Mild electronics question

Discussion in 'The Projects Forum' started by CDC, Sep 30, 2010.

  1. CDC

    Thread Starter New Member

    Sep 30, 2010
    3
    0
    Hi there my name is Michael.

    I have a Mains Voltage to 24VDC 2.5Amp transformer. I currently have hooked up to a long skinny LED light~30LED's for the purposes of lighting a fish tank.
    Can i dimm LED's like i would a lightbulb? i understand basic electronic principles(ie built a electric motor and transformer, used picaxe and made and programmed a basic chip) but its been a long while since ive done any of it.

    The lights too bright and i basicly want to know if i can put a dimmer on it?

    thank you for your time.
     
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Yes you can use a dimmer, using some kind of PWM modulation will get you the best results.
     
  3. CDC

    Thread Starter New Member

    Sep 30, 2010
    3
    0
    pulse width modulation talks alot about wave forms, my LED's run of 24V DC. I have no idea how u are going to use pwm to dim my lights, do you?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    See the attached.

    If you're in the States, you can get the components at your local Radio Shack.

    The LM317 is a voltage regulator. R1 and R2 set the output of the regulator to ~12v.

    The LM555 is a timer; the way I've wired it up allows a reasonably constant frequency output where the duty cycle can be varied from perhaps 5% to around 98% duty cycle. With VR1=10k, the frequency is roughly 1kHz. You can use a 20k or 50k pot instead; the output frequency will be lower (~220Hz for 50k), but you will have a slightly wider PWM range.

    LED1 and RLED represent your string(s) of LEDs.

    I do hope you have used a resistor in series with your LEDs to limit their maximum current.

    Calculating the maximum number of LEDs that you can wire in series:
    LED_count = INT(Vsupply / Vf_LED)
    where:
    Vsupply = 24v - (24v/10) = 24v-2.4v = 21.6v
    Vf_LED = The typical Vf of the LED at a specified current.
    INT = The rounded-down integer value of the result of the equation within the parenthesis.

    Calculating RLED:

    R >= (Vsupply - (Vf_LED * LED_count)) / DesiredCurrent
    where:
    Vsupply = 24v - (24v/10) = 24v-2.4v = 21.6v
    Vf_LED = the typical Vf specification at the current rating of the LED.
    LED_count = the number of LEDs that you have in series.

    If R is a negative number, you must reduce the number of LEDs that you have wired in series.
     
  5. CDC

    Thread Starter New Member

    Sep 30, 2010
    3
    0
    thank both of you for your interest in helping me.

    sgt.wookie i cant make sence of your equations although i think i under stand the idea behind them. 1 thing i didnt mention is that thei got the LED light with its power supply ex shop front display.


    maybe i should scrap the idea of using and electronic solution to my problem and just get some frosted glass or plastic to case the light it. A diffuser or something
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Oh, ok - it's a pre-made string. Don't worry about my calculations then. Just use a 50k pot instead of a 10k pot.
     
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