Return-Path: Received: (qmail 3265 invoked from network); 28 Oct 2002 12:21:35 -0000 Received: from marstons.services.quay.plus.net (212.159.14.223) by mailstore with SMTP; 28 Oct 2002 12:21:35 -0000 Content-Transfer-Encoding: 8bit Received: (qmail 4330 invoked by uid 10001); 28 Oct 2002 13:27:44 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: from post.thorcom.com (193.82.116.70) by marstons.services.quay.plus.net with SMTP; 28 Oct 2002 13:27:44 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from majordom by post.thorcom.com with local (Exim 4.10) id 1868u0-00043b-00 for rsgb_lf_group-outgoing@blacksheep.org; Mon, 28 Oct 2002 12:21:52 +0000 Received: from [147.197.200.9] (helo=hestia.herts.ac.uk) by post.thorcom.com with esmtp (Exim 4.10) id 1868u0-00043S-00 for rsgb_lf_group@blacksheep.org; Mon, 28 Oct 2002 12:21:52 +0000 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 1868sT-0003Kt-00 for rsgb_lf_group@blacksheep.org; Mon, 28 Oct 2002 12:20:17 +0000 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 1868sI-0000rh-00 for rsgb_lf_group@blacksheep.org; Mon, 28 Oct 2002 12:20:06 +0000 Message-ID: <5.1.0.14.0.20021028111826.00a8f110@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Mon, 28 Oct 2002 12:20:04 +0000 To: rsgb_lf_group@blacksheep.org From: "James Moritz" In-reply-to: <000001c27dfb$bb8c69c0$0ee086d4@erica> References: <001b01c27cd3$a229fe80$0f6468d5@oemcomputer> <000401c27cc9$ea5b06e0$edce28c3@erica> <000101c27d94$53aeed80$07ce28c3@erica> MIME-Version: 1.0 X-MailScanner: No Virus detected Subject: Re: LF: Testing FETs Content-Type: text/plain; charset=us-ascii; format=flowed X-Spam-Status: No, hits=-0.4 required=5.0tests=DEAR_SOMEBODY,IN_REP_TO,REFERENCES,SPAM_PHRASE_00_01version=2.42 Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group At 20:58 27/10/2002 +0000, you wrote: >I have a faulty Decca PA module and I suspect that one or more of the >IRF250s have blown. Dear Peter, LF Group, 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. Cheers, Jim Moritz 73 de M0BMU