Check of NPN transistor

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 

Hi Hiro_Hamada,

Thank you very much.

torben

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

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.

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.