P mosfet

G'Day All,

I'm trying to use a P type mosfet (IRF 9540) as a PWM switch to charge a battery. I cant get it to turn on completely, as a test I have connected the + supply to the source and the drain to the + of the battery and the neg of the battery to the neg of the supply. If I take the gate to the neg it does not turn on completely, there is a volt drop of several volts across it at 2 amps. This happens with the 2 devices I've tried. They turn off OK when the gate is held high. What am I missing here ???

Cheers ....... Rheilly

Reply to
Rheilly Phoull
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"Rheilly Phoull"

** What WE are missing is the battery type and voltage and the nature of the supply.

Devil, details .....

.... Phil

Reply to
Phil Allison

Ya right of course, 12v battery and a regulated variable supply that I crank up to set the current flow. The real use will be with solar panels.

Cheers ......... Rheilly

Reply to
Rheilly Phoull

"Rheilly Phoull

** You have got a " stopper " resistor is series with the gate - right ?

Say 2.2 kohms - to stop HF oscillation.

... Phil

Reply to
Phil Allison

Thanks Phil, I didn't but putting one in has not changed things. One other thing is that these are a batch from Hong Kong, I'm wondering if perhaps they are bodgy ??

Cheers .......... Rheilly

Reply to
Rheilly Phoull

"Rheilly Phoull"

** Looks a lot like it.

Rds(on) is about 5 times higher than the spec of 0.2 ohms.

... Phil

Reply to
Phil Allison

Yup, beats me. I'll wander down town and get a known brand and see how that goes. Thanks for the help.

Cheers ......... Rheilly

Reply to
Rheilly Phoull

What's the measured Vgs when it is supposed to be turned on?

Reply to
swanny

Well I just grounded the gate so it was around 14v or 0 depending on how you view it. As it turns out I was ripped off once again by the fiendish oriental trader :-) I've just come back with a brand type one and all is well. (As Phil A and I were discussing)

Cheers ........ Rheilly

Reply to
Rheilly Phoull

There are a couple of Thai dealer who are very good BUT some of the honkers and Singa boys sell rubbish If you find a good one stick with him

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Reply to
atec77

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