What is the expected resistance of a transistor in cutoff mode?

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Discussion Overview

The discussion revolves around the expected resistance of a transistor, specifically an NPN BJT, when in cutoff mode. Participants explore measurement techniques and the implications of observed resistance values in the context of circuit diagnostics.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that a collector-emitter resistance of around 1k ohms in cutoff mode seems low, suggesting potential damage to the transistor.
  • Another participant emphasizes that the accuracy of the measurement depends on the method used, recommending out-of-circuit testing for reliability.
  • It is mentioned that in-circuit measurements can be complicated by other components and signals present, which may affect the readings.
  • A follow-up question asks for clarification on how the collector-emitter resistance was determined, requesting details about the measurement conditions.

Areas of Agreement / Disagreement

Participants express differing views on the implications of the measured resistance, with some suggesting it indicates damage while others highlight the importance of measurement context. The discussion remains unresolved regarding the actual condition of the transistor.

Contextual Notes

Limitations include the dependence on measurement techniques and the potential influence of circuit conditions on resistance readings, which are not fully explored in the discussion.

Harrison G
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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, which 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

 
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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 into account any signal present and even if there is a parasitic oscillation throwing the measurements off.

Have fun!
 
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Tom.G said:
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 into account any signal present and even if there is a parasitic oscillation throwing the measurements off.

Have fun!
Thank you :-)
 
Harrison G said:

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.
 
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Thank you everyone for the replies, but the situation is already under control :-)
 

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