❓ How to check a field effect transistor (mosfet)


To check the health of the field-effect transistor, you can use any digital multimeter with the function of “dialing” diodes. This function works in such a way that allows you to measure the direct voltage drop at the pn-junction, which will be displayed on the display of the multimeter during testing.

During this test, the multimeter is able to pass a current within a few milliamperes through the circuit being tested, and if the voltage drop is too small, then if the device has an audible warning function, it will sound. And since pn-junctions are present in any field-effect transistor, you can count on a completely adequate result.

Before checking the field-effect transistor for serviceability, short-circuit all its leads with foil for a second in order to discharge a static charge in order to discharge all its transient capacitances, including the gate-source capacitance.

Check the built-in reverse diode

Practically in any modern field-effect transistor, with the exception of their special types, an internal “protective” diode is connected in parallel with the drain-source circuit.

The presence of this diode inside the field is due to the peculiarities of the production technology of high-power transistors. Sometimes it interferes, it is considered parasitic, but in most field-effect transistors it is impossible to do without it, as part of the integral structure of the electronic component. Therefore, in a serviceable field effect transistor, this diode must also be in good order. In an n-channel field-effect transistor, this diode is connected to the drain by the cathode, to the source by the anode, and to the drain by the anode to the drain, and to the source by the cathode.


Turn the multimeter into the dialing mode of the diodes. If the field-effect transistor is n-channel, then apply a red multimeter probe to its source (source) and a black one to the drain (drain).

Usually, the drain is in the middle and connected to the conductive substrate of the transistor, and the source is the right pin (check this in the datasheet). If the internal diode is in good condition, the multimeter will display a direct voltage drop across it – in the region of 0.4-0.7 volts. If now the position of the probes is reversed, the device will show infinity. If everything is so, then the internal diode is healthy.

Drain-source circuit test

The field effect transistor is controlled by the electric field of the gate. And if the gate-source capacity is charged, the conductivity in the direction of the drain-source will increase.

So, if the transistor is n-channel, attach the black probe to the gate (gate), and the red one to the source, and in a second change the location of the probes to the opposite — red to the gate, and black to the source. So we probably first discharged the bolt, and after – charged it. The shutter is usually on the left and the source is on the right (see datasheet).

Now move the red probe from the shutter to the drain, and let the black one remain at the source. If the transistor is intact, then as soon as you move the red probe from the gate to the drain, the multimeter will show that there is a voltage drop in the drain (not infinite, but may increase) – this means that the transistor has passed into a conducting state.

Now the red probe is at the source, and the black one is at the shutter (we discharge the shutter of opposite polarity), after which the red probe is again at the drain, and the black one is at the source. The device should show infinity – the transistor is closed. For a p-channel field effect transistor, the probes simply change places.

If the device stumbles

If at the stage of checking the drain-source the device squeals, this can be quite normal, because in modern field-effect transistors the drain-source resistance in the open state is very small. The main thing is that there should be no shutter-source and drain-source ringing, especially at that moment when the shutter is charged with the opposite polarity. Alternatively, you can connect the gate with the source and in this position call the drain-source (for n-channel red to drain, black to source), the device should show infinity.

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