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Old 03-18-2010, 04:58 PM
johndoe45 johndoe45 is offline
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Default testing a FET

i am testing a FET

here is spec sheet

http://forum.allaboutcircuits.com/at...0&d=1268639919

now here is what one website says.

using the diode function on your DMM.
probe with - on the drain and + on the gate. this should arm the FET.

then keep - on drain and put + on source. multimeter should beep.

my FET passed this SO FAR.

now it says to test the FET's ability to switch off touch all three pins with finger. then put - on drain and + on source again and should get a high reading.
well my FET still beeps. is the FET defective????
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Old 03-18-2010, 05:01 PM
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Your finger could be defective.

Does your DMM have a npn or pnp test area/function?
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Old 03-18-2010, 05:03 PM
johndoe45 johndoe45 is offline
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lol

i don't think so
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Old 03-18-2010, 05:07 PM
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ok, try this instead:
Quote:
Testing MOSFETs

Field Effect Transistors are difficult to test with a multimeter, but "fortunately" when a power MosFet blows, it blows big time: all their leads will show in short circuit. 99% of bad MosFets will have GS, GD and DS shorted. In other words - everything will be connected together.

Note: When measuring a MosFet hold it by the case or the tab and don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed. Do not allow a MosFet to come in contact with your clothes, plastic, etc. because of the high static voltages they can generate.

You'll know a MosFet is good when the Gate has infinite resistance to both Drain and Source. Exceptions to this rule are FETs with protection circuitry - they may act like there is a diode shunting GS - a diode drop for gate reverse bias. Connecting Gate to Source should cause the Drain to Source act like a diode. Forward biasing GS with 5V and measuring DS in forward bias should yield very low ohms. In reverse bias, it will still act like a diode.

Another simple test procedure: connect the multimeter's negative lead to the source of the MosFet. Touch the MosFet's Gate with the meter's positive lead. Move the positive probe to the Drain - you should get a low reading as the MosFet's internal capacitance on the Gate has now been charged up by the meter and the device is turned-on. With the meter's positive lead still connected to the Drain, touch the source and gate with your finger. The Gate will be discharged through your finger and the reading should go high, indicating a non-conductive device! This simple test is not fail proof, but it's usually adequate.
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Old 03-18-2010, 05:08 PM
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If you are getting continuity between drain and source, its a gonner.
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Old 03-18-2010, 05:23 PM
johndoe45 johndoe45 is offline
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gate has infinite resistance to both drain and source

and no continuity between drain and source

so i'm guessing it is still good?

i don't understand this part of third paragraph
Forward biasing GS with 5V and measuring DS in forward bias should yield very low ohms. In reverse bias, it will still act like a diode.
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Old 03-18-2010, 05:28 PM
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It is still good.

The third paragraph is explaining how to test protected FETs as diodes.
No continuity, no problem. If you had continuity, then you would do a diode check. If you get the diode drop, its still good. If no diode drop, its failed shorted
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Old 03-18-2010, 05:33 PM
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oh ok. thanks a lot. spent thirty minutes reading up on it and you helped me in only about 5 minutes.
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Old 03-18-2010, 05:53 PM
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Any time I can help, i'll try.
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Old 03-22-2010, 08:40 PM
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i can't figure out if it is not switching off???

here is what i did. hold black probe at drain whole time. touch gate with red and then touch source with red while touching gate and drain with finger. a reading shows.
now to switch it off do i keep my fingers on the drain and gate???

cause i read you have to touch gate once with red and then touch source again. but doesn't specify if i keep my fingers on drain and gate.
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