While taking apart a 2009 Apple Time Capsule, I found a 2N3904 NPN transistor on a 2-wire cable with the emitter connected to one wire and the base and collector connected together and to the other wire. The transistor was physically attached to the back of the hard drive with Kapton tape. What is the transistor being used for?


simulate this circuit – Schematic created using CircuitLab

  • 5
    \$\begingroup\$ thermal measurement maybe..? \$\endgroup\$
    – Wesley Lee
    Commented Nov 25, 2015 at 23:05
  • \$\begingroup\$ electronics.stackexchange.com/questions/137960/… \$\endgroup\$
    – Wesley Lee
    Commented Nov 25, 2015 at 23:06
  • \$\begingroup\$ That could probably be what it is, although why would they not use a simple analog temperature sensor like a LM335? \$\endgroup\$
    – 3871968
    Commented Nov 25, 2015 at 23:11
  • \$\begingroup\$ @jBot-42 Have a look at LM84... \$\endgroup\$
    – Daniel
    Commented Nov 25, 2015 at 23:14
  • 1
    \$\begingroup\$ LM335 is more than 10x more expensive than 2n3904 in 100pc quantities at mouser \$\endgroup\$
    – Wesley Lee
    Commented Nov 25, 2015 at 23:29

1 Answer 1


I am 99% sure it is a temperature sensor.

App Note AN12.14 from Microchip can tell you a little bit about it.

(This is a pretty common cheap way to measure temperature on a circuit board)


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