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Old 06-02-2005, 09:03 PM   #1
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Default Question about Resistors, current, and voltage.

I have some basic electrical knowledge, but it's been a very long time since I had any EE classes. I need someone to help refresh my memory.

I'm thinking of making an LED light.

I have a bunch of bright white LED's. As I recall, there will be a drop in voltage across the LED when it is energized. Say I use a 12VDC source, and voltage drop across the LED is 2VDC.

Connected in series, I can power 6 LEDs with my 12VDC source. Alternately, I can connect as many as I want in parallel until I'm pushing the amp-hour rating of my battery or the size of some of the wires won't support the current flow.

But I've heard there is a need to limit the current that flows to the LEDs, otherwise they can burn up quickly.

Here's where my memory gets fuzzy.

I *think* I remember that a resistor can be used to reduce the voltage if wired one way, or it can be used to limit the flow of current wired another. If that is true, how do you wire in the resistor for each case?

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Old 06-02-2005, 09:22 PM   #2
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An LED must have a current limiting resistor wired in series if you're connecting it to a 12V source, or it will burn out almost immediately. The exception is LEDs that are supplied with a built-in resistor which your white ones probably aren't.
To connect multiple LEDs each one should have its own resistor, and the LED/resistor combos are connected in parallel across your voltage source.
To determine the proper resistance value you need to know the specified operating current of the LED. Typically it will require around a 1 or 2K resistor.
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Old 06-02-2005, 09:54 PM   #3
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This page is pretty helpful for visualizing/figuring out LED circuits.
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Old 06-02-2005, 11:50 PM   #4
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In addition to the setups on that site, I've seen a few setups with LEDs using a zener to clamp the voltage at a safe level, but that's more if you are going to use it on a 12v automotive system where the regulator might provide too much voltage for the setup. I've run 6 standard red leds in series off an automotive 12 supply without failure and a number of single blue LEDs in series with 1 330 ohm resistor each. I think the regular blue LEDs provide more of a voltage drop so when running single reds it's safer to use 470 ohm resistors in series. With a white LED I'm not sure, but it'll be around that range.
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