Date:         Tue, 7 Jan 2003 14:51:27 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: [Syncro] Fridge burner indicator light bright, then dim?
Comments: To: "Loren A. Busch" <lbusch@IX.NETCOM.COM>
In-Reply-To:  <000f01c2b67b$c77d8580$e3c84b43@hewlett2ih5nie>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 01:36 PM 1/7/2003, Loren A. Busch wrote:
>on the behavior of the LED I have always assumed that it was driven by a
>thermocouple that was heated by the pilot light/flame:  If the flame
>goes out so does the LED.

It's driven by one of the eight op-amps (4 each in two chips) in the LED
panel.  The op-amp takes one of its inputs from the thermocouple, and the
other from the LED panel ground lead.  In the standard panel the op-amp is
set to a voltage gain of 370 by a pair of resistors (see
<http://pws.prserv.net/synergy/Vanagon/Pilotmod_schematic.gif>  for the
circuit).  This means that it will output two volts if the thermocouple and
ground are different by about 5.5 millivolts, and four volts for 11
millivolts etc.

If you want to eliminate artifacts from changing ground levels, lift pin 3
of the right-hand chip and connect it directly to the gas valve on the
fridge, where the thermocouple is screwed in.  NOTE: I haven't considered
the possibilities, it could be that under some conditions it could blow the
chip if that lead weren't disconnected before the fridge, or the LED panel
ground was bad or some such.  But the chips are cheap, about a buck at
Radio Shack, and you can look up the data sheet on the web for the input
limits...the chip is an LM324N, where the N simply says that it's packaged
in molded epoxy.

david

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"