Return-Path: Received: from rly-dd07.mx.aol.com (rly-dd07.mail.aol.com [172.19.141.154]) by air-dd04.mail.aol.com (v124.15) with ESMTP id MAILINDD041-b884a3ca24025; Sat, 20 Jun 2009 04:48:20 -0400 Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by rly-dd07.mx.aol.com (v124.15) with ESMTP id MAILRELAYINDD072-b884a3ca24025; Sat, 20 Jun 2009 04:48:01 -0400 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1MHwDy-0003Ea-7h for rs_out_1@blacksheep.org; Sat, 20 Jun 2009 09:46:58 +0100 Received: from [83.244.159.144] (helo=relay3.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1MHwDx-0003ER-NG for rsgb_lf_group@blacksheep.org; Sat, 20 Jun 2009 09:46:57 +0100 Received: from brigitte.telenet-ops.be ([195.130.137.66]) by relay3.thorcom.net with esmtp (Exim 4.63) (envelope-from ) id 1MHwDv-0003Ex-TR for rsgb_lf_group@blacksheep.org; Sat, 20 Jun 2009 09:46:58 +0100 Received: from [192.168.0.100] ([84.196.202.224]) by brigitte.telenet-ops.be with bizsmtp id 68mu1c0064r06vG0G8muHY; Sat, 20 Jun 2009 10:46:55 +0200 Message-ID: <4A3CA1F8.7020009@telenet.be> Date: Sat, 20 Jun 2009 10:46:48 +0200 From: "Pedro M.J. Wyns (netbook)" User-Agent: Thunderbird 2.0.0.21 (Windows/20090302) MIME-Version: 1.0 To: rsgb_lf_group@blacksheep.org References: <582241.74739.qm@web28516.mail.ukl.yahoo.com> <000a01c9f17f$16506d40$0301a8c0@mal769a60aa920> In-Reply-To: <000a01c9f17f$16506d40$0301a8c0@mal769a60aa920> X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,HTML_MESSAGE=0.001 Subject: LF: Killing FET finals... Content-Type: multipart/alternative; boundary="------------070604080305060100070705" X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.5 required=5.0 tests=HTML_40_50,HTML_MESSAGE autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-AOL-IP: 193.82.116.20 X-Mailer: Unknown (No Version) --------------070604080305060100070705 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Not relevant. Most FET's are going QRT due to feedback issues in bad parallel circuits... The _golden rule_ is to put SUFFICIENT series resistance in every gate separately, do NEVER put gates in parallel. Some nice literature on this is the fantastic book on HF amplifier design by Helge Granberg from Motorola. Pedro ON7WP-AA9HX /Licensed transceiver doctor and radio guru/ :-) mal hamilton schreef: > Chris > Use a separate pwr supply for the fan, not the same supply that runs > the FETS or driver ccts. This is what I do and never had a problem. > mal/g3kev > > > ----- Original Message ----- > *From:* Chris Osborn > *To:* LF Group > *Sent:* Saturday, June 20, 2009 7:34 AM > *Subject:* LF: 500 kHz - Fans > > LF, > I've nearly completed my new PA for 500 kHz and have now managed to > stop destroying FET's. > > Unfortunately I have an unusual problem in that the heatsink > cooling fans > are going U/S. > > I have used 24 volts brushless PC-type fans and after a while they > either die completely or start to go backwards slowly. . . > > The fans may be slightly over-run but only by a volt or so and on > key-up > (unregulated supply) and are situated about 50 mm (2 inches) from > the heatsink. > I have rf decoupled them from the supply but maybe not well enough ! > > Any suggestions in curing this problem would be most welcome. > > 73 Chris G3XIZ > > > > --------------070604080305060100070705 Content-Type: text/html; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Not relevant.  Most FET's are going QRT due to feedback issues in bad parallel circuits...

The golden rule is to put SUFFICIENT series resistance in every gate separately, do NEVER put gates in parallel.

Some nice literature on this is the fantastic book on HF amplifier design by Helge Granberg from Motorola.

Pedro ON7WP-AA9HX
Licensed transceiver doctor and radio guru :-)




mal hamilton schreef:
Chris
Use a separate pwr supply for the fan, not the same supply that runs the FETS or driver ccts. This is what I do and never had a problem.
mal/g3kev
 
----- Original Message -----
Sent: Saturday, June 20, 2009 7:34 AM
Subject: LF: 500 kHz - Fans

LF,
I've nearly completed my new PA for 500 kHz and have now managed to
stop destroying FET's.
 
Unfortunately I have an unusual problem in that the heatsink cooling fans
are going U/S.
 
I have used 24 volts brushless PC-type fans and after a while they
either die completely or start to go backwards slowly. . .
 
The fans may be slightly over-run but only by a volt or so and on key-up
(unregulated supply) and are situated about 50 mm (2 inches) from the heatsink.
I have rf decoupled them from the supply but maybe not well enough !
 
Any suggestions in curing this problem would be most welcome.
 
73 Chris G3XIZ
 
 
 


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