More LEDs, less current - LED Driver

Discussion in 'The Projects Forum' started by okiedokie, Jul 5, 2010.

  1. okiedokie

    Thread Starter New Member

    May 20, 2010
    2
    0
    Hello,

    I am a completely newbie at electronics and got a mission to build a couple of led back-lit letters, the problem is that we dont have room for a descent power supply (because of heat in the very cramped space inside the letters) and the amout of LEDs I need it too high for my power supply ( i need 200 20ma leds for a 1amp 6v power source).

    The solution I thought for this was blinking three led chains in diferent intervals like:

    loop{
    chain3.LOW
    chain1.HIGH=20ms
    chain1.LOW
    chain2.HIGH=20ms
    chain2.LOW
    chain3.HIGH=20ms
    }

    Since the humam eye has a frequency of ~24hz, this would work pretty well (i think), what about the power supply demand, would that work without overloading the power supply?


    Sorry for the poor explanation, but like I said, I am a complete idiot in this subject...

    thanks!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hi okiedokie,
    Well, first things first - we need to know the typical Vf @ current rating for your LEDs.
    Vf = the forward voltage of an LED at the rated continuous current.
    If you have a manufacturer and part number for the LEDs, that would be very helpful; a datasheet can then be located for them.

    Is your plug supply (wall wart) voltage regulated, or is it simply rated for 6v@1A? If the latter, you'll find that the output voltage varies quite a bit between no-load and full load conditions.

    Sounds like you are using a uC (microcontroller) in this project, too. What brand and model? You know that most uC's have a working voltage range of 2.5 to 5.5v, right? If you go over that, you risk frying the uC. Your best bet is to supply the uC with a constant 5v, unless it is one of the low-voltage types.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. okiedokie

    Thread Starter New Member

    May 20, 2010
    2
    0
    Bill, I didnt see that, and I will go through that right now... thank you.


    StgWookie, the ic I plan to use is actually the arduino platform, since I already got the board and is, aparently, quite easy to use. The led vf is 3.2v. AS for the power source, I have no idea if it is regulated, it´s one of those generic dremel chargers. Is there any way to find out if it is regulated or not?

    thanks!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Battery chargers generally aren't very suitable for DC power supplies. Many of them will provide intermittent power if subjected to a load near their rating.

    For a cheap supply, you might consider converting an ATX computer power supply to a bench supply. Google "ATX Bench Supply" for lots of ideas.

    Using 12v would mean that you can operate several LEDs in series rather than just one with a resistor. You would save a LOT on wiring.
     
  6. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    Follow the power (it's like "Follow the money").

    1amp, 6v means 6 watts.

    20ma leds means 20ma times about 2V drop per LED, or 40mw per LED, or 1/25 watt, which means 150 LEDs before you're out of power. And that's assuming that 100% of the available power drives the LEDs, which it won't.

    Maybe the LEDs don't have a full 2V drop and you'd use less power. Check the specs. But your conversion from 6v to whatever the LEDs take needs to be very very efficient. Conventionally an LED operates with a resistor in series to maintain the right current, which is simple but inefficient. If you're starved for electrical power, you won't be able to afford that.

    Chopping the output so not all the LEDs are on all the time just means that your output intensity will be reduced. Maybe that's OK, but be aware that it's going to happen.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    JohnP,
    In the OP's last post, they revealed that the LED Vf is 3.2v, so that means 12.8W as a minimum is required to light them all at once with 20mA current. Proper current regulation will add to that requirement, as without it, melt-downs are likely.
     
  8. John P

    AAC Fanatic!

    Oct 14, 2008
    1,632
    224
    All right then, so much the worse. The power needed and the power available just don't match; not even very close. And efficiency is never going to be 100%.

    Okiedokie, you have to decide what's going to give. Supply more power, or live with less brightness.
     
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