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

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?

JazzMan

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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:

The datasheet shows a few more typical hookups, at: 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.

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--Larry Brasfield
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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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"Rats and roaches live by competition under the laws of
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A closer study of the MAX629 datasheet shows that it is not rated for the input voltage you need for an automotive application.

If you are eager to build a power converter to run your LED's, the MAX668 or MAX669, depicted at:

and whose datasheet is at: can be made to do it.

Its current limit is not well-defined in the datasheet, so, if you are interested in this alternative, say so and I (or somebody faster on the draw) will outline or explain how to set the current sense resistor to get approximately the current you want. This current should be predictable and stable enough for your purposes, although it is not strictly regulated.

"Larry Brasfield" wrote in message news:m40Zd.26$ snipped-for-privacy@news.uswest.net...

Replace these two links with above recommendation:

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Above views may belong only to me.

Well, if you are putting these in a vehicle, be warned that you are plugging into the power supply from hell (to quote an app note from Linear Tech).

As a designer of equipment that ends up in vehicles, I can assure you that the 'nominal voltage' you get is there *most* of the time, but can be as high as 60V or so under load dump conditions (when loads get suddenly disconnected the reverse emf on the alternator windings generate a significant voltage), to say nothing of having large spikes at most times anyway.

Because of that, I'd put a current limiting resistor in the sequence, otherwise as noted by others, you will regularly (and spectacularly if there's a really good load dump - as happens quite regularly at cold crank, engine pre-stall, accelerator kick). Most cars will dump the A-C if it's on for a maximum acceleration request, known to most of us as lead-footedness :)

We'd usually generate our own power from the vehicle with a nice protected supply and use that, although you can use a string of LEDs with current limit.

Cheers

PeteS

Ok, I bought 100 LEDs yesterday to do this project. Specs on the LEDs are as follows:

Vf @ 20mA 2.0V, max 2.4V output 11,150mcd

How do I get one of these chips?

JazzMan

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**********************************************************
Please reply to jsavage"at"airmail.net. 
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**********************************************************
"Rats and roaches live by competition under the laws of
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live under the laws of justice and mercy." - Wendell Berry
**********************************************************

If you want to drive them with 2.0V @ 20mA, then you can have 5 in a string, giving 10V total. That will require 20mA for each string, which means 400mA total current. Each string will require current limiting. You can do that with a resistor, but as others have pointed out, spikes will damage the LEDs over time.

Another way to do it would be to use transistors to limit the current. A trivial 'current mirror' scheme might work here.

A TL431A, with the reference connected to the cathode, serves as a nice

2.5V voltage reference. Using that, you can build a little current source by setting the base of an NPN transistor to 2.5V, and using a resistor to limit current. This circuit is independent of the voltage input, and will draw a dependable current.

By using that current reference, you can then use a set of PNP transistors to limit current across those strings. The base of all of these PNP transistors are tied together, and referenced using the current source above.

12V -o---------o-------------o---------o--------------------. | | | | | | .-. .-. .-. .-. | | |10 | |10 | |10 | |10 | | | | | | | | | | '-' '-' '-' '-' | | | | | | | | | | | | | | .-. >| |< |< |< | | 10k |----o----| .--| .. .---| | | /| | |\ | |\ | |\ '-' | | | | | | | | | o------)----o----)--------------' | | | | | | | | | |< | | | | o----| Q1 | | | | | |\ LEDS LEDS LEDS | | | | |/ | o-------| | TL431A | |> | .---o | | | | K | | | R - .-. | '-- ^ | | 82 | | A | | | | '-' | | | | ----o---------o------o-------------------------------------------o GND (created by AACircuit v1.28.5 beta 02/06/05

If there are spikes of 60V, the transistors and 10 ohm resistors will absorb them. I simulated a spike of 60V, and the LEDs went from 20 to

22mA. That is well within their tolerances.

You can adjust the brightness by adjusting that 82 ohm resistor, which sets the current limit for all of the strings. The transistors will be slightly different, in which case the current may be a bit different in the strings. However, the 10 ohm resistors will tend to even things out a bit. You could even go up to 22 ohm resistors without affecting things too badly, which would give you a larger margin for error, and also a bit more protection against spikes.

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Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.