Solve NPN Transistor Puzzle: -0.4V?

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Discussion Overview

The discussion revolves around a puzzle involving an NPN transistor where the base is grounded, and a positive voltage is applied to the emitter through a resistor. Participants are trying to understand the reported voltage of approximately -0.4 volts at the collector when the collector is left open.

Discussion Character

  • Exploratory
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about the origin of the -0.4V reading at the collector and seeks clarification.
  • Another participant notes the potential for damage to the NPN transistor due to reverse biasing the emitter-base junction, especially with high voltages.
  • Several participants share links to external resources that may provide additional context or information related to the problem.
  • Multiple participants express enthusiasm for the problem, describing it as "cool" and "amazing."

Areas of Agreement / Disagreement

Participants do not reach a consensus on the explanation for the -0.4V reading, and there are differing views on the implications of the experiment on the transistor's functionality.

Contextual Notes

There are concerns about the assumptions regarding the transistor's behavior under reverse bias conditions, and the discussion does not resolve the mathematical or physical reasoning behind the reported voltage.

donpacino
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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
 
Engineering news on Phys.org
Cool problem. Amazing effect.
 
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.
 
meBigGuy said:
Cool problem. Amazing effect.
My thoughts exactly. Thanks for the links everyone. Great reads!
 

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