A bipolar NPN transistor is an electronic device having electric charges in negative-positive-negative order. It acts as a valve, controlling large electrical currents with relatively small ones. A transistor's specifications measure this in units called HFE or beta. This is the ratio of the currents flowing to the transistor's collector and to its base, with typical values being between 20 and 1,000. Today, inexpensive digital multimeters can test a transistor's HFE. You can measure only loose transistors this way, not those wired in a circuit.
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Things you need
- Multimeter with transistor tester
- Assortment of NPN transistors
- Transistor data sheets
Turn the multimeter on. Set it to read HFE by turning the function selector knob.
Find the pin arrangement on the data sheet of the transistor you want to test. Insert the base, collector and emitter pins in the corresponding NPN transistor socket on the multimeter. If you don't have a data sheet, rearrange the pins in the socket until you find the right combination. When you have the right combination, you should see a positive HFE reading on the multimeter display.
Read the measured HFE on the multimeter display. Try different transistors and compare the results. Two transistors of the same kind may have different HFE measurements.
Tips and warnings
- If the multimeter reads a low HFE (less than 10) even when you have the transistor connected correctly, the transistor might be damaged.
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