Check of NPN transistor

Torben Friis's picture

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Joined: Nov 29, 2019

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Check of NPN transistor
November 29, 2019 - 6:15pm

Hi,

How can I check if a 2N2221A transistor that I want to use as a switch is OK?

best regards

torben

 

Hiro_Hamada's picture

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Multimeters will have that option
November 30, 2019 - 9:59am

A transistor is noting but two diodes connected in reverse direction, so you can simple use a multimeter and check two terminals at a time to check if it is okay. There are many videos online explaining you how to do it.

Alternatively you can also connect an LED with current limiting resistor to collector and emitter and apply a base voltage to check if the transistor is switching 

Torben Friis's picture

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Hi Hiro_Hamada,
November 30, 2019 - 2:12pm

Hi Hiro_Hamada,

Thank you very much.

torben

Torben Friis's picture

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Hi
December 1, 2019 - 3:53pm

Hi
I have made a switch with a 2N2221A (NPN) transistor and a red LED as load. I get the following voltages:

+ side of load: 4.59v
- side of load/C: 2.720v
B: 1.092v
RB: 1,740 Ohm

I have the following questions:

1. why does the LED not light up (it lights up at a voltage difference of 4.59v)
2. changing base voltage to 0v or 4.59v does not change anything
3. what should the voltages typically be (the base voltage)?

Is the 2N2221A destroyed?

best regards
torben

jaksonlee's picture

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using a DMM to get a basic
December 16, 2019 - 3:33pm

using a DMM to get a basic pass/fail reading from a suspected faulty NPN or PNP bipoloar transistor is a simple and quick task. Some multimeters have a built-in transistor testing function, if yours does, you can skip this blog post – simply insert your transistor into the socket on the multimeter and set the meter to the correct mode. You will probably get information such as the gain that could be checked against the datasheet as well as a pass/fail reading. If your meter does not have a transistor testing function, fear not – transistors can easily be checked with the “Diode” testing setting. 

Hook the positive lead from the multimeter to the to the BASE (B) of the transistor. Hook the negative meter lead to the EMITTER (E) of the transistor. For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V.

marry roser's picture

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Hook the positive lead from
December 26, 2019 - 3:22pm

Hook the positive lead from the multimeter to the to the EMITTER  of the transistor. Hook the negative meter lead to the BASE (B) of the transistor. For an good NPN transistor, you should see “OL” (Over Limit). If you are testing PNP transistor, the meter should show a voltage drop between 0.45V and 0.9V. These versitiale devices can be used to test and diagnose a wide range of circuits and components. fail reading from a suspected faulty NPN or PNP bipoloar transistor is a simple and quick task. Some multimeters have a built-in transistor testing function, if yours does, you can skip this blog post – simply insert your transistor into the socket on the multimeter and set the meter to the correct mode.