Hi,

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.

 

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.

 

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?

 

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

 

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