Transistor Basic Circuit

How to Know if a Transistor Defective

Transistor is an active electronic part. An active electronic part is something can perform amplifications or signal processing. Transistors are basic elements on power amplifiers, audio amplifiers, switching converters, power supplies and so on. A transistor term is somewhat generic. It could be a BJT, MOSFET of JFET. But for common people (electronic hobbyist), it is commonly referred to a BJT. So, in this tutorial we will concentrate on how to know if a transistor defective that refers to a BJT. If you are interested to know about MOSFET, read the article: “How to Know if MOSFET is Defective”. BJT is short for Bipolar Junction Transistor. A transistor is can be an NPN or PNP type. It is an active device capable of amplification and even switching action.

Below is a simple illustration on how the NPN and PNP differ from each other. If you are interested to know more on the basics, read the article “NPN Transistor Principles and Practical Uses”.

NPN and PNP Transistor Configurations

How to Know if a Transistor Defective – NPN

If the transistor is known to be an NPN type, below are the troubleshooting steps on how to know if a transistor defective.

Steps:

1. Get a DMM and set it to diode mode

2. Connect the positive probe of the DMM to the base or the “P” or the base of the above figure for the NPN type. Connect the negative probe to the “N” or the emitter. A good transistor will read a voltage around 0.7V. A bad transistor will read otherwise. Otherwise means a value that is far away the 0.7V level. A good transistor usually reads around 0.3V-0.7V for germanium and silicon variants.

3. Transfer the negative probe of the DMM to the other “N” or the collector pin. The judgement must be the same as item 2 above.

4. Try reversing the probe connections of item 2 and 3, the reading now must be 0V. This means the transistor junction is not conducting as it is reversed biased. If the reading is otherwise, then the transistor is faulty.

5. A faulty transistor may have zero resistance when measured across collector-emitter.

How to Know if a Transistor Defective – PNP

If the transistor is known to be an PNP type, below are the troubleshooting steps on how to know if a transistor defective.

Steps:

1. Get a DMM and set it to diode mode

2. Connect the positive probe of the DMM to the base or the “P” or the emitter of the above figure for the PNP type. Connect the negative probe to the “N” or the base. A good transistor will read a voltage around 0.7V. A bad transistor will read otherwise. Otherwise means a value that is far away the 0.7V level. A good transistor usually reads around 0.3V-0.7V for germanium and silicon variants.

3. Transfer the positive probe of the DMM to the other “P” or the collector pin. The judgement must be the same as item 2 above.

4. Try reversing the probe connections of item 2 and 3, the reading now must be 0V. This means the transistor junction is not conducting as it is reversed biased. If the reading is otherwise, then the transistor is faulty.

5. A faulty transistor may have zero resistance when measured across collector-emitter.

If the Transistor Type is not Known, How to Start the Diagnosis?

Nowadays, it is easy to get any electronic component datasheet once the part number or the body marking is available. However, in the absence of these, it will be difficult. The possible remedy is to have understanding on the circuit diagram if it is available. NPN and PNP type has different biasing configuration. The NPN type always has positive supply injected to its base section while ground on the emitter and positive supply again to the collector. On the other hand, the PNP type has ground on the base section and positive source on the emitter section.

Sample NPN and PNP Circuits

How about there is no circuit diagram available? A trial and error will do. Follow below steps.

Steps to identify an NPN transistor:

  1. Connect the positive probe of the DMM to any pin or leg of the transistor. Connect also the negative probe to any leg or pin of the transistor but not to the pin/leg that has the positive probe. Ensure the DMM setting is in the diode mode. Observe the DMM reading.
  2. If the DMM reading is within 0.3V-0.7V, this means one of the diodes on the transistor junctions has been forward biased.
  3. Do not remove the positive probe on its location while remove the negative probe to the leg/pin that has no probe connection. If the DMM reading is still around 0.3V-0.7V, then the transistor is an NPN type.
  4. If above tests result to otherwise, then consider below steps.

Steps to identify an PNP transistor:

  1. Connect the positive probe of the DMM to any pin or leg of the transistor. Connect also the negative probe to any leg or pin of the transistor but not to the pin/leg that has the positive probe. Ensure the DMM setting is in the diode mode. Observe the DMM reading.
  2. If the DMM reading is within 0.3V-0.7V, this means one of the diodes on the transistor junctions has been forward biased.
  3. Do not remove the negative probe on its location while remove the positive probe to the leg/pin that has no probe connection. If the DMM reading is still around 0.3V-0.7V, then the transistor is a PNP type.

 If above trials result to otherwise, the transistor is can be faulty. It is worth replacing it. Above tutorials are maybe basic only. Experience will tell more How to Know if a Transistor Defective.

Transistor Failure Mode

Common transistor failure modes are shorted base-emitter junction, shorted base-collector junction, shorted collector emitter, open collector-emitter, open base-emitter junction or open base-collector junction.

If you want to know how to bias a transistor, read the articles “Complete Analysis of a Fixed Bias Circuit using NPN Transistor”, “How to Select a Transistor For Switch and Linear Applications”, “Transistor Operation Mode Determination”, “How to Select a Transistor For Switch and Linear Applications”, “How to Saturate a PNP Transistor”, “How to know if a Transistor is Saturated?”, “How to Drive a Transistor into Hard Saturation”.

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