How to Test MOSFET in Circuit? A Step by Step Guide!

MOSFET is an acronym for Metal Oxide Semiconductor Field Effect Transistor. MOSFETs are the most popular transistor type and are used in a wide range of applications such as amplifiers, switching regulators, motor controls, and power supplies. In this article, I will discuss everything you need to know about MOSFETs including how to test them in a circuit!

So, if you don't know how to test MOSFET in Circuit this guide is for you. But first, let's have a look at MOSFET and their types.

What is a MOSFET?

The word MOSFET is derived from metal–oxide–semiconductor field-effect transistor as the name suggests it is an advanced transistor made up of metal oxide and semiconductor. It can be either n-channel or p-channel type. MOSFETs are unipolar devices because they consist of only one kind of charge carrier, electrons or holes.

MOSFETs are used in a variety of applications such as Amplifiers, Switches, Motor controls, Power supplies, Inverters and many other home appliances.

How do MOSFETs Work?

The operation of a MOSFET is based on the movement of charge carriers in the semiconductor. When a MOSFET is turned ON, the electric field generated by the gate terminal repels the carriers in the semiconductor. This creates a low-resistance path between the source and drain terminals, allowing current to flow through.

MOSFET pin configuration

When a MOSFET is turned OFF, the electric field generated by the gate terminal attracts the carriers in the semiconductor. This creates a high-resistance path between the source and drain terminals, preventing current from flowing through.

There are two types of MOSFETs: n-channel and p-channel.

n-channel MOSFETs are made up of semiconductor material with n-type doping. The n-type doping creates an excess of electrons in the semiconductor.

p-channel MOSFETs are made up of semiconductor material with p-type doping. The p-type doping creates an excess of holes in the semiconductor.

Now that we know the basics of MOSFETs, let's learn how to test them in a circuit.

How to Test MOSFET in Circuit?

The most authentic way to test a MOSFET is to test it with a multimeter. But sometimes if the damage is big MOSFET might not work at all and in that case, you might see burns or carbon deposition on the MOSFET. In that case, you need to replace the MOSFET.

Tools Required to test MOSFET in a Circuit

  • Digital multimeter (Best multimeters for Electronics work)
  • Some Alligator clips
  • Nose Plier

Testing Mosfet Transistor with a multimeter (Don't Miss A Single Step)

  • First of all, you need to identify the pins of the MOSFET. The gate terminal is always labeled with the letter "G". The source and drain terminals are usually labeled with the letters "S" and "D". The best way to identify these terminals is to see the MOSFET manufacturer datasheet.
  • Now set the multimeter to diode mode and connect the positive lead of the multimeter to the gate terminal and the negative lead to the source terminal. The gate to source should have zero voltage. In the off position, the voltages must be zero, because the gate is insulated from the drain.
  • Now do the same testing procedure between the gate and drain voltage terminals. You should see zero volts in the off position between the drain and gate voltage. This shows that the MOSFET is good.
  • Now set the multimeter to resistance mode and connect the positive lead of the multimeter to the drain terminal and the negative lead to the source terminal. In the off position, you should see a high resistance between these terminals.
  • Now turn on the MOSFET and touch probe to the drain and source terminals. You will see a low resistance. This shows that the MOSFET is good.
  • Touch the black probe to the source, lift the red probe from the drain and touch it to the MOSFET gate momentarily, then return it to the drain. The meter must show continuity or you can say a short circuit here meaning your MOSFET is good. If there is no continuity then your MOSFET is bad.

This is the easiest method of checking defective MOSFETs. Because you can see the value of each terminal separately and conclude results accordingly. If you want to know how to test diodes with multimeter then Click here!

Safety Guide:

Safety Guide while using multimeter

Be careful while testing the MOSFET in a circuit as there is a high chance of damaging the MOSFET or multimeter if not done properly. Mosfets are used in power circuits so make sure you take all the safety precautions while testing MOSFETs.

  • If you see any damage or burn then just replace it without testing as it is already damaged.
  • Always use gloves and safety glasses while working with MOSFETs.
  • Make sure you know the maximum voltage and current ratings of the MOSFET before testing it in a circuit.
  • Never try to test a MOSFET with a multimeter if you are not sure about what you are doing. Always take help from an experienced person.
  • Make sure that the probe tips of the multimeter are always clean. A dirty probe tip can damage the MOSFET.

Conclusion:

Mosfets are just like transistors so the testing procedure is also similar. It's like to be riskier to test a MOSFET in live circuitry so make sure you take all the safety precautions while testing MOSFETs. Both n-p-n and p-n-p-type MOSFETs can be easily tested with the help of a multimeter. If you see any damage or burn just replace it without testing as it is already damaged. I hope after reading this guide you may understand how to test MOSFET in-circuit helpful. Stay safe and keep learning. Thanks for reading!

Frequently Asked Questions:

Why testing a MOSFET is important?

Testing a MOSFET is the most authentic way to check if it is working properly or not. It is also important to test a MOSFET before using it in a circuit as it can be damaged if not used properly. Most of the MOSFETs physically look perfect but they're not. So, testing is the only way to be sure about its performance.

What are the voltages drops in "drain to source" and "gate and drain"?

The voltage drop in "gate to drain" should be zero and the voltage drop in "drain and source" should be high when the circuit is energized.

Why diode mode is used for testing a MOSFET?

In diode mode, the multimeter measures the voltage drop between two terminals. So, in order to test a MOSFET, diode mode is used to measure the voltage drop between the gate and drain terminals. If the voltage drop is zero, then the MOSFET is good.

Why you should use a Digital multimeter?

The most appropriate device to check electronics components in close and open circuit is a Digitial Multimeter. Because digital multimeter show gate, source and drain voltages in digital figure.

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