How to Test MOSFET Using a Digital Multimeter

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Have you ever heard of a MOSFET? In case you didn’t know, it’s a type of transistor that comes from metal oxide semiconductor FET. In today’s guide, we will talk more about this unique type of transistor and find out how you can test it with a digital multimeter.

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Are you ready? Without further ado, let’s dive in! 

Introducing MOSFET: What is it really?

MOSFET is the device where the variation in the voltage identifies the device’s conductivity. It’s a semiconductor device, which belongs to the FET family. 

Tae notes that MOSFET is also called IGFET or insulated-gate field-effect transistor. Nonetheless, the term MOSFET is utilized because many devices are made using gate electrodes of metal oxide and Si for semiconductors. 

You will find three-terminal devices in a MOSFET that has a gate terminal, a drain, and a source. Those are the voltage-controlled devices where the current flowing between the drain and the source is proportional to the given input voltage. 

It’s worth mentioning as well that MOSFET is an innovative FET created to overcome the drawbacks of FET. You see, FET provides the big value of drain resistance along with delayed operation and moderate input impedance. Meanwhile, MOSFET has a tinier value of capacitance, not to mention its input impedance is more than that of FET because of tiny leakage current. 

MOSFET finds its applications commonly in amplification and switching of electrical signals due to its capability to change conductivity along with the applied voltage. It’s also normally utilized transistors in digital circuits, thanks to its small size. The applied voltages transform the channel width. The wider channel width also offers improved conductivity of the device. 

How Does It Work?

In simplest terms, MOSFET is a voltage-controlled field-effect transistor that differs greatly from a JFET. You see, the gate electrode is electrically insulated from the main semiconductor by a small layer of insulating material.

The best part here is that the insulated metal gate is much like a plate of a capacitor that has a high input resistance. You won’t find any current flow into the MOSFET from the gate because of the gate’s isolation.

Once the voltage is applied at the gate, it immediately adjusts the width of the drain-source channel where charge carriers flow. Simply put, the wider the channel, the better the device conducts. Isn’t it amazing? 

Typically, MOSFETs are utilized differently compared to other standard junction FETs. 

The infinite high input impedance of MOSFET makes it practical for power amplifiers. The devices are well fitted to high-speed switching applications as well. Other integrated circuits or ICs have small MOSFETs and are commonly utilized in computers.

Furthermore, the MOSFET could be damaged by built-up electrostatic charges as the oxide layer is relatively thin. MOSFET devices don’t normally function in weak signal radio frequency as well as other types of FET. 

Malfunctioning MOSFET

It’s a fact that MOSFETs are powerful and strong, but they can also malfunction if their ratings are exceeded. When these transistors fail, they frequently go short circuit drain-to-gate. That could revert the drain voltage into the gate where it feeds, not the drive circuit. Potentially, it will blow that section.

On top of that, it will get to any other twinned MOSFET gates, raging them too. Hence, if the MOSFET is minimized, make sure you check the drivers. 

Other problems that may cause a MOSFET to fail are the following:

  • defective battery
  • short-circuit load
  • rapid deceleration or acceleration 
  • jammed or blocked motor
  • foreign objects
  • too much current
  • too much power dissipation
  • dV or dt failure 
  • avalanche failure 

What’s the Best Way to Test the Functionality of a MOSFET? 

The answer lies to your digital multimeter. This is the best device you can use if you like to learn if you have a malfunctioning MOSFET. 

You will find some digital multimeters with a diode testing setting, putting three to four volts across the device you are testing. If your MOSFET is properly working, the voltage of your digital multimeter will switch it on and off. On the other hand, a bad and malfunctioning MOSFET will not respond to the voltage. Always keep that in mind before you start your test. 

Now, let’s continue with the steps you need to do. Follow all of them, so you’re guided.

  1. Attach the ground connector of your anti-static wrist straps to a solid electrical ground. It can be a conduit or a cold-water pipe. Slide the strap into your wrist.
  1. Now, determine which of the three leads of your MOSFET are the drain, gate, and source. If needed, you can refer to the datasheet of your MOSFET.
  1. For the third step, turn on your digital multimeter and set its selector knob to the diode test setting. 
  1. Hold the MOSFET by the case, and make sure you avoid touching the leads.
  1. Next, attach the multimeter’s negative probe to the source lead. Make sure you also attach the positive probe to the gate lead. 
  1. Turn the positive probe to the drain lead and carefully read the display provided by your multimeter. It must show a low resistance. 
  1. Now, keep the positive probe at the drain lead. Hold the gate and source with your fingers, and your DMM must display a high resistance reading. If it does not give anything, it means your MOSFET is bad and needs immediate replacement. 

Final Thoughts

To sum up, a MOSFET is a voltage-controlled device that has a source, gate, and drain, terminals rather than having an emitter, collector, and base terminals in a bipolar transistor. It produces an electric field to regulate the current flow through the channel between source and drain by applying a voltage at the gate. You will not find current flow from the gate into the MOSFET. 

There you have it! We hope everything we explained in this guide allows you to learn more about MOSFET in general and the best way to test it using a multimeter. Are you ready to test yours? We wish you the best of luck, and don’t forget to practice safety precautions!

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