This post will cover how to test a MOSFET transistor using an analog multimeter.
The inner workings of the MOSFET are a clear departure from the BJT in terms of configuration but still use N and P junctions with enrichment or depletion channels where the conductivity occurs. For an explanation of the construction and operation of MOSFETs in general, review Digi-Key Electronics’ article on the eewiki. MOSFET symbol showing the integral reverse p - n junction diode- 30 A Pulsed Diode Forward Currenta ISM -110 Body Diode Voltage VSD TJ = 25 °C, IS = 30 A, VGS = 0 Vb-1.6 V Body Diode Reverse Recovery Time trr TJ = 25 °C, IF = 30 A, dI/dt = 100 A/μsb - 120 180 ns Body Diode Reverse Recovery Charge Qrr. Digi-Key Part Number Manufacturer Part Number Manufacturer Description Quantity Available Unit Price SGD Minimum Quantity. RF MOSFET N-CH JFET 10V SOT23 3,000 - Immediate 12,000 - Factory Stock Available: 3,000 $0.19118. Discrete Semiconductor Products – Transistors - FETs, MOSFETs - Single are in stock at DigiKey. Discrete Semiconductor Products ship same day.
MOSFET (also known as FET) is short for Metal Oxide Semiconductor Field Effect Transistor. FETs are widely used for switching and amplifying electronic signals in the electronic devices. The FET is a three-terminal device such as source, gate, and drain. You will find them in power supplies and many electronic devices. FET failure and leakage are quite high in a circuit and you need to know how to accurately test it.
First you will need to find which lead is the gate, drain and source pinout from semiconductor cross reference book or a datasheet.
Once you have the cross reference or diagram for each pin of the FET, then you can use your analogue multimeter set to times 10K ohm range to check the FET. Assuming you are testing the n channel FET then put the black probe to the drain pin.
Touch the gate pin with the red probe to discharge any internal capacitance in the FET. Now move the red probe to source pin while the black probe still touching the drain pin. Use your right finger and touch the gate and drain pin together and you will notice the analogue multimeter pointer will move forward to center range of the meter’s scale.
Use your finger to touch on the gate and drain pin.
P Channel Fet
Lifting the red probe from the source pin and putting it back again to the source pin, the pointer will remain at the middle of the meter’s scale. To discharge the FET you must lift the red probe and touch just one time on the gate pin. This will eventually discharge the internal capacitance again.
At this time, use the red probe to touch on the source pin again, the pointer would not kick at all because you have already discharge it by touching the gate pin. These are the good FET characteristic. You need to practice more by taking some FET from your bench or from your component’s compartment. Once you know the secrets, testing other FET is as simple as testing diode.
Digikey P Mosfet
Testing Mosfet In Circuit
If you notice that all the result that you measured kicked towards zero ohms and will not discharge, then the FET is considered shorted and need replacement. Testing the P channel FET field effect transistor is just the same way as when you check N channel FET. What you do is to switch the probe polarity when checking the P channel. Some analog multimeter have the times 100k Ohm range, this type of meter can’t really test FET due to the absent of 9 Volt battery inside the multimeter. This type of meter will not have enough power to trigger the FET. Make sure you use a meter that have the times 10k ohm range selector.