The post explains how to test mosfets using multimeter through a set of steps, which will show help you to accurately learn the good or faulty condition of a mosfet
Mosfets are Efficient but Complex Devices
MOSFETs are outstanding devices when it comes to amplifying or switching of various kinds of loads. Though transistors are also largely applied for the above purposes, both the counterparts are hugely different with their characteristics.
The amazing efficiency of mosfets are to a great extent neutralized by one drawback associated with these devices.It is the involved complexity which makes these components difficult to understand and configure.
Even the simplest of operations like testing a good mosfet from a bad one is never an easy task especially for the beginners in the field.
Though mosfets usually require sophisticated equipment for checking their conditions, a simple way using a multimeter is also considered effective most of the time for checking them.
We take the example of two types of N-channel mosfets, the K1058 and the IRFP240 and see how these mosfets can be tested using an ordinary digital multimeter through slightly different procedures.
How to Check N-Channel Mosfets
(Remember the following explanation is for the above mentioned Mosfets, for other mosfets the pinout assignments may change, and you may have to proceed accordingly)
1) Set the DMM to the diode range.
2) Keep the mosfet on a dry wooden table on its metal tab, with the printed side facing you and leads pointed towards you.
3) With a piece of insulated wire stripped at the ends, short the extreme left lead and the extreme right leads of the mosfet (gate and drain). This will initially keep the internal capacitance of the device completely discharged.
4) Now Touch the meter black probe to center lead (source) and the red probe to right lead (drain) of the device.
5)You should see an "open" circuit indication on the meter.
6) Now keeping the black probe touched to the center lead (source), lift the red probe from the right lead and touch it to the left lead (gate) of the circuit momentarily and bring it back to the right lead (drain) of the mosfet.
7) This time the meter will show a short circuit (sorry, not short-circuit rather "continuity).
The results from the point 5 and 7 confirms that the mosfet is OK.
Repeat this procedure many times for proper confirmation.
For repeating the above procedure each time, you will need to reset the mosfet by shorting the gate and the source leads using a piece of wire as explained earlier.
How to Check P-Channel Mosfets
Follow the procedure exactly as given in the steps 1,2,3,4 and 5.
Next remove the red and the black probes from its positions and touch the red to the left pin (gate) and the black to the center pin (source) of the mosfet.
After a few seconds, put the red probe and the black probe back to the center (source) and the right pin (drain) of the mosfet respectively.
This time the meter should show a short circuit, proving that the mosfet is in a good condition.
If you any further doubts regarding the procedures please feel free to express your thoughts in the comment section.
How to Test an IRF540 Mosfet
The procedures are exactly similar to the above explained N-channel mosfet testing procedures. The following video clip shows and proves how it may be implemented using an ordinary multi-meter.