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Old 07-07-2008, 05:54 PM   #1
ricodued
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Default Having an issue working out my circuit for cupholder LEDs

I've two blue LEDs which I will place into the bottom of my two cupholders (one each). They're 5 volt, 30mA LEDs and I'm having an issue recalling my high school physics.

I've got a circuit diagram with my two LEDs, taking up a sum total of 10v from the 12v tap in the cigarette lighter, so I need a resistor in the circuit that will knock out the remaining 2 volts. However, without the amperage of the cigarette lighter tap, I've no idea how to calculate the ohms with V=IR since I have no I (the amps).

Ideas?
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Old 07-07-2008, 05:59 PM   #2
ricodued
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Actually, I may have solved it. If I remember right, only the 'requested' amount of current flows through the circuit, so.. sum total, it'd be 0.06A for the LEDs flowing through the circuit. If I base my resistor calculation on that, I should need a 34 Ohm resistor.

I hope that's right :P
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Old 07-08-2008, 01:26 PM   #3
skingfreak
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type in good resistor calculator it will save you some time next time
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Old 07-08-2008, 11:00 PM   #4
Cougar4
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It sounds like you are running your diodes in series with each other since you say that the total voltage needed for the diodes is 10 volts. If that is the case you need to remember that current is the same in a series circuit and so it will be 30 ma., not 60. A 68 ohm, 5 watt resistor will do the job I think. The actual power dissipation is 2 watts but you wouldn't want to use a 2 watt at the max limit.
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Old 07-08-2008, 11:28 PM   #5
ricodued
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Thanks for the tips, especially that calculator! :P

Cougar4: The LED calculator I used from google said a 1/4 watt, 68ohm resistor. Since Radioshack doesn't carry 68ohm resistors, I just got seven 10 ohm resistors in series.

My project should be completed tomorrow, I ran into some complications today. Eventually, when the whole thing is completed, my cupholders will be illuminated through a plexiglass bottom with the regular plastic bottom underneath that. And between the plexiglass and the regular bottom, I'm putting some custom graphics. It should be pretty dope. Pics when I'm done

Thanks for the help guys
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Old 07-09-2008, 12:38 PM   #6
matt.britt
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Is 30 mA the maximum current rating? Likewise, is 5 V the maximum forward voltage drop? You usually don't want to assume LED bias at either spec if you want them to have a long life... You could be over-driving them; refer to their datasheet.
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Old 07-09-2008, 10:06 PM   #7
Cougar4
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My power calculation was way out. I knew it didn't seem correct when I did it and I don't remember how I came up with it. You're welcome for the help.
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Old 07-10-2008, 08:35 AM   #8
skingfreak
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Quote:
Originally Posted by ricodued View Post
Thanks for the tips, especially that calculator! :P


Thanks for the help guys
Np i would have no idea how to figure out values if it weren't for those online calculators
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Old 07-11-2008, 03:11 AM   #9
div
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Any pictures of product yet?
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