Re: Just checking if my thinking is right on an automotive LED lighting design

From: JazzMan (No_Spam_at_airmail.net)
Date: 03/13/05 

Date: Sun, 13 Mar 2005 12:03:13 -0600

Larry Brasfield wrote:
>
> "JazzMan" <No_Spam@airmail.net> wrote in message
> news:423465BE.486D@airmail.net...
> >I want to make a set of amber DRLs for my car using LEDs in
> > arrays. The LED is an AND185HAP, specs are If 50 mA MAX,
> > iF 20mA typ @ 2.0-2.4V. Measured output on my alternator
> > is 14.5V typ, so I want to run four parallel strings of
> > six LEDs each, no resistors.
> >
> > Does this sound workable?
>
> I believe you would be disappointed with the
> result and could lose (destroy) the LED's often
> enough to wish you had done it differently.
>
> The problem is that the LED current will vary
> quite a bit with battery voltage, which ranges
> from 11-12 Volts during engine off conditions
> to 13-16 Volts when the engine is on and the
> battery is being charged after a start. By the
> time you keep the upper end of that range
> from hurting your LED's (by using a limiting
> resistor), they will be too dim near the lower
> end of that range. Without the limiting resistor,
> over-voltage transients (exceeding the range
> mentioned) may well destroy or degrade your
> LED strings.
>
> The LED has the same exponential current
> versus voltage function as other semiconductor
> junctions, with a relatively small series resistance.
> If you can get a plot of the E/I characteristic, you
> will see the problem with voltage variation.
>
> If efficiency is your concern, rather than simplicity,
> a switching converter with controlled current
> output might be attractive. If you are not so
> concerned with efficiency, 1 or 2 fewer LED's
> per string and limiting resistor is a good route.
>

Efficiency is irrelevant to me, what I'm after is maximum
brighness. The cost of the LEDs is trivial. The existing front
marker light is a dual filament bulb that uses the dim filament
for a parking light and the bright filament for intermittent use
when signaling. A friend of mine bought a module that
inverted that function and ran both filaments all the time
with lights off, cycled the bright filament when signalling,
and reverted to dim-on and bright-signal when the headlights
are on. The problem he had was that his light housing melted
because of the extra heat from running the bright filament
full time as a DRL. My thought was to replace the bulb with
as many LEDs as I can physically fit into the housing, which
looks like it will take a 4" x 1.5" circuit board. Now that
I think of it, I can probably run twenty strings of 6 LEDs
if I pack them in edge to edge like the new traffic signal
LED bulbs are built. 120 high-output LEDs should be bright
enough. :)

My experience with electronics beyond basic DC and some TTL
is very, very limited from a design standpoint. I'm really
good at laying out boards and fabricating complex circuits,
if I have a schematic and parts list to work from, but I'm
fairly deficient when starting from scratch.

Anyone feel like referring me to a relevant schematic? :)

JazzMan 

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