Saturday, April 21, 2012

How to Check a MOSFET Using a Digital Multimeter

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 checked using an ordinary digital multimeter through slightly different procedures.

STEPS for type K1058 :

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. This will initially keep the internal capacitance of the device completely discharged.

4) Now Touch the meter black probe to center lead and the red probe to right leads 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, lift the red probe from the right lead and touch it to the left lead of the circuit momentarily and bring it back to the right lead of the mosfet.

7) This time the meter will show a short circuit.

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 you will again need to short the gate and the source leads using a piece of wire as explained earlier.

STEPS for type IRFP240:

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 and the black to the center pin of the mosfet.

After a few seconds, put the red probe and the black probe back to the center and the right pin of the mosfet respectively.

This time the meter should show a short circuit, proving that the mosfet is in a good condition.




14 comments:

  1. According to alldatasheet.com the pins of K1058 are GSD, yet U stated here GDS. So I'm confused.
    http://www.alldatasheet.com/view.jsp?Searchword=K1058.P.S. excuse my english. A big thanks to you Swagatam

    ReplyDelete
    Replies
    1. OK...? I may be wrong with the pin assignments, but the procedure is right, just follow the procedures as given in the article.

      Thanks!

      Delete
  2. hi. after test, need to dicharge pins? Before use like short circuit? Or reverse probs? Thx

    ReplyDelete
  3. HELLO. is this is applicable for all kind of n-channel mosfet?

    ReplyDelete
    Replies
    1. yes but first make sure about the pin assignments, might be different for different brands.

      Delete
  4. Hi. how to check for p-channel mosfet?

    ReplyDelete
    Replies
    1. just change the prod polarity in the above explained procedures

      Delete
  5. hi swagatham this srinivas
    ihave to know the six band resister value of these colours brown,black,black,silver,brown,green
    plese tell me the above resister value
    thanks & regards

    ReplyDelete
    Replies
    1. Hi Srinivas,

      It's

      1 ohms, 1 % tolerance, 20 ppm temperature coefficient

      Delete
  6. Hi sir i am girish. i brought two IRF540N and two IRFZ44 it was new MOSFETs , for my inverter. first i used IRFZ44 it gave me very low output. i checked it with multimeter first 5 points given above was ok , but it did't give any shot circuit. then the two unused MOSFETs IRF540N gave the exactly same result i,e it did't give any short circuit . then i again brought two MOSFETs IRF540 from the same shop but, it gave the exactly same result i,e it did't give any short circuit. i checked all the MOSFETs without using in my inverter so ,we can say that the inverter did't damage it. i have all ready checked many MOSFETs successfully. but here totally confusing .and i have conformed the pin outputs of the MOSFETs. there could be any possibility of manufacture's defect?

    ReplyDelete
    Replies
    1. Hi Girish,

      The best and simple way to check them is by testing them practically. You can make the first circuit shown in the following article. Put a 12 lamp in the "load" section. If pressing the switch triggers the load and vice versa would mean your mosfets are OK.

      http://homemadecircuitsandschematics.blogspot.in/2013/04/simple-mosfet-switch-circuit-with-delay.html

      Delete
  7. HI sir .

    I tested the MOSFETs in the above link the 12v bulb(two 6v / 300mA in series ) the bulb glows with full bright. i think the MOSFET is all right.

    sir ,can you tell why these 7 points given above was not applicable with multimeter, for these MOSFETs.

    -GIRISH.

    ReplyDelete
    Replies
    1. Hi Girish,

      Can't say actually why it's not working for you, because when I checked the mosfets k1058 and j162 with the above methods, it gave me perfect results as explained in the article.

      Delete

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Readers are advised to proceed with the construction of the presented circuits only after understanding the concepts from the core. Not adhering to this can lead to failures and frustrations.