Date:         Fri, 6 Aug 2004 05:45:02 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: LED replacement for westy
Comments: To: MC <gipsyflies@comcast.net>
In-Reply-To:  <004601c47b4d$0bcc4090$c200a8c0@vsovaio>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 20:33 8/5/2004, MC wrote:
>Although the driver for the flame sense LED is set up as an amplifier,
>in operation it either is nearly completely off or full on driving to
>the 5V rail. The thermal couple sees either ambient or a few hundred
>degrees.

On my panel I added a 5M pot to the feedback resistor so I could crank up
the gain, because the green flame LED was just barely lighting.  It worked,
and I haven't really looked at the circuit since (that was ten-twelve years
ago).  It is odd though -- seems to me a few years ago I measured the
thermocouple output for a different reason and it was considerable.  I may
have to revisit that circuit.

About the five volt rail though -- there isn't one.  LM324 swings its
output to (Vcc - 1.5v) and Vcc is the regular 12-volt nominal system voltage.

>The 5V blue LEDs from Radio Shake have built-in resistance. Why? Typical
>blue LED has forward diode drop of between 3.4V - 3.5V. Without current
>limit resistor it will burn out at 5V almost immediately.

I hear you.  Although it is a 430nm device and they take higher
voltages.  The highest I could find was from NTE, 4.5typ/5.5max.  Trouble
is I'm looking at the thing and there's no resistor (see enclosed
picture).  Of course I also couldn't drive it anywhere near five volts, and
I ruined it trying.  Perhaps RS slipped on the ratings?  Or changed the
part?  I have another one still sealed in the package, and it has no
resistor either...

david


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