How to test MOSFET transistors with digital multimeter

There are basically two types of MOSFET (Metal Oxide Semiconductor Field Efect Transistor), the N channel and the P channel. The difference is the type of doping that conducting channel receives. This proposed test, will be illustrated with a transistor with TO220 package which is widely used for this type of component.

Figure 1

This test proposed by eletronPi is verified in laboratory.


The MOS transistors usually have 3 terminals: Gate, Drain and Source. The Gate terminal has a similar function to the base of bipolar NPN and PNP transistors, controlling the amount of current flowing through the transistor. The Drain and the Source are defined depending on the direction of charge carriers: the electron or the hole. If the MOS channel is P-type(excess of holes), the source must be connected to the positive terminal of the circuit, an analogy to "holes source". If the channel is type N (excess of electrons), the source must be connected to the negative terminal of the circuit, an analogy to "electrons source". The Drain is the terminal's left. The current in a MOSFET runs between Source and Drain. The gate current is typically very small (approaching zero for DC signals). We will use this feature in this proposed test.

To perform the test, you will need a digital multimeter with the "diode" function (fig 2) and 2 electrolytic capacitors, ideally 1000uF.

Figure 2

Testing MOS type N

Step 1 - Verifying Short Drain Source:

In TO220 package, the metal part is normally connected to the middle terminal (drain in our case). So, for facility, we will use the metal part to connect the Drain.

Set the multimeter to the "diode", then connect the leads as shown in figure 3.

Figure 3

If the transistor is OK, the meter reading will be between 0.3V and 0.6V. These values ​​can change depending on the transistor caractheristics. This value corresponds to the conduct voltage of the transistor intrinssec diode. This diode is formed during transistor manufacturing and is normally present in most of MOS transistors. If the transistor Drain and Source are short circuited the Meter reads a very low voltage, close to 0V. If the Drain Source channel is broken, the reading will be the same when the multimeter leads are disconnected (1 ----).

Reverse the multimeter leads, as shown in figure 4.

Figure 4

First, discharge Gate terminal touching it in Source terminal. This is necessary to turn off the transistor. After connect meter leads as shown in figure 4. If the transistor is OK the Meter shows maximum reading (1----). If the transistor Drain and Source are shorted, the Meter reads a very low voltage, close to 0V.

Step 2 - Loading the capacitors with Gate Voltage:

To turn on a MOS transistor, it is usually necessary to apply a voltage to the gate in relation to the source, greater than 3V. The polarity will depend on whether the channel is N or P. But multimeters, function "diode", usually apply a maximum voltage in range of 2V. This voltage is not sufficient to turn on most MOS transistors. So we will use the capacitors for duplicate this voltage.

First connect the capacitors in series as shown in figure 5(positive of first capacitor to the negative of second).

With meter in "diode" function connect the leads as shown in figure 5, for charging first capacitor. Wait until the meter indicates maximum reading (1 ----).

Figure 5

Then repeat the operation, as shown in Figure 6, to charging the second capacitor.

Figure 6

Do not touch capacitor terminals with your fingers to don't discharge then.

To check if your meter generates the voltage required, adjusting scale to 20Vdc and measure the loaded value, as in Figure 7. Ideally, the charged voltage is above 4V. But most of the MOS transistors turn on over 3V.

Figure 7

Step 3 - Checking if transistor turn on:

Gate current required to turn on MOS transistors is very small. So, even the resistance of your body is sufficient to conduct this current. Connect the capacitors to the Gate and Source, through you, as shown in Figure 8. Take cary about polarity of capacitors. Your right hand should touch only the source terminal. With his left hand, just hold the negative terminal of the capacitor, as shown.

Figure 8

Some current from capacitors goes to gate and turn on the transistor. To see if it really turned on, connect the multimeter leads again as shown in figure 4. Now the reading should indicate a very low value, close to 0V, indicating that the transistor is turned on. Important: You do not need to keep the capacitors connected to the Gate. As the gate impedance is very high, the charge remains there even with the capacitors disconnected.

Step 4 - Checking if the Transistor turn Off:

To turn off the transistor from the previous step, repeat the condition of Figure 8, but with the reversed polarity of the capacitors. You can also simply touch the Gate to Source.

To check if transistor turned off, position the meter again as shown in figure 4. In this case the reading should again accusing maximum value.

Testing MOS type P

The procedure is basically the same as above, but with all polarities reversed.

Fig 9

Reverse the multimeter tips, as shown in figure 10.

Fig 10

Fig 11

Figure 12

Fig 13

Figure 14