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Hot, Burning Resistor Question Answered

Ok....I have 24 leds in parralel attached to a 9 volt battery - and a 10 ohm resistor - as reccommended by LEDCALC.comLEDCALC.com - but, when i connect the circuit and hold the resistor in my hand, it becomes INCREDIBLY hot. Each led is in parallel, individually. So...is it normal for it to be that hot, or did I do something wrong?

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Tool Using Animal (author)2007-12-27

You're dissipating couple of watts there, yes it's going to get hot, and your battery is going to run down quickly

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gmoon (author)Tool Using Animal2007-12-27

What he said. Your calculator page recommends a 3 watt resistor with that circuit. Also, the resistor alone dissipates 2430 mW (2.5 watts.)

(assuming 3.6 V drop, 9V supply, 20 mA per LED.)

Using the same calculator page, two LEDs in series with a 100 ohm resistor, the resistor dissipates only 32 mW. If you create 12 identical circuits (2 LEDs and one resistor), the total power lost in the 12 resistors is only 384 mW (0.4 watts, 32 * 12) vs. 2430 mW for the single resistor...

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Joe426 (author)gmoon2007-12-28

dang...didn't realize that...I should have used the guru on that site. Anything I can do at this point, being that I already soldered everything up and don't want to take it apart?

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Patrik (author)Joe4262007-12-28

Well, luckily you only need to desolder *half* of the connections. You have 24 LEDs, with all the anodes soldered together, and all the cathodes soldered together, right? Separate those LEDs into two groups: 12 LEDs with the anodes soldered together, and 12 LEDs with the cathodes soldered together. Now solder 12 resistors in between the loose anodes and cathodes.

I'm assuming you're using white LEDs with a 4.5V voltage drop, so use resistors of 1-10 ohm each. (Note that if you use the "Guru" with a 9V supply and 4.5V drop per LED, it will actually tell you to put them all in parallel, with a 270 ohm resistor each. So plug in a 4.49V drop, and save yourself 12 resistors to solder...)

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Joe426 (author)Patrik2007-12-31

any chance the resistor could explode? or burn something?

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Patrik (author)Joe4262007-12-31

Depends. With the setup you have, the resistor dissipates about 2Watts. Most "off-the-shelf" resistors are rated for 1/4 or 1/8 Watt. It's actually unlikely your resistor would explode if it's rated too low, but it probably won't last very long, and will burn through due to overheating.

Regardless of whether you use a resistor rated for 1/8W or 5W, in this circuit it will still dissipated 2Watts of heat - which is a good amount of heat in a very small amount of space. If you have anything flammable nearby - paper, cardboard, plastic, etc - you might conceivably set something on fire.

Also, by putting all your LEDs in parallel without a separate limiting resistor for each, some of you LEDs may burn out. The "20mA @ 4.5V" is only a *typical* value. The voltage drop for individual LEDs may deviate from that by quite a bit, and a small change in voltage can cause a big change in current . - that's exactly why you need a resistor in series with each LED: to make them "better behaved". Here's a nice explanation:

Why do I need a resistor with an LED?

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gmoon (author)Patrik2007-12-31

any chance the resistor could explode?

Explode? Well, actually--yes. But it's more of a little fzzzzzzzzzz..pop, as the molten insides find a way to the outside.

But as Patrik says, it's as likely the resistance element will just fail, leaving you will an open circuit.

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Joe426 (author)gmoon2007-12-31

thanks for the replies, it's only a 10 ohm resistor so i don't think it would do any damage if it failed.

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gmoon (author)Joe4262007-12-31

Sure. 10 ohms is not a big deal if used in series with 2 white LEDs, not with voltage drop of the LEDs. But in a general sense, remember: the lower the resistance value, the greater the load (current draw.) 10 ohms is pretty low. Shorting one of these directly across the current path would be problematic...

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Joe426 (author)Patrik2007-12-28

thanks, thats true, I dont have to desolder all of them. Actually they are blue Leds on a sign, spelling a word.

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tech-king (author)2007-12-31

when some resistors overheat, their resistive properties change. this could blow out the leds. you could try adding a heat sink to the resistor

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Joe426 (author)2007-12-30

Any chance the resistor will explode or something?

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