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85 LEDs taking 20ma each individually wired with a resistor, what ma power supply is needed? Answered

For 85 LEDs each taking 20ma wired individually with a resistor off of a single 9v power supply I would need at least a 1700 ma power adapter correct??...I would probably go with a 2000ma one though just to be safe.?

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orksecurityBest Answer (author)2011-05-23

85 * 20 = 1700. Yes. 1.7 amps. You shouldn't need to go much above that, but it generally won't do any harm if you do.

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verence (author)2011-05-23

While orksecurity is right, the solution to wire each LED individually is just a waste of energy.

What kind of LEDs do you use? (The options you have depend on the forward voltage which is different for different LED colours)

Have a look here: http://led.linear1.org/led.wiz

E.g. with red LEDs (Uf=2.0V) you can go with only 440mA totally consumed current!

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lamboboy732 (author)verence2011-05-24

Yea I know its kinda a waste but with varying colors meaning varying voltage I just wanted to be safe and wired them each with their own aproprite resistor. Thanks though for confirming my question.

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frollard (author)verence2011-05-23

+1

You want as many series leds as you can fit on your power supply. If you have 9v supply and 2v leds, you need 4 in series plus a resistor to burn off the last volt to make 9.

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iceng (author)frollard2011-05-23

+2
And if you find 85 red LED with 1.7 Vf then you can run on 340 ma

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