Date: Mon, 1 May 2000 17:48:19 -0400
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: Blue highbeam indicator success
In-Reply-To: <39090643.4DF74FE5@ican.net>
Content-Type: text/plain; charset="us-ascii"; format=flowed
At 23:32 4/27/2000, Miguel Calvin wrote:
>I picked up a blue LED from Neutron electronics and some wire and a 40pak of
>resistors from Radio shack today. Using David Beierl's instructions I soon
>had a working blue high beam indicator to replace the green one that came with
>the van. The LED was rated at about 3.5volts so a resistor was necessary. I
>found, after some experimentation, that 22k ohms provided about the right
>brightness. My efforts to create interior lighting have met with less
>success, but I am not finished yet...
>
>Many thanks to David for his straightforward explanation of the electronic
>details.
Great! and you're welcome. In case someone else finds it useful, here it is:
At 23:40 4/19/2000, Miguel Calvin wrote:
>I think I could use some help with the formulas.
>I want to replace my green high-beam indicator LED with a blue one.
That's pretty easy, but you have to experiment a bit. Easiest to get will
be Radio Shack p/n 276-311 which is rated at 5 volts and 30 milliamps. So
let's do the numbers for that:
The formula for the resistor value is Ifwd = (Vss - Vfwd) / Rlim.
Here's what the names mean.
Ifwd = desired current in Amps. This LED is rated at max 30 milliamps, or
0.030 Amps.
Vfwd = voltage drop across LED at working current, in Volts. This one is
unusually high, 5 volts.
Vss = System voltage -- about 14 volts when charging.
Rlim = Limiting resistance in ohms, which is what we have to figure
out. So the formula turns into:
Rlim = (Vss - Vfwd) / Ifwd.
To put the numbers in, Rlim = (16v - 5v) / 0.030 amps = 367 ohms. I used
16 volts as a worst-case if your regulator fails. Now resistors don't come
in 367 ohms unless you want to pay a lot, so you take the next larger
standard value which is 390 ohms.
The formula for the resistor wattage is:
P = E**2 / R, that is Power in watts = voltage squared / Resistance. The
voltage here is the difference between Vss and Vfwd = 16 - 5 = 11 volts.
So P = 11 volts**2 / 390 ohms = 0.310 watts. That is the power wasted as
heat in the resistor, and the resistor needs to be big enough not to
overheat. A conservative rating would be twice the actual dissipation, but
since we'd only reach this value under emergency conditions we can get by
with a half-watt rated resistor. At the actual operating voltage of 14
volts, the power would be 9 volts**2 / 390 ohms = .208 watts and the
current (using the first formula) would be Ifwd = 9 volts / 390 ohms =
0.023 amps or 23 milliamps.
So you need a 390 ohm, half-watt resistor. *But* if you run this LED at
20+ milliamps it will be unbearably bright at night. That's where the
experimentation comes in. By my quick tests you'll be wanting to run this
down around .001 amps -- 1 milliamp at 14v. That would give you a resistor
of around 9,000 ohms, and power dissipated in the resistor would be P = 9
volts **2 / 9000 ohms = 0.01 watts, so a quarter-watt resistor would be plenty.
There's another little wrinkle here -- this LED is meant to project a
narrow bright beam, 24 degrees wide at the half-intensity point, so the
lens is clear instead of diffused blue. You might end up wanting to cover
it with translucent blue dye or paint, which would mean that you'd want the
lamp brighter to get through the covering. Or you might want to use an LED
with a diffused lens -- here's a data sheet for one. They probably won't
sell to you, but can tell you who might. In that case you'll be running
the LED much closer to its actual rating -- you can follow the same methods
as here and I'll check your numbers if you like.
http://www.microelect.com/blueled/mbb51d-l.htm
Anyway, continuing with the assumption that you'll be using the Radio Shack
part -- buy the LED p/n 276-311 US$2.99, a 390 ohm half watt resistor p/n
271-1114, US$0.49 for five, and a 10 K-ohm multiturn trimmer, p/n 371-343,
US$1.49.
Hook them in series (you can tack-solder them together -- you may find very
handy a set of ten little 14" clip leads, p/n 278-1156 US$3.99. If you
solder, use the ends of the LED leads which will be trimmed away
later). The trimmer has three leads -- you only need two of them, probably
the middle and either end. If that's not it, try until you get a pair
where the resistance changes when you turn the little screw in the end five
turns one way and then five turns the other way (it's fifteen turns from
one end to the other, but the screw doesn't stop at that point; you can
keep on turning forever. So to be safe if it doesn't change in one
direction, try the other). If you have an ohmmeter use that, otherwise
just hook up and see what combination changes the light intensity. Because
of the 390 ohm resistor in series, you won't burn up the LED.
Now set the trimmer in the middle and take it outside in the dark. Run the
engine and hook up to a voltage source in the van. If it doesn't light up
at all, connect it the other way around. Move the trimmer until the light
intensity suits you. Try again in daylight to find whether you can see it
then.
Now you can do one of several things -- measure the resistance of the
trimmer plus 390 ohms, and get a quarter watt resistor near that value. Or
leave the trimmer alone, solder it properly to the 390 ohm resistor and use
that. Or remove the resistor and readjust the trimmer to your satisfaction
and use it alone.
To attach your new resistor pack to the panel, trace the conductors from
the LED socket to the existing resistor. Leaving it in place, make a good
mechanical connection from the new one to the old, so they are connected in
parallel. Clean the leads of the existing resistor first, and make your
connections near the resistor body. Quickly solder both connections with
rosin solder and a small iron -- you don't want to melt the plastic of the
panel circuit. If you aren't good at electronic soldering, either learn
how or get someone else to do it, 'cause it will be expensive if you ruin
the panel. Now clip out the old resistor from between the new connections,
or crush it so the ends are separated. Clip the leads on the LED to the
proper length and stick it in the socket, orienting the little flat on one
side the same as the other LEDs on the panel. Test it and reverse the LED
if necessary -- the flat is *almost* always on the negative side.
Whee! Easier to do it than to tell you.
>I also would like a third stop light and I ame considering LED's for engine
>bay lighting.
Ok, now you know how to figure the resistors for the stoplight. For that
one you want super-bright clear reds. I think Radio Shack p/n 276-086
US$2.49 is bright enough. It's a big one too, 10mm across. BTW, in the US
you get a noticeable price break if you buy ten at once.
Once you have that done you can think about engine bay lighting. That one
won't be so easy, because LEDs don't actually produce a lot of light. 20
milliamps at 1.8 volts (typical for red) is only 35 milliwatts, after
all. If it's focused in a narrow beam it can be pretty bright, but it
doesn't cover much area. But by that time you'll be used to fiddling with
LEDs, and you may very well come up with something that suits you.
cheers
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
