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Thread: Constant Current Supply

  1. #1

    Default Constant Current Supply

    While my trike is down for painting and upgrades, I've been thinking about doing some more work on my lighting system. Posted here, #127

    Currently (pun intended), I am using LM317s in constant current configuration to drive the LEDs. I know this is not the most efficient way of doing it. In looking around here and on MTBR forums, I've seen reference to the AMC7135 boards. Being these are comparable in cost to the 317s with resistors, I am considering upgrading? to them.

    All the LEDs I am using are 1W buttons on either stars or triple MCPCBs, driven at 320 ~ 340mA. The individual buttons are all rated to be 80 -100 lumen, and I think I got lucky with them. All seem to be very close in light output. They are from a cheap china seller (eBay, of course) who claims to sell chips from Cree, Paury, and Epistar. I'm not sure which these that I have are.

    How much more efficient would this changeover be?

    Since I am using a 12V8Ah SLA as my supply --I can't afford Li chemistry, and NiMH would be a constant battle to keep charged --, I am wanting to explore options to extend my run time. Right now I get about 5 hours before B+ drops below 10V from full charge. Additional note -- Weight is not much of a issue, since the trike weighs in at ~90lbs empty.

    Would this changeover actually do me any good?

    Yes, I will probably be adding a few more lights to the trike, for side visibility. Will be amber strobes on the front, red strobes on the rear, facing out from the sides, along with the same color steady on each side.

    When I finally get the trike put back together, I'll post some beam shots of the headlights, as well as other pics to show overall lighting.

    Another question regarding the headlight set:

    Would adding a short hood over the front of the headlight set give me a sharp cut-off line? Or would I need to make some changes to the optics? Right now I get an incredible amount of spill upwards, and I want to kill most of that. Down and side spill I want to keep for the most part, but will need to wrap the light bar anyway, for a more finished look.

    Thanks in advance for any help and/or suggestions you might have.


  2. #2
    Flashaholic* georges80's Avatar
    Join Date
    Oct 2002
    Sunnyvale, CA

    Default Re: Constant Current Supply

    AMC7135 based boards will provide no efficiency benefit over what you have. Basically they and your lm317 design are linear regulator based designs that take any excess voltage and turn it into heat...

    To improve efficiency of your light you need to use a buck or boost driver that utilises an inductor and switcher IC controller to drive the LED with a constant current while converting DC : DC the input voltage to the appropriate level to maintain the target current to the LED load.

    Some folk seem to think the AMC7135 is much more than it actually is. You can think of an AMC7135 as an optimised version of your LM317 design that integrates the parts necessary to provide a constant 350mA output into a single chip. Obviously it can only dissipate so much heat before it will thermally shutdown or limit output current to protect itself.

    LED drivers
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  3. #3

    Default Re: Constant Current Supply

    Thanks for that, George.

    Guess I didn't do enough research about that one. From the descriptions for the premade boards, it looked to be more than what it is.

    Looks like I get the opportunity to increase my meager knowledge in power supplies if I want a more efficient electrical system.

    Does anyone have any thoughts about the other question in my post, about the hood for the lights?

    Last edited by charlie_r; 06-10-2012 at 03:23 AM.

  4. #4
    Join Date
    May 2009
    Queensland, Australia

    Default Re: Constant Current Supply

    To enable a nice cut off like a car headlight is quite hard. I have been doing some experiments with both 30mm diameter and 80mm diameter aspheric lenses and can achieve a sharp cut off by putting an opaque screen mounted midway between the led and the aspheric and mounted at a height so you still get good throw but a smooth cut off. This method however limits you to 1 led per aspheric and you also loose some of the available light output but an XML at 3A is still bloody bright so not a big deal.

    Another good optic I have used is the oval beam type. they are very effective if you want a beam that does not spill too much light into the air

  5. #5

    Default Re: Constant Current Supply

    Thanks for that. I suspected that it would have to be done between the lens and led, but was hoping someone had an idea on other ways of doing it.

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