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Transistor in cutoff mode

  • Thread starter Harrison G
  • Start date
  • #1
41
2

Homework Statement


Hi! I learned how to work with transistors a long time ago. However, it is good to fortify youre knowledge. I was checking a circuit board for issues and found a npn bjt, wich Colector-emitter resistance while in cutoff mode, is arround 1k ohms. So my knowledge of bjt's says me that that is a pretty low resistance for a transistor in cutoff mode (it should be almost infinite). I mean, that thing is damaged for sure, right?

Homework Equations




The Attempt at a Solution

 

Answers and Replies

  • #2
Tom.G
Science Advisor
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Probably... depends on how you "measured" it.

Most accurate of course is out-of-circuit with a transistor tester.
Out-of-circuit again, a few ohmmeter tests will usually spot a bad one. First, check that the junctions are actually diodes, that Collector-Emitter are not shorted, check Base-Emitter when forward biased then short Collector to Base and the indicated resistance should drop slightly.

In-circuit is tough because you have to account for EVERY component that the transistor is connected to, and if the circuit is powered-up you also have to take in to account any signal present and even if there is a parasitic oscillation throwing the measurements off.

Have fun!
 
  • #3
41
2
Probably... depends on how you "measured" it.

Most accurate of course is out-of-circuit with a transistor tester.
Out-of-circuit again, a few ohmmeter tests will usually spot a bad one. First, check that the junctions are actually diodes, that Collector-Emitter are not shorted, check Base-Emitter when forward biased then short Collector to Base and the indicated resistance should drop slightly.

In-circuit is tough because you have to account for EVERY component that the transistor is connected to, and if the circuit is powered-up you also have to take in to account any signal present and even if there is a parasitic oscillation throwing the measurements off.

Have fun!
Thank you :-)
 
  • #4
rude man
Homework Helper
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Homework Statement


Hi! I learned how to work with transistors a long time ago. However, it is good to fortify your knowledge. I was checking a circuit board for issues and found a npn bjt, with collector-emitter resistance while in cutoff mode, is around 1k ohms. So my knowledge of bjt's says me that that is a pretty low resistance for a transistor in cutoff mode (it should be almost infinite). I mean, that thing is damaged for sure, right?

Homework Equations




The Attempt at a Solution

How did you determine the collector-emitter resistance? Include details such as powered-on or not, instrumentation, schematic, etc.
 
  • #5
41
2
Thank you everyone for the replies, but the situation is already under control :-)
 

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