simple LED resistor question

freddotu

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I've scanned through what I think are the appropriate sections and didn't come up with an answer. It's probably out there, but my keyword selection didn't yield any success.

I suppose this could be considered a flashlight... It's a single yellow LED powered by a 3v lithium flat disk cell, a CR2032, I think. The item is used to illuminate a bike computer mounted on the handlebars.

I have 12vdc available (13.7 nominal) and would like to bypass the lithium cell. Mechanically, it's not a problem, but I understand that I should have a current limiting resistor in the circuit.

I cannot determine visually if there is already a resistor in the circuit, but would there be, with only 3v powering the light? Perhaps that question displays my limited understanding of LED circuits.

I have the capability of installing the resistor in series with the LED or in parallel, but which is the proper method?

What value and what power rating (?) resistor do I need for such a modification?

thanks in advance for your consideration

fred
 

wasBlinded

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Probably no resistor in that existing circuit. You can use this calculator to get an idea of the resistor needed.

http://led.linear1.org/1led.wiz

Assume 3 volt forward voltage and 30 mA drive current and you get a 1/2 watt 330 ohm resistor. That resistor will be consuming 3 times as much power as the LED will. Pretty wasteful, but if you have plenty of 12v capacity it should not be an issue.
 

freddotu

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I was hoping for such an answer. I tried the calculators in the thread, but I lack any of the parameters and also the proper knowledge to use them. You noted a 1/2 watt resistor, but my stash of leftover electronic goodies only contain 1/4 watt resistors. I do remember how to combine resistors in series and parallel, so if I put matching pairs in series/parallel, will that give me 1/2 watt capacity?

Barring that, I think Radio Shack no longer sells real electronic components in our local stores. Is there a small parts source on the internet for two or three of the 330 ohm 1/2 watt resistors?

thanks for such quick responses. Just for entertainment value, this lighting device will allow me to read my bike computer and Drain Brain in my electric-assist velomobile when the rain cover is on and/or it's too dark otherwise. I have a 5ah 12v NiMH aux pack for lights and alarm system, and a 13ah 36v NiMH for assist.

fred

wasBlinded said:
Probably no resistor in that existing circuit. You can use this calculator to get an idea of the resistor needed.

http://led.linear1.org/1led.wiz

Assume 3 volt forward voltage and 30 mA drive current and you get a 1/2 watt 330 ohm resistor. That resistor will be consuming 3 times as much power as the LED will. Pretty wasteful, but if you have plenty of 12v capacity it should not be an issue.

p.s. I just searched Radio Shack and found they still do sell resistors, specifically the value and rating noted, in five packs for less than a buck. I'll be pedaling my velomobile right past one of those places tomorrow and will have to stop in and see what they have!
 
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wasBlinded

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And if the LED then seems too dim or too bright, adjust the resistor down or up a bit to suit.
 

gorn

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Radio Shack has a real limited supply of resistors. You might be lucky. I had to go to my local Fry's Electronics to get the resistors I needed for an LED project.
 

freddotu

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I want to thank all of you for your support. I was able to snag a couple packages of resistors at ye olde radio shack. After dinner today, I assembled the usual suspects and the result is quite satisfactory. I could not detect any increase or decrease in brightness when running on 12v.

I'm looking forward to my next pre-dawn or post dusk ride to see how it checks out in the real world.

thanks again

fred
 

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