Having a hard time sorting transistors as good or bad? Read through this article on how to test a bipolar transistor.
Introduction
Last time, you had a primer on bipolar transistors. This time, you’ll learn how to test and troubleshoot them. It’s very annoying to have a bad batch of transistors lying around on your component box.
Things You may Need
- A Multimeter. Can be Analog or Digital. The test is easier if you have a diode tester function on the multimeter.
- A convenient place to test the transistor leads. You may use a bread board if you like.
How to test the Transistor
First, determine if it is a Good or Bad Transistor
A transistor is a three-layer semiconductor device. It is basically made up of two PN-junction diodes connected back-to-back. See the illustration above. The diodes share a common region or terminal called the base. With this information, you can easily test if a transistor is good or bad.
Additionally, in order for a transistor to do amplification, the Base-Emitter junction should be forward, while the Base-Collector reverse biased. Note that the Base-Emitter junction is more heavily doped than the Base-Collector junction. With this, the base current can be amplified a hundred times more than the collector current.
For an Analog Meter
Remember, the polarity of your probes determine the actual biases you apply to the BJT.
Testing either the Emitter-Base or Collector-Base Junction
- If you know the pinout of your transistor, test the Emitter-Base, and then the Base-Collector junction using a multimeter. You can use the Ohmeter. If you don’t know the pinout, you can do trial and error.
- The Emitter-Base in forward bias will have a low resistance while having high resistance in reverse.Â
- Similarly, the Collector-Base in forward bias will have a low resistance while having high resistance in reverse.
- Find the two back to back diodes. If any of the characteristics of a diode fail on your test, your transistor is likely bad.
Finding the Collector and Emitter Pins
- Having known the two diode junctions and the base pin from the previous steps, you are left to find which are the Collector and Emitter pins.
- Measure the resistance between the base and the other two pins.
- Â The emitter usually shows slightly lower resistance than the collector due to heavier doping.
For a Meter with a Diode Test Function
Find the diode test function on your DMM.
- It’s easier to test a BJT using the diode test function of a multimeter.
- Find the Emitter-Base and Base-Collector forward bias diode junctions. Since these are forward bias junctions, you should see a voltage drop of about 0.5V – 0.7V.
-  To identify which is Collector and which is Emitter: The pin tapped showing a higher voltage drop (~0.65–0.7V) is the emitter while the pin showing lower voltage drop (~0.55–0.6V) is the collector.
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