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.

LED with built-in resistance in the Westy Vanagon days were very
expensive - around a few bucks apiece.

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.

http://www.eetimes.com/pw/potw/showArticle.jhtml?articleId=18309928&kc=2
609

- Vince

-----Original Message-----
From: David Beierl [mailto:dbeierl@attglobal.net]
Sent: Thursday, August 05, 2004 9:29 AM
To: MC
Cc: vanagon@GERRY.VANAGON.COM
Subject: Re: LED replacement for westy

At 17:10 8/4/2004, MC wrote:
>The driver of the monitor board delivers 5V.

That depends...the stock panel is wired with a 2K7 input and 1M feedback

resistor giving a gain of minus 370, so if your thermocouple is
delivering
-13.5 mv relative to the panel ground you'll get +5v output.  The chip
will
swing its output to whatever the supply is less a volt and a half, given

sufficient input.

>  However the factory LEDs
>are without built-in current limiting resistors.

True -- LEDs with internal resistors (or even better, internal voltage
regulators) do exist but they're not common.

>  On the PCB there are a
>current limiting resistor for each LED.

On my schematic <http://pws.prserv.net/synergy/Vanagon/LEDpanel.htm>
I've
got it as ~500 ohms which if correct means it's 470R.

>  Both blue LEDs I found at Radio
>Shack are 5V (with built-in current limit resistor) this make the
>current limiting resistor on the PCB unnecessary.

Erm...blue LEDs require a higher voltage (ultimately because there's
more
energy in the photons of blue light, I think) than other colors (Reds
typically run around 1.8v, yellow and green around 2.2-2.3v, blue >3v
for
rated output.  The Radio Shack 276-311 I was just looking at is rated at

5.0v typical, 6.0v max, and 30 mA.  Under test it shows a barely visible

glow, rather greenish, at about 2.8v and 50 uA.  It's distinctly blue at

3.0v/100uA.  At 3.34v/1mA it's easily visible with a 75-watt bulb
shining
on it from a foot away.  A bit hard to look at at 3.57v/5mA, and painful
at
4.07v/30mA.  I ran out of courage at 4.55v/60mA, twice the current
rating
and still half a volt shy of typical voltage).  Not at all likely that
there's a built-in resistor (overall behavior is 400R between 50-100uA,
down to 16R between 30 and 60mA).   [Note: after running a while it's
now
needing 4.33v to get 30 mA and it's no longer painful to look at; 60 mA
is
about 4.66v.  Either it didn't like the overcurrent the first time or my

eyes have adapted, or there's some long-term effect I don't know
about.  It's probably over a year since it had power on it.]

david

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