Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Cheap DMM to also be able to test MOSFETs

Status
Not open for further replies.
T

treez

Guest
Hello,
We want to buy this cheap DMM . (Tenma 72-7770A)
https://www.farnell.com/datasheets/1993713.pdf
We want to also be able to test N type MOSFETs with it by putting it on OHMS reading, then touching gate and source with the probes (red to gate obviously). –Then seeing if its short from drain to source…then touch gate to source with our hands, so as to dishcharge it, and then checking if its open from drain to source.
The thing is, the manual doesn’t assure us that when on ohms reading the probe voltage would go up above 5V to turn the mosfet on. Do you know if it does?
 

Hi,

A DMM may be one part of a suitable test circuit.

"Touching with fingers" is a no go. It gives no useful results.

Depending on what parameters and with what precision under what conditions you want to measure .., the circuit becomes more and more difficult.

Look into a Mosfet's datasheet to see some test circuits.

Klaus
 

    V

    Points: 2
    Helpful Answer Positive Rating
Thanks, we could use an external psu to put 8v on the Vgs, and then DMM the D-S.....then change it to zero volts, and again check D-S is open that time.....but its time consuming and easier if we can just do it with a dmm alone.
As you know, The "fingers" bit is to discharge the CGS with our body.
 

Hi,

Use a 24V supply
A 100k resistor
a DVM
And the DUT

Circuit:
Short circuit D and G --> DG
+24V to resistor
Resistor to DG
S to GND
DVM at DG and GND

The measured voltage should be close to V_gs_th.
If the voltage is close to zero, then the Mosfet is defective
If the voltage is too high, then the Mosfet is defective

This gives good, reliable and repeatable information about the health status of the Mosfet...with a simple test circuit.

Klaus
 

Thanks it sounds great, did you deliberately select 24V , knowing it would be outside the likely VGS max.....because if the nfet was OK, then its gate would never see the 24V anyway
 

I use a cheap LCR and ESR tester from that well known sales site. Cost almost nothing and has an LCD that identifies not only the component type but it's pin outs, including MOSFETs.

20171008_232646a.jpg
That is actually a P5K50 N channel MOSFET in the socket.

Brian.
 

    V

    Points: 2
    Helpful Answer Positive Rating
Thanks, The good thing about testing mosfets with a simple DMM whilst in an unpowered PCB, is that you can say that if D-G and/or G-S are short cct as seen by a DMM on low ohms setting , then the mosfet is dead...no doubt about it......however, if D-G and/or G-S are not short, the mosfet may still be dead, but we have to desolder it in order to find out if its dead by doing the “gate-source voltage and measure D-S resistance test”.. do you agree?
 

Whether or not any such tests are valid will depend on the circuitry surrounding the MOSFET. There are applications where it would be quit normal to measure only a few Ohms between pins so in-circuit testing cannot be relied upon.

Brian.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
Thanks, the following concerns in-circuit-testing of N type mosfets, in circuits that are powered off (and all capacitors discharged)..

-do you believe we would agree that in the following current regulator circuit, the MOSFET would be deemed dead if (when powered off)…………
1…..Drain to gate was seen as short circuit or low ohms with a DMM on low ohms setting
2….Gate to source was seen as short circuit or low ohms with a DMM on low ohms setting

Also, if we first probed the NFET in circuit with a DMM on low ohms setting, and with black probe to gate and red to source……..then……if we next probed from Drain to source on a low ohms setting and saw a short circuit, then the fet would be dead
Do you agree with the above?

(schem and ltspice attached)
 

Attachments

  • LED current regulator.txt
    3.6 KB · Views: 41
  • LED current regulator.pdf
    21.8 KB · Views: 94

Assuming a 'normal' DMM with low voltage across it's probes while measuring resistance:

1. You should measure a high resistance as the LEDs will have insufficient voltage from the DMM to allow them to conduct.
2. Impossible to say without knowing the output characteristics of U2. If the DMM voltage was high enough it might force conduction through U2 output circuit leaving only the extra resistance of R1 and R2 readable.

conclusion: use those tests at your own risk, they are not conclusive and results depend on other factors, in particular the DMM probe voltage.

Brian.
 

Thanks, if i test 100 of the products of post #10, and 99 are "open circuit" from D-S and from D-G, ..but then one product is found to be having short circuit from D-G and/or short cct from G-S, then categorically you can say that that one product definetely has a failed mosfet?

(i am speaking of testing with a DMM on low Ohms setting; -Also, i am speaking about testing the nfet "in-circuit")
 

A significant difference in measurement in 1% of the products certainly indicates something is wrong with them but pinning it down to a specific component requires a more in-depth investigation. Probability says it's the MOSFET but certainty doesn't!

Brian.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top