Return-Path: Received: (qmail 11292 invoked from network); 27 Oct 2002 22:31:40 -0000 Received: from murphys.services.quay.plus.net (212.159.14.225) by mailstore with SMTP; 27 Oct 2002 22:31:40 -0000 Received: (qmail 19576 invoked from network); 27 Oct 2002 22:30:16 -0000 Received: from post.thorcom.com (193.82.116.70) by murphys.services.quay.plus.net with SMTP; 27 Oct 2002 22:30:16 -0000 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 4.10) id 185vuo-0001Sw-00 for rsgb_lf_group-outgoing@blacksheep.org; Sun, 27 Oct 2002 22:29:50 +0000 Received: from [194.73.73.93] (helo=rhenium.btinternet.com) by post.thorcom.com with esmtp (Exim 4.10) id 185vuo-0001Sn-00 for rsgb_lf_group@blacksheep.org; Sun, 27 Oct 2002 22:29:50 +0000 Received: from host213-122-132-233.in-addr.btopenworld.com ([213.122.132.233] helo=main) by rhenium.btinternet.com with smtp (Exim 3.22 #8) id 185vun-0002jc-00 for rsgb_lf_group@blacksheep.org; Sun, 27 Oct 2002 22:29:50 +0000 Message-ID: <001901c27e08$642b6980$e9847ad5@main> From: "Alan Melia" To: "LF-Group" Date: Sun, 27 Oct 2002 22:28:16 -0000 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 4.72.3110.5 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Subject: LF: testing FETs Content-Type: text/plain; charset=iso-8859-1; format=flowed Content-Transfer-Encoding: 8bit X-Spam-Status: No, hits=1.0 required=5.0tests=SPAM_PHRASE_02_03,USER_AGENT_OEversion=2.42 X-Spam-Level: * Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Hi Peter, I think you said when we chatted that you couldnt tell the difference with a test meter (ohm-meter) The reason ofr this is the battery in an Ohm-meter is usually 1.5 volts. This is sufficient to turn-on a BJT but not sufficient to turn on a power FET of the type used in the Decca. The Audio FETs in the B&K may be different as they are used in linear audio applications.....they probably start to conduct at much lower gate source voltages. I suggest a little test rig with an ohm-meter across Drain-source (I dont think it matters which way round as the channel is resistive). Then use an external battery (PP3) or a PSU to put +9v through a 10k0 (series....i.e.current limit) resistor on the gate. Most of these power switching FETs take about 6 volts gate-source to switch on....hence no reading either way with an 'AVO'. The meter in d-s should be high resistance and swing to very low resistance when the 9v is applied to the gate..my IRF640s read about 2.6ohms on an Altai h/h DMM when on and fullscale when off. I think that any failure would mean that this sequence could not be repeated. I am not sure how they fail, but the aftermath of the initial fault could well be a ball of melted and recryatalised silicon. The high resistor in the gate lead should ensure that if the gate has broken down there will not be enough volts to switch the device on. A quick check could be made directly across g-s to show that all the 9v was being dropped across the 10k0 resistor. If the FET has suffered from over-current it will show resistance between d-s with no gate drive I think. Maybe those who have fryed fets will confirm.....I have not lost a single FET to my Class E experiments !! Cheers de Alan G3NYK alan.melia@btinternet.com I hope this helps