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# Power Supply for LED Aquarium Fixture

## Power Supply for LED Aquarium Fixture

#1
07-23-14, 10:08 AM
Member
Join Date: Jul 2014
Posts: 2
Power Supply for LED Aquarium Fixture

I have a 12v 204w 17A power supply laying around and I would like to use it to power a marine LED hood. The only issue I see is the amperage. The supply has five +12v lines that I can use to power my various arrays. Is there anyway to configure the 12v supply lines for the following LED arrays?

Source 1: 3 10w 20000k(ish) high power LEDs - these can take 12v directly and operate at 4.5A

Source 2: 10 3w Blue LEDs these operate at 3.6V and 600mA.

Source 3: 5 1W UV LEDs these operate at 3.6v and 350mA

Source 4: 10 1W Red and Green LEDs these operate at 3.6v and 350mA.

Source 5: 4 12v Fans to ran through a 12v digital fan controller.

I know it would be easier to buy 5 LED drivers that are pre configured but I would like to know if anyone knows a way to make this work! Thank you all for any help!

#2
07-23-14, 11:30 AM
Member
Join Date: Jul 2013
Location: USA
Posts: 57
If these LEDs are individual LEDs then you need to choose a drive current and limit them accordingly by wiring them in series and using resistors, etc. If they have supporting circuits or internal power supplies then you do not need to worry about the drive current as much.

Basically the formula for each 12V line is:
Vf (Forward Voltage -- the voltage required to turn on the LED, this should be on the datasheet)
Ic (Operating Current -- this is the design current at the Vforward voltage, usually there is a max current too that you want to stay well under to maximize LED life)
R (resistor value to choose to set drive current)

R = (12v - (Vf * #LEDs)) / Ic

where R must be positive (so reduce the #LEDs till it is)
Also R must be sized to dissipate the wattage it draws:

RWatts = (12v - (Vf * #LEDs)) * Ic

Vf = 3.6v
Ic = 0.6A
#LEDs = 3

R = (12v - (3.6v * 3)) / 0.6 = 2 Ohms
RWatts = 0.72 Watts

So you'd need a 2 Ohm resistor rated at 1.5Watts (2x design factor)... and maybe a heatsink for the resistor

Other options are to use settable regulators and drive them at exactly the Vf * #LEDs voltage, or find individual LED drivers online for each LED current draw

#3
07-23-14, 11:49 PM
Member
Join Date: Jul 2014
Posts: 2
Thanks mnmattmn! I appreciate you breaking it down for me. I thought about resistors but I read all of these "DON'T USE RESISTORS FOR HIGH POWERED LED" warnings so I was hesitant to try it. I also thought about installing a overload protection circuit on each of my 5 DC lines and then installing a DC-DC CC regulator behind it to drive each line. That method would cost about \$30.00 vice your method that would cost about \$5.00. Thanks again!

#4
07-24-14, 06:45 AM
Member
Join Date: Jul 2013
Location: USA
Posts: 57
If you're looking for high reliability and efficiency then I love a current controlled power supply because the LED Vforward can actually change slightly from LED to LED and over temperature and the current source will adjust for those changes.

The best way to get life out of an LED is to keep it cool, so try to have good air circulation or heat-sinking, and make sure to measure the voltage drop across the resistors to verify you are getting what you predicted.