Why an LED driver?

Discussion in 'The Projects Forum' started by donjohnston, Nov 29, 2009.

  1. donjohnston

    Thread Starter New Member

    Nov 29, 2009
    5
    0
    I've searched through the posts but can't find an answer (at least a simple one ;)).

    I am replacing some incandescent courtesy lights used in a 24v system. In the housing I have mounted 4 white LEDs (in series) that are rated at 3.7v @ 30ma. Using the LED calculator at http://ledz.com/?p=zz.led.resistor.calculator I added a 600ohm, 1/2w resistor.

    I've tested it and it seems to work fine. Then I thought about adding a variable resistor so I could also dim the lights.

    That's when I ran across "LED dimmers" and "LED drivers".:confused:

    So here are my questions:

    1. Is there a problem powering these LED's with just a simple resistor?
    2. Is dimming them with a variable resistor acceptable?
    3. What does the "LED driver" give me that a fixed resistor doesn't
    4. What does the "LED dimmer" give me that a variable resistor doesn't
    5. If I go with an LED driver, will I need one for each fixture or can I drive multiple fixtures with a single driver.
    Thanks,
    Don
     
  2. lihle

    Active Member

    Apr 12, 2009
    83
    3
    The problem with LEDs is that you have to avoid driving them at maximim current rating. what i would recommend is that use each LED with its own resistor value.

    for the dimming part i think you can use a microcontroller, where by each uotput pin is connected to each led. to vary light intensity you can use adjustible port if possible. but using the potentiometer i dont think it would work.
     
  3. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Mounting LEDs in series is completely acceptable, it is wasteful in power not to do so.

    You don't need a microcontroller to dim LEDs, that is major overkill for a simple function.

    The reason a driver is preferred is power. Resistors work perfectly well, as long as you have designed it correctly.

    LEDs are pretty nonlinear as to brightness vs current, most people want it to change smoothly from full brightness to completely dark. There is a technique called PWM that does this, traditional dimmers don't use it. I expect as LEDs become more common you will see a new class of dimmers that do use it.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If it is a battery-powered system, the nominal voltage will be 25.4v when the batteries are charged, and 23.5v when the batteries are discharged.

    When the batteries are being charged (if this is in a vehicle), the nominal system voltage will be approximately 27.6v to 28v. You must calculate your resistor for the maximum system voltage. I'll use 28v for the calculation.

    Rlimit >= (Vsupply - VfLED) / DesiredCurrent
    Rlimit >= (28v - (4 x 3.7v)) / 30mA
    Rlimit >= (28-14.8) / 30mA
    Rlimit >= 13.2/0.03A
    Rlimit >= 440 Ohms
    A table of standard resistance values is here:
    http://www.logwell.com/tech/components/resistor_values.html
    Bookmark that page.
    Standard values up to the E24 range (green columns) are generally easy to obtain. E48 and up values you will generally have to order from somewhere.
    470 Ohms is the closest standard E24 value that is equal to or greater than 440 Ohms.
    13.2v/470 Ohms = 28.1mA (rounded off)
    Power requirement is 13.2v x 28mA x 2 = 739mW; use a 1 Watt or higher rated resistor.

    You used the top calculator for just a single LED.
    That's OK, your LEDs will have a very long life. However, they will not be as bright as they could be.

    No. If you have a stable power source, a resistor is cheap and effective. It isn't as efficient as some solutions, but it's certainly cheap.
    You would need a variable resistor (potentiometer or rheostat) with a 1W or higher rating, but you could do that. The pot/rheostat will be somewhat expensive. Usually, PWM or other methods are used, but with your system being 24v/28v, the complexity is increased somewhat
    Efficiency, and very closely regulated current.
    Efficiency. Rather than dissipating power as heat via a resistor, the current is PWM'ed at various duty cycles.
    Well, an LED driver is generally for just a single string of LEDs. Its' purpose is to limit the LED current.

    Some LED drivers have an enable/disable input. This could be used as an input for PWM. Some LED drivers have a current control input, which could be adjusted by a remote potentiometer.

    But if you wanted to keep it really simple, you could just use one 470 Ohm resistor per string of 4 LEDs.

    And then use a PWM controller as a dimmer. The PWM controller could be designed to turn the grounds for the lights on and off rapidly, or to turn the power on and off rapidly. If the PWM frequency is around 200Hz or higher, you won't be able to notice any flickering.
     
  5. donjohnston

    Thread Starter New Member

    Nov 29, 2009
    5
    0
    Sarge,

    Thanks! Exactly what I was looking for. I used the series calculator but went over to make sure I didn't over power them. I also used 28v as the source power.

    Thanks again,
     
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