# Question for people who understand how to wire LED's



## mikaila31 (Feb 24, 2006)

So I want to make a moonlight using some blue LED's I already own. These are the exact ones I have. If I understand all that correctly they are 3.5V LED's. I will be running off a standard 12V adapter. I'm confused on resistors . I have one 47omh that I would like to use. I only have this one so I don't really want to buy a pack of resistors just for one resistor.

Say I wire 3 LED's in one series, would this resistor be OK because a online site suggested 56omh. Is 47 close enough?

What if I wire 4 LED's in one series. I know this goes over 12v so they won't all be fully powered but I'm OK with that. Would I still need the resistor?

Thanks for any help.


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## ianryeng (Dec 22, 2008)

Have you measured the output voltage of the 12v adapter? Most are rated for a voltage at a specific current. At lower currents inexpensive power supplies typically have higher voltage (could be 13-14 volts).

3 led's with that resistor will be fed a current around 30mA assuming that the power supply actually supplies 12v. 

What I would try is 4 led's with that resistor and see what happens, underdriving them is not a bad thing. They will last much longer and will most likely still be really bright.

My moonlight is a severely underdriven white led that I covered in hot melt glue to diffuse the light (works really well to spread the light and prevent spotlighting for this type of application)

Hope that helps, my opinion is that 3 led's with that resistor is a bad idea. Try 4 and see if you are happy with it.


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## FishBeast (Jan 7, 2010)

You will need alot of resistance for 12v down the 3.5. Do a search for led resistor calculator on the web and that should help you.

Here is one I found just now.

http://ledz.com/?p=zz.led.resistor.calculator


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## mikaila31 (Feb 24, 2006)

Thanks for the help! Its all up and running.

I ran all four in a series and just used the one resistor. Thanks for the link fishbeast, I was using another one like that. The only issue I had with is is it wouldn't help me with running the LED's underpowered. Yours helped more.


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## MoonFish (Feb 12, 2006)

You need to run a multimeter in series with the circuit and read the current(amps) to get the spec. 

The specs for the led mean the proper drive current is 30ma and it'll be somewhere around 3.5v+/- 

Underdriving them is fine.


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