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# What power supply and wiring setup should I use for an LED lighting fixture? Answered

I'm trying to build a light from LEDs. I managed to pick up 40 Bright white LEDs for \$4 and want to put them all together into one light fixture. I know a bit about electronics, but I'm stumped on what kind of power supply I should build and how I should wire them. They are 3V 20mA. When building it should I multiply the voltage or the amperage? And should I run them in series or parallel? Any help would be appreciated. And please don't just give a "Do this" answer. I'm looking for more of "Do this because this is how it works." Thank you.

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## Discussions

I always Use some 12V light strips, but if you want to use individual Led's...

12V solution, You can wire them in groups of 4 leds and a 1ohm resistor in Series, then 10 groups in parallel. Be sure to use a 250Ma or higher power supply.

Or 36V Solution, Wire 10 Leds in Series with a 330-Omh resistor, Then 4 Groups in parallel. You can find a cheap 36V power supply from old printers, that one will only require a 150ma or higher supply.

And to answer you math question You add the forward Voltage (Led's required Voltage) together, Current stays constant.

1 Led = 3V,20ma

2Led = 6V,20ma

3Led = 9V, 20ma ....Ect

You then use a resistor to dissipate the remaining voltage. 12V - 9V(3-Led's) = 3V left over. You would need a 150-Ohm resistor to dissipate the remaining voltage.

I use the same calculator the Verence liked to, Best one on the web !!

Wired Mist, I'm using the individual LEDs because I got them for next to nothing. I'm essentially wanting to make my own strips(in a circular design though) to run on either a 12V battery system, or a transformer I have somewhere in my scrap pile(still gotta locate it lol) So what I gather from your response is wiring them in series ups the required voltage, and parallel ups the required amperage. Right?

Yes each series of 4 leds adds up to you Source Voltage (Good design practice says add a 1-Ohm resistor also)

Each string of four leds will only consume 20ma, Even though it's now 12V.

So 40leds / 4 led's per string = 10 strings. 10 strings * 20 ma each = 200ma curret draw. Again good design practice says Use a supply that provides the exact voltage, But is able to deliver Extra current (Even if it won't be used) . for a 200ma draw i'd use a 250ma or higher.

you could, with all due caution run these directly on mains electricity.

Although that sounds *Really* Sketchy, It could work...

You'd still need a Bridge Rectifier to covert AC to DC.

If you don't it will still light up, but will seem to flicker as be half as bright.

And please use a fuse too !

If you use a reactive component to limit the currrent, its actually pretty good. If you ALSO wire the leds in anti-parallel pairs, then it doesn't flicker, though you can use a bridge rectifier.

On US, 60Hz 120V mains, 20 LEDs in two chains, with a 400V 1 uF X class capacitor on each chain, with a 1.2 VA rating, this ought to work.