P mosfet

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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

Re: P mosfet

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

   Devil, details .....



....  Phil







Re: P mosfet
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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

Re: P mosfet

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

    Say 2.2 kohms -  to stop HF oscillation.



...   Phil




Re: P mosfet
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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

Re: P mosfet

"Rheilly Phoull"
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** Looks a lot like it.

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



...   Phil




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

Cheers ......... Rheilly


Re: P mosfet
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What's the measured Vgs when it is supposed to be turned on?


Re: P mosfet
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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

Re: P mosfet
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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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