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Powering LED array with 12V AC/DC converter? Answered

Hi! Long time lurker here! I have a project in mind that involves the construction of an array of 121 LEDs in an 11x11 configuration. I want to power the LEDs off a 12V AC/DC adapter, but I'm a little unsure about how current works in this case. The adapter is a regulated 12V 1A adapter. The LED array, however, needs only 820 mA. I know the maximum DC current is 30 mA so if the full amp is spread across the 41 parallel series it should be passing ~24 mA through the LEDs. Will this cause any major problems? Is there a way to limit the current to the suggest 20mA? Furthermore, if anyone has any good ideas as to how to mount 121 LEDs in a square grid of inch spacing, I'm all ears. I considered using pegboard, but I can't seem to find any, and veroboard large enough is also proving difficult to source. I don't really have the resources necessary to make my own PCB. Maybe I could drill some pegboard of my own and attach some thin conductive strips for the 12V and 0V rails. Thanks for any help! I've taken a practical electronics course at university and done some soldering (just op-amps and the like), but this is the first real electronics project I've taken on, so any help is greatly appreciated.

4 Replies

LargeMouthBass (author)2011-03-07

I'm assuming you are using individual LEDs, and not LEDs that are already arranged. You made a reference to a number of strings, but I didn't know if that part was carved in stone.

Each LED will need about 2 volts across it, maybe a little more or less, depending on the color. This will limit the number of LEDs that can be put in series in a string. If the voltage drops across the LEDs exceed 12 volts, they will not light, or the will be dim.

If you put say, 4 LEDs in a series string, you would need a total of about 30 strings, (plus one odd string with only 1 LED) which will then be placed in parallel. Each string should have a resistor in series with the LEDs, to set the current. You can set the current in each to whatever you want based on the following.

Resistance = (12Volts -Number of LEDs in each string * Voltage drop across each LED) / Desired Current

So, to use 4 LEDs per string and each has a voltage drop of 2 volts, and you want it to carry a current of 20 mA, the resistor used in each string should be : (12 volts - 4*2volts)/20mA= 200 ohms.

If you used this arrangement, you would have 30 strings, each with 20mA flowing in it, for a total of 600mA, which should work fine with your supply.

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bwrussell (author)2011-03-07

Any luck with this LED array?

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lemonie (author)2009-04-27

I'd drill your own board if you want then 1" apart. Be sure to post an I'ble eh? L

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Goodhart (author)2009-04-27

sinking such a small amount of current is easy (if it is actually necessary).

Ohm's Law.

And here is a Calculator that may help you along.

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