Mar 16 at 2:10
  • 1
    \$\begingroup\$ Mysterious at least. I'm normally pretty negative on component identification questions, but have to upvote the quality of that reverse engineered and hand drawn schematic. \$\endgroup\$ – Chris Stratton Mar 16 at 2:16
  • \$\begingroup\$ Never mind my comment above. As spehro points out in their answer, the marking code is that of an MMBT3904 BJT. I'd love to know what the point of that resistor is... \$\endgroup\$ – Hearth Mar 16 at 2:18
  • \$\begingroup\$ A lot of people like to put pull-downs on BJT's. I have had people review my schematics and insist on a pulldown. Hopefully in the OP's case, the base input current is limited somehow in the part of the schematic not shown. \$\endgroup\$ – mkeith Mar 16 at 4:37
1
\$\begingroup\$

It's a jellybean NPN BJT, specifically an MMBT3904. The "E" (in a different size or font) is the date code.

enter image description here

Check that the flyback diode is not shorted, particularly if it's a glass type.

\$\endgroup\$
  • 1
    \$\begingroup\$ Not a FET? That resistor is a bit odd there then. Why the pulldown resistor on the base, if it's a BJT? \$\endgroup\$ – Hearth Mar 16 at 2:13
  • \$\begingroup\$ @Hearth. Could be to deal with leakage from whatever is driving it. Hard to tell without a complete schematic. That's the correct marking code. \$\endgroup\$ – Spehro Pefhany Mar 16 at 2:16
  • \$\begingroup\$ Yeah, the marking checks out, that resistor placement just feels wrong for a BJT being used as a switch. \$\endgroup\$ – Hearth Mar 16 at 2:17
  • \$\begingroup\$ Also, BJTs are cheaper than MOSFETs so you would expect them to be used if possible. Cost is about 1/2 to 1/3 in moderate quantity. \$\endgroup\$ – Spehro Pefhany Mar 16 at 2:22
  • 1
    \$\begingroup\$ True. My background is in research, not production, so I'm not as used to that mindset of shaving off every penny for mass production. \$\endgroup\$ – Hearth Mar 16 at 2:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.