Message-ID: <001401c27eda$1b6b91c0$391686d4@erica>
From: "g3ldo" <g3ldo@ukonline.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <001b01c27cd3$a229fe80$0f6468d5@oemcomputer> <000401c27cc9$ea5b06e0$edce28c3@erica> <000101c27d94$53aeed80$07ce28c3@erica> <5.1.0.14.0.20021028111826.00a8f110@gemini.herts.ac.uk>
Date: Mon, 28 Oct 2002 23:30:24 -0000
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Subject: Re: LF: Testing FETs
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>
> I have in the past found power MOSFETs can partially fail, where they
> appear to be working when simple go/no-go tests are applied, but quickly
> get hot and fail when put back in the circuit. so it is certainly a good
> idea to do the tests Alan and Stewart have suggested. In particular,
pretty
> well no gate current should flow, so if you connect a clip lead to the
gate
> to bias it on, then remove the lead without touching the gate, it should
> stay biased on for several seconds at least, probably minutes or more..
The
> "on" resistance of an IRF250 is 0.085ohms maximum (with 10V Vgs bias), so
> too small to measure with a multimeter - better to put a known current,
say
> 1A, through it and measure the voltage drop between source and drain leads
> (should be below 85mV).
>
> Also check all the other components in the PA circuit - all the diodes and
> zeners, the driver transistor etc. When one component goes, it tends to
> take others with it. When you put it back together, with no "HT" supply
> check there are reasonably clean 136kHz square waves driving all 4 gates
at
> about 15 - 20V pk-pk, symetrical around 0V. Start off running the module
on
> a low "HT" voltage, say 5V (but don't reduce the driver supply voltage!!!)
> - the current drain should be down in proportion to the reduction in
supply
> voltage with a class D amplifier. When you are sure it is working OK at
low
> voltage, gradually increase to the full supply voltage, checking that
> things don't get hot. Starting off with a low supply greatly reduces the
> possibilities for blowing things up again.


Many thanks to Jim, Alan and Stewart for their advice regarding testing
FETs.
>From this I find that you can test Decca Tx module FETs (IRF250s) in situ
using a variable PSU with voltage and current control and indication.
Disconnect the transformer wire from the gate but leave the protection
diodes in place (BZX79C10s). Set the PSU voltage to 10 or 12 volts and the
current limiting to (say) 0.5A. Connect the positive and negative leads from
the PSU to the drain and source respectively. Touch the gate with your hand
(best done with the blade of a small screwdriver). This causes an indication
in the current reading. To get full current touch the positive lead
(connected to the drain) with the other hand. This should cause the current
to go to the max limit set on the PSU. Repeat for all other suspect FETs.
Using this method I quickly located the faulty FETs.

Regards,
Peter, G3LDO

e-mail <g3ldo@ukonline.co.uk>

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