I am new to electronics and I'm understanding most of what I'm reading but I have a mosfet on my board that I've identified as a P-channel Trench Mosfet. I haven't a clue how to test this and what I should expect to see with the multimeter probes. I am hoping someone with years more experience than myself can help me understand how to use a multimeter to read this. Please look at the drawing I have attached to this and can you try to explain to me what I should be doing with my multimeter in diode mode and what results I should expect from a good mosfet?
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If I am reading this properly and in diode mode, putting your black probe on Pin 3 Drain and Red Probe on Pin 2 Source you will get nothing. Put your black probe on Pin 2 Source and your red probe on Pin 3 Drain you will get a reading perhaps in the 500 range. If you put your black probe on Pin 1 Gate and your red probe on Pin 2 Source you will show a short the way it looks to me and your beeper will sound on your meter, and vice versa put your red probe on Pin 1 Gate and your Black Probe on Pin 2 Source again you will read a short and your beeper will go off. Is my interpretation of this Mosfet correct? It's weird because on the gate and source pins all you can see are the anodes and the cathodes seem attached internally inside the mosfet. This truly has me stumped. I'm sure an engineer will know how to interpret this chip. -
If I am reading what you wrote then what has happened you turn on the mosfet to conduct the flow of current source to drain you should be able to turn it off again and you should get the same results you did when you first started checking it
I personally have not tested mosfets very much so I can not tell you accurately how to turn it offComment
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Correct.Originally posted by dj8452000 View Post
With that test, you're essentially reading the internal protection diode between the Source and Drain pins (and also confirming that Source and Drain are not shorted together). For N-ch MOSFETs it's the same, but with probe polarity reversed to what it is for P-ch MOSFETs.
No, it's usually bad news for the MOSFET any time you get continuous beeping on your meter (indicating short-circuit / low resistance / low voltage drop) between Source and Gate, regardless of the probe orientation - at least if testing the MOSFET out of circuit. In circuit, there can be another component interfering with the measurement, so the MOSFET may not necessarily be bad... though in most cases, beeping between Gate and Source in circuit is also still indicative of a failure.Originally posted by dj8452000 View Post
I haven't looked at this particular MOSFET's datasheet yet, but those are probably Zener protection diodes on the Gate.Originally posted by dj8452000 View Post
Continuing with the rest of the tests for this MOSFET...
If you put your red (+) MM probe on the Source and black (-) MM probe on Gate, then move the black (-) probe to the Drain, you should now see a low voltage drop / low resistance / your multimeter continuously beeping to show continuity, as this charges the Gate, which opens the conduction channel between Source and Drain (for current to flow.)
The above is applicable to P-channel MOSFETs only. For N-channel MOSFETs, the test is the same, but with the MM probe polarity reversed (i.e. red on Gate and black on Source, then move red on Drain, and you should get beeping). And of course, worth noting that this works only for MOSFETs out of circuit. In-circuit, there will usually be a discharging / pull-down resistor between Gate and Source, so this test won't work there.Comment
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Thank you for your input and for clearing some of this up for me.
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