How to test MOSFET? with Digital Multimeter

What is MOSFET?

The MOSFET, also known as a metal-oxide-semiconductor field-effect transistor, is a type of FET with an insulated gate that is assembled by the controlled oxidation of that semiconductor.

The semiconductor used in it is generally silicon.

Source(S)
Gate (G)
Drain (D)
Body (B)

In a general overview, the body of a MOSFET is in connection with its source terminal hence forming a three-terminal device such as a field-effect transistor.

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

  There are two types namely;

1.Enhancement MOSFET,
2.Depletion – MOSFET.

The (MOSFET) is a transistor used for amplifying or switching electronic signals.

Although the MOSFET is a four-terminal device with source (S), gate (G), drain (D), and body (B) terminals, the body (or substrate) of the MOSFET often is connected to the source terminal, making it a three-terminal device like other field-effect transistors.

Because these two terminals are normally connected to each other (short-circuited) internally, only three terminals appear in electrical diagrams.

The MOSFET is by far the most common transistor in both digital and analog circuits, though the bipolar junction transistor was at one time much more common.

By applying a transverse electric across an insulator, deposited on the semiconducting material, the thickness and hence the resistance of a conducting channel of a semiconducting material can be controlled. 

The MOSFET is most widely used than scr and is excellent switching device in megahertz range.

The switching speed of the device is decided by the duty cycle and frequency of the PWM generator.

MOSFET is the  familiar with is the metal oxide semiconductor field-effect transistor You must be extremely careful when working with MOSFETs because of their high degree of sensitivity to static voltages.

MOSFET SYMBOLS

• IRF 540 N-Channel PDF Details: MOSFET IRF540-datasheet

 

• N-Channel MOSFET PDF Details:  N-Channel MOSFET-datasheet

 

• MOSFET TEST CIRCUITS: Details:        IRF540-TEST CIRCUIT-datasheet

BEFORE TESTING MOSFET SOME TIPS:

1. Soldering iron should be grounded.

2. A metal plate should be placed on the workbench and grounded to the ship’s hull through a 250-kilohm to 1-megohm resistor.

3. You should also wear a bracelet with an attached ground strap and ground yourself to the ship’s hull through a 250-kilohm to 1-megohm resistor.

4. A vacuum plunger (solder sucker) must not be used because of the high electrostatic charges it can generate. Solder removal by wicking is recommended. It is also good practice to wrap  MOSFETs in metal foil when they are out of a circuit.

Animation for how MOSFET is working:

FIRST TIME USING DIGITAL MULTIMETER : DMM means Digital multimeter

TESTING WITH DMM – (Diode Mode)

1. Never exceed the protection limit values indicated in specifications for each range of measurement.

2. When the value scale to be measured is unknown beforehand set the range selector at the highest position.

3. When the meter is linked to measurement circuit, do not touch unused terminals. Before rotating the range selector to change functions, disconnect the lest leads from the circuit under test.

Never perform resistance measurements on live circuit. Always be careful when working with voltage above 60v DC of 30v Ac RMS.

4. KEEP THE FINGERS BEHIND THE PROBE BARRIERS WHILE MEASURING. BEFORE ATTEMPTING TO INSERT TRANSISTORS FOR TESTING, ALWAYS BE SURE THAT TEST LEADS HAVE BEEN DISCONNECTED FROM ANY MEASUREMENT CIRCUIT. COMPONENTS SHOULD NOT BE CONNECTED TO THE hfe SOCKET WHEN MAKING VOLTAGE MEASUREMENTS WITH TEST LEADS.

IMPORTANT: 

 

A. If the resistance being measured exceeds the maximum value of the range selected or the input is not connected, an over range indication  will be displayed.

B.

 

When checking in-circuit resistance, be sure the circuit under test has all power removed and that all capacitors have been discharged fully.

C.

 

For measuring resistance above 1 Mohms the meter may take a few seconds to get stable reading., this is normal for high resistance measurements.

 

 

STEP-1.  SELECT DIODE MODE IN DIGITAL MULTIMETER (DMM)
Connect DMM  positive test lead  to PIN-1 (Gate)
DMM Negative test lead to PIN-2 (Drain)  open or ‘1’ or OL
DMM Negative test lead to PIN-3 (source)  OL or ‘1’ open

STEP-2. Connect DMM  Negative test lead to PIN-1 (G)
DMM  positive test lead to  PIN-2 (D)      DMM READING SHOWS OL  or ‘1’ or OPEN
DMM  positive test lead to  PIN-3 (S)       OL or ‘1’ or open

STEP-3.
Connect DMM  Negative test lead to PIN-2 (D)
DMM   positive test lead  to PIN-3 (S) DMM READING SHOWS =(0.427=427mV) GOOD.

STEP-4.
Connect DMM  positive test lead  to PIN-2 (D)
DMM Negative test lead to PIN-3 (S) open or ‘1’ or OL

Verification: 

If the DMM above reading shows the condition is  GOOD.

ANY OTHER READING GIVES THAT DEVICE MAY FAULTY

Properly testing a MOSFET requires a lot of expensive test gear, but if you have a suitable digital multimeter you can do a pretty accurate go/no go test which will fail very nearly all dud MOSFETs.

Nowadays most multimeters have a diode test range. On most multimeters (but by no means all!) this puts about 3-4v across the device under test.

This is enough to turn on most MOSFETs – at least partially, and enough to test. The meters which use a lower open-circuit test voltage (sometimes 1.5v) will not make this test!

Plastic package IRF 540

Step 1.  Connect the meter negative probe to the MOSFET’s source. This is arrowed in the drawing above which shows the most popular TO220 MOSFETs. Hold the MOSFET by the case or the tab if you wish, it doesn’t matter if you touch the metal body but be careful not to touch the leads until you need to.

Step 2. Touch the meter positive probe on to the gate. Now move the positive meter probe to the drain. You should get a low reading.
The MOSFET’s gate capacitance has been charged up by the meter and the device is turned on.

Step 3. With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you wish, it matters not).

The gate will be discharged through your finger and the meter reading should go high, indicating a non-conducting device. Such asimple test cannot be 100% – but it is useful and is usually adequate.

What the test above actually measures is the pinch-off voltage: the highest voltage that can be put on the MOSFET’s gate without it starting to conduct.

The circuit below shows a better way

MOSFET TESTING WITH CIRCUIT

Faulty MOSFET: