LED power supply help

Steviecuk · Dec 11, 2014

Hi. I would like some advice on an LED fish tank light that I am trying to make. I am not sure if to use the CREE style 3w SMD's mounted on a 20mm star PCB or the larger square 10w LED's, I will decide later. But either way I need help working out the PSU if anyone would kindly steer me right.

The LED details are as follows.

10w, 0-12v, 1400mA.

Or

2.3w (CREE), forward voltage 3.3v, 700mA.

What kind of power supply would I need? Is it as simple as just one that is rated at 5x the voltage, current and wattage above? Or something else?

i have seen this driver with the following spec:
Model:LA50P-700
Input: 100-240Vac
Output: 36-72Vdc 700mA
Rated Loading: 11---20LED
Efficiency (at full load): >87%
Dimensions (LxWxH): 148x40x30mm

http://m.ebay.co.uk/itm/131346072951?cmd=VIDESC

Would I be right in thinking that this driver could be plugged into the wall at 240v (uk mains) and then run out to the LEDs in series. For example say 20x 3.3v, 700a, 2.3w LEDs as this should total 66volts.

In essence, am I also right in thinking that the current of the LEDs must match the driver, if it's less then a resistor must go in (for if I say have 6 blue at 700mA and 6 white at 900mA) and the total voltage of all LEDs added together may be within the drivers operating range?

if that bits right, I'm also thinking, if I have 900 and 700mA LEDs, get a 900 driver, run to the 900 leds first, then add a resister before going on to the 700 ones? Resister being V/I=R (2.3v/0.2A=3 ohm resister to bring it down 200mA to 700? Or is it the calculation of total voltage pulled/current? 66v/0.2A=330 ohm. Or then again, is it 66v/0.7A=94 ohm?

Oh it all gets very confusing.

I look forward to your responses.

Thanks, Steve.

1Justin · Dec 20, 2014

Two possibilities.

He was using LED's which were packaged with their own buck mode SMPS driver chip, hence could individually be connected directly to 12V, in the same way as all MR16 spot light LED's are. Had he done this he'd have needed to connect the LED's in parallel to the 12V (not, as you say in a series "string").

Or if using "naked" LED's it could be that with four in series, he happened on the "sweet spot" whereby the back EMF of the four LED's in series just about matched the 12V output, and the natural current limiting of the PSU prevented runaway current from the LED's and kept them running at the right level. Messy and not a nice engineering solution, but it could work out. Experimentation needed, possible risk of damage to power supply, or LED's.

Steviecuk · Dec 21, 2014

Hi Justin. Just when I think I'm getting the hang of it, something frazzles my logic. Thanks for bearing with me. I've just emailed him and asked how he did his. All I know is that he used a PSU from a computer, and about 26 of the square 10w naked less. (like these AQUARIUM 10w LED ROYAL BLUE 450nm - 455nm, LIGHT UNIT, DIY, MARINE, MULTI CHIP | eBay)

I also went into a shop and looked at my mates proper bought light and looked at the PSU for them. These units consist of 2x 35w square LEDs in white, and about 30 of the 3w CREE SMD's on star PSB's (various colours) I looked at the supply thinking right, it'll be something like 700mA - 1A and something like 110v (30x3v (small) + 2x10v (square) yet I was surprised as the label read OUTPUT 24V, and I think 2A. Which kind of confused me as I thought the voltage got added together and the current remained the same throughout. :-/