Transistor in cutoff mode

In summary, a transistor with a collector-emitter resistance of 1k ohms is damaged and should be replaced.
  • #1
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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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  • #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 into account any signal present and even if there is a parasitic oscillation throwing the measurements off.

Have fun!
 
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  • #3
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 :-)
 
  • #4
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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  • #5
Thank you everyone for the replies, but the situation is already under control :-)
 

1. What is cutoff mode in a transistor?

Cutoff mode is a state in which the transistor is turned off, meaning that no current flows through it. This occurs when the base-emitter voltage is below the threshold voltage, causing the transistor to act as an open circuit.

2. How does a transistor enter cutoff mode?

A transistor enters cutoff mode when the base-emitter voltage is below the threshold voltage. This can be achieved by either reducing the voltage applied to the base or increasing the resistance in the base circuit.

3. What is the purpose of cutoff mode in a transistor?

The purpose of cutoff mode is to control the flow of current through the transistor. By turning the transistor off, it allows for precise control of when and how much current flows through the circuit.

4. Can a transistor remain in cutoff mode indefinitely?

Yes, a transistor can remain in cutoff mode indefinitely as long as the base-emitter voltage remains below the threshold voltage. This is often used in power-saving applications, where the transistor is turned off until it is needed.

5. How does a transistor exit cutoff mode?

To exit cutoff mode, the base-emitter voltage must be increased above the threshold voltage. This can be achieved by increasing the voltage applied to the base or decreasing the resistance in the base circuit.

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