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Is this the right configuration for LEDs? Answered

I am building some LED aquarium lights and I need to know the configuration pictured below will work? The LEDs are white with the following specs: Forward Voltage 2.8-3.5V, 20mA. I used an online calculator and the resistor pictured as 'R1' is 1 ohm, does this sound right? The power supply is 9VDC 800mA. I believe that if each of the 3 LEDs in a set pull 3V, then no resistor would be required, am I right? I have basic electronic knowledge, but have never designed a circuit requiring resistors.

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gmxx

Best Answer 10 years ago

yes, mostly... the layout is correct...

using http://led.linear1.org/led.wiz i get 33 ohm resistors.

i know this calculator works, so ymmv with 1 ohm resistors.

led4willard2-0.png
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Willard2.0gmxx

Answer 10 years ago

Thank you for the calculator link. It is better than the one I was using. With 3.0V I get 1 ohm resistor. If the LEDs have a max V of 3.5, then running them at 3.0V should be fine, right?

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gmxxWillard2.0

Answer 10 years ago

running the leds at 3.0 volts would be fine. the only reason i said 33 ohm resistors, is because the source voltage you specified is 9 volts. if you have a 3 volt power supply, then by all means use it.

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Willard2.0gmxx

Answer 10 years ago

If I want the LEDs to run at 3.0V then I use a 1 ohm resistor right? The source Voltage is 9.0V. I appreciate the help.

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gmxxWillard2.0

Answer 10 years ago

no... source voltage is always 9.0 volts unless you change your power brick... unless you are wiring the leds into a microcontroller, or using a voltage regulator, then you want to use 33 ohm resistors. so: power brick output 9v @ 800ma. use 33 ohm resistors power brick outputs 3v @ 800ma (doesnt matter how many amps, as long as enough is provided.) use 1 ohm resistors.

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Willard2.0gmxx

Answer 10 years ago

Thanks for the quick reply, and thank you very much. I am now ordering 33 ohm resistors ( hadn't bought any yet). Thanks again

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gmxxWillard2.0

Answer 10 years ago

no problem... be sure to make an instructable on your lights...!

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Willard2.0gmxx

Answer 10 years ago

My first instructable is on the way!!

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frollard

10 years ago

Zero and Matt are correct in that if your leds run at 2.8 volts, you want 33 ohms/series string of 3. If they are okay to run at 3 volts then go for it.

The reason for the 1 ohm resistor is just that = 0 ohms = potentially infinite current. 1 ohm is not much, but any overvoltage wont cause the leds to deteriorate quickly. In fact, the smaller resistor you use the more efficient the system, so long as the leds closely match the source.

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mathews

10 years ago

I'd be inclined to use 33R resistors.

9-(3*2.8) = 0.6v

0.6v/0.02a = 30R, with the nearest E12 being 33R

You might also want to look into PWM, and with this many, you may want to use a decade counter to divide them up and light them up in sections. It'll reduce the amount of power used, and the waste heat dissipated.