Re: LED's

[Wallace White <wallace@remdesign.com>]

Todd -

I have no idea about the possibility of putting a A6 center high-mounted
stop light (CHMSL, I think it's called officially) onto a 4k; sounds
expensive, even if possible. Using a 4kq CHMSL with newer internals
would work, though. If you weren't able to get A6 internals (if they are
indeed LED's), the only thing you need to keep in mind when replacing
incandescents with LED's is that while incandescents are essentially
resistors (albeit with funny characteristics over time and temp), LED's
are, well, diodes. They do not have any considerable resistance, once
the voltage across them is high enough to open the diode, and as such
you need a current-limiting resistor to go along with your LED.

The LED will have a specified voltage drop, like a diode. Say it's 2.0
volts at a current of 30 mA (pretty standard current). (They have higher
drops than plain old diodes and they vary widely depending on the
elements involved, so check your specs. Also note that the drop will be
current dependent, too, to a greater or lesser degree, so get a
datasheet that has the I-V graph if you're really going to be exacting.)
So you want a resistor to drop the voltage by 10 volts (assuming 12 V
across the assembly) at this 30 mA. V=I*R -> R=V/I~=330 ohms. 

In this scheme, you would connect each LED to its own resistor and put
many of those subassemblies in parallel. (Don't put multiple LED's in
parallel with one resistor--their impedances will all be very low but
slightly different, and you'll probably have one LED getting all the
current.) The other alternative is to stack up several LED's (in series)
with a single resistor (in series). Say you use 4 of those 2.0V-drop
LED's in series, giving a total drop of 8.0V, with a single resistor
sized for the remaining 4.0V drop. Then you're wasting a lot less power
in the resistors and you have a much greater total efficiency. The
tradeoff is in stability of performance as your input voltage (assumed
to be 12V) fluctuates. With a single LED per circuit and a resistor
dropping most of the voltage, changes in the input 12V won't affect the
current through the resistor and LED much, while a very efficient setup
of many LED's in series will really vary in brightness when that nominal
12V varies. I would leave a 4V drop in the resistor. Efficiency really
isn't a priority here, anyway.

As for Brett's comment:

> More likely, this is the pulse driver circuit coming up and it just appears 
> that the light "comes up flashing"; LED's are many times more efficient 
> when pulse-driven at higher voltages, because you can take advantage of a 
> lower junction temperature but higher voltage and current.  No burnout, and 
> much brighter light; the pulses are so fast that the human eye can't see 
> anything but continuous light(however, you do get an eerie feeling that 
> they're blinking, say  when you look side to side.  Cool effect.)

I hadn't heard this before. The eye tends to average the light that it
sees, if it's happening fast enough, so for the same amount of power
consumed by the LED, you'll see the same amount of light. And that
doesn't count the power consumed by the fancy drive circuit. I once read
a HP application note, which had an entire heading of "DC Operation Is
Always Better Than Pulsed Operation For Visible Applications." (I found
that an amusing heading, especially since I'd been wondering about that
idea.) Now if it happens that your particular LED is really far more
efficient at higher current/voltage, which I believe is usually not
true, then pulsed operation would let you operate in the highly
efficient range, without overheating the LED. They really do this,
Brett? Very interesting.

- Wallace
  '87 5kcstq 150k, no LED's

Todd Phenneger wrote:
> 
> I have a few thoughts regarding this thread.
>         First, I believe the A6 uses an LED rear Third Brake Light.  Has
> like 8 Leds or so in there.   I know when I follow my Dad in his A6 I can
> visibly notice that the 3rd light comes on much sooner than the others.
> Great safety feature IMO.  I wonder if this could be retrofitted to an
> older Audi.  My 4kq does not have a 3rd brake light but I would like to
> put one on my early 85' for saftey reasons.  I was going to just do a 4kq
> rear light but thought about doing an LED setup.  Do they require any sort
> of Voltage regulator to step down or up the power or is just a (Hook up
> +/- 12v and go) thing.
>         Second, if you are worried about the rears icing up just put a
> defroster element in there.  YES, that would defeat the idea of not using
> much power but you would still have the Bright/fast reacing LED's and when
> defroster was not need like 90% of the time, you wouldn't need to worry
> about the power thing.  L8R
> 
>         Todd Phenneger
>         1984 4000s quattro / modified/ awaiting Turbo Transplant.
>         1985 4000 quattro / Silver / Fixing it Up.
>         1987 4000cs quattro / Saphire Metallic Blue/ Girlfriend's
>         1996 A6q / Volcano / Dads Car
>    *****1985 5kt / PARTING OUT!