Which Power supply for led (PWM)

Discussion in 'General Electronics Chat' started by sebdehne, Feb 5, 2011.

  1. sebdehne

    Thread Starter New Member

    Feb 5, 2011

    I'm planning to controll (PWM for dimming & ON/OFF) a set of LEDs using a MOSFET with a micro-controller connected to the gate. The LEDs need a current of 0.7A and have a V-drop of 3.15V. I'll be using 20 of them.

    My question is: which power supply do I need. Do I need to worry about PWM having an affect on the stability of the PSU output? Do I need a good quality PSU would a cheap AC/DC converter do the job?

    I'd like to discuss the above question for both scenarions:

    A) Using constant-voltge PSU with current limiting resistors in a certain array configuration
    B) Or driving the LEDs directly using a constant-current power supply.

    Thanks for any advice.
  2. SgtWookie


    Jul 17, 2007
    Well, you might use a surplus 3A 19V laptop switching supply for starters, they're available pretty cheaply nowadays. You could operate 5 LEDs per series string, 4 strings total. However, 700mA per string is a bit much to be using resistors to limit current; you'd be better off using something like a BuckPuck (one per string) that has options to control the output current.

    If you don't care about wasted power, you might use current limiting resistors - but that's going to dissipate a fair amount of power as heat.
  3. sebdehne

    Thread Starter New Member

    Feb 5, 2011
    Thanks for the reply!

    The question whether I need to worry about PWM having a negative affect on the stability of the PSU output remains though. Would a cheap laptop PSU keep the output stable when cycling frequently between 60W load and 0W load?

    The reason why I prefer a constant-voltage PSU over the constant-current (BuckPuck) PSU is that I would only need one PSU for all LEDs. As I understand (but I'm not 100% sure), when using affordable constant-current PSU (like BuckPuck), I'd have to use multiple ones to drive all the 20 LED's, which will be more expensive. Or?
  4. TierOne

    New Member

    Feb 5, 2011
    In my opinion, micro-controller and LEDs only require a 5V supply.
  5. tom66

    Senior Member

    May 9, 2009
    You might be better off using a constant current supply which has a PWM input to modulate the current.