Npn transistor puzzle

  • Thread starter donpacino
  • Start date


Gold Member
I was just sent an email with the following text.
Here is a puzzle I just encountered.

Take any NPN transistor and ground the base. Now apply +12 volts to the emitter through a series resistor of 1k. Yes, it is a Zener diode of about 6 volts so about 6 ma of current flows.

What is the voltage between the collector and ground?

I couldn't guess. Well I could guess it was zero volts, but then, that would not have been a puzzle. So I gave up.

Answer: about -0.4 volts ! (Yes negative.) Why?

I am at a loss for understanding where they are getting this supposed -0.4V. Any takers?

edit: it is important to note my interpretation of the problem is the the collector is left open


Gold Member
Cool problem. Amazing effect.


Science Advisor
Note that most NPN transistors will be damaged by reverse biasing the E-B junction, especially with such large voltages. I'm not discouraging you from doing the experiment (these things are cheap, after all), but don't expect the transistor to function properly afterward.

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