Date: Thu, 5 Aug 2004 12:29:01 -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
In-Reply-To: <002f01c47a67$8b7901c0$c200a8c0@vsovaio>
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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"
