X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sat, 09 Feb 2008 18:43:10 -0500 Message-ID: X-Original-Return-Path: Received: from wind.imbris.com ([216.18.130.7] verified) by logan.com (CommuniGate Pro SMTP 5.2.0) with ESMTPS id 2722384 for lml@lancaironline.net; Sat, 09 Feb 2008 10:45:49 -0500 Received-SPF: none receiver=logan.com; client-ip=216.18.130.7; envelope-from=brent@regandesigns.com Received: from [192.168.1.100] (cbl-238-80.conceptcable.com [207.170.238.80] (may be forged)) (authenticated bits=0) by wind.imbris.com (8.12.11/8.12.11.S) with ESMTP id m19Fj38f021711 for ; Sat, 9 Feb 2008 07:45:08 -0800 (PST) (envelope-from brent@regandesigns.com) X-Original-Message-ID: <47ADCA7E.2090800@regandesigns.com> X-Original-Date: Sat, 09 Feb 2008 07:45:02 -0800 From: Brent Regan User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 X-Original-To: Lancair Mailing List Subject: VM1000 Failure Content-Type: multipart/alternative; boundary="------------000607060405030409060909" This is a multi-part message in MIME format. --------------000607060405030409060909 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit A few weeks ago Christer Wretlind reported that his VM1000 system would shut down at low engine speeds. Through correspondence with his electronics technician, Sune Nilsson, and investigation using a spare VM1000 DPU I had on hand, we discovered the root of the problem. It seems the VM1000 DPU was designed with a linear regulator to supply the 5 volts that powers the processor AND the sensors. There is a 5 ohm, 5 watt series resistor that limits the current being supplied. Unfortunately, under short circuit conditions, the resistor has nearly all the 14 volts across it (the regulator is trying to supply 5 volts) so is dissipating nearly 35 watts! This causes the resistor to adopt some of the characteristics of a fuse. In Christer's case it is speculated that at some point the 5 volt sensor supply was shorted to ground, causing the resistor to overheat and change resistance to ~30 ohms. The normal system loads were now required enough current to cause a high enough voltage drop across the resistor so there was not enough input voltage to the 5 volt regulator and it browned out. The fix was simply to replace the resistor. It is interesting to note that the DPU I had on hand also showed signs of an overheated current limiting resistor, so of all the DPUs I have seen, 66% have overheated resistors. FYI. Regards Brent Regan --------------000607060405030409060909 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit A few weeks ago Christer Wretlind reported that his VM1000 system would shut down at low engine speeds. Through correspondence with his electronics technician, Sune Nilsson, and investigation using a spare VM1000 DPU I had on hand, we discovered the root of the problem.

It seems the VM1000 DPU was designed with a linear regulator to supply the 5 volts that powers the processor AND the sensors. There is a 5 ohm, 5 watt series resistor that limits the current being supplied. Unfortunately, under short circuit conditions, the resistor has nearly all the 14 volts across it (the regulator is trying to supply 5 volts) so is dissipating nearly 35 watts! This causes the resistor to adopt some of the characteristics of a fuse.

In Christer's case it is speculated that at some point the 5 volt sensor supply was shorted to ground, causing the resistor to overheat and change resistance to ~30 ohms. The normal system loads were now required enough current to cause a high enough voltage drop across the resistor so there was not enough input voltage to the 5 volt regulator and it browned out. The fix was simply to replace the resistor.

It is interesting to note that the DPU I had on hand also showed signs of an overheated current limiting resistor, so of all the DPUs I have seen, 66% have overheated resistors.

FYI.

Regards
Brent Regan
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