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

From: Larry Brasfield (donotspam_larry_brasfield_at_hotmail.com)
Date: 03/13/05 

Date: Sun, 13 Mar 2005 10:59:37 -0800

"JazzMan" <No_Spam@airmail.net> wrote
 in message news:42348061.11AD@airmail.net...
> 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? :)

Given your requirement, I would just cut the string
length to 4 LED's and use a 1/2W 180 to 240 Ohm
limiting resistor for each string. This will typically drive
your LED's at 24 to 33 mA depending on the resistor.

If you want better efficiency and are willing to build a
power converter, take a look at this SMPS IC to get an
idea of what would be necessary:
  http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1705
The data*** shows a few more typical hookups, at:
  http://pdfserv.maxim-ic.com/en/ds/MAX629.pdf
The part has a 250 mA current strapping option, and
if you were to divide your LED collection into N strings,
where N = 0.25 / (IperLED), and use a small resistor
in each string to equalize the current sharing among the
strings, your circuit would not be much more than what
can be seen at the above links. (You choose IperLED
according to how hard you want to drive the parts.)

If you decide to go with a convertor, come back for
a few tips on layout, or a review of your planned
circuit prior to committing layout effort or buying parts. 

-- 
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.