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Finding proper voltage for LED
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Finding proper voltage for LED

by MrGlass on Tue Nov 09, 2010 5:44 pm

I have a box of parts, including an assortment of random LEDs. Unfortunately I was not smart enough to keep track of the power specs for them. Is there a way to determine the proper power to use? I'm mostly concerned about them burning out too quickly.
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Re: Finding proper voltage for LED

by stinkbutt on Tue Nov 09, 2010 5:52 pm

Throw a big (1K, 10K) resistor in series with each one and then hook it up to 5V. The difference between the 5V and the voltage drop across the RESISTOR tells you the voltage drop across the LED.
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Re: Finding proper voltage for LED

by zener on Tue Nov 09, 2010 5:58 pm

Your title says voltage, then you ask about power. The issue is really current. Most "standard" LED's can take 15 or 20 mA. What LEDs you have I don't know. But if they look fine at 10mA or less then just run them there.

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Re: Finding proper voltage for LED

by stinkbutt on Tue Nov 09, 2010 6:02 pm

I was assuming he wanted to know what the voltage drop for each LED was. As for how much power these things were rated for, you're probably shit out of luck. I can't think of any way to differentiate between two LED's that are the same color.
Red M&M, Blue M&M: They all wind up the same color

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Re: Finding proper voltage for LED

by MrGlass on Tue Nov 09, 2010 6:05 pm

Looking to figure out what battery I can hook them up to & make them glow, and what resistor I might need inline. the voltage drain bit is a good tip though, thanks.
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Re: Finding proper voltage for LED

by zener on Tue Nov 09, 2010 8:02 pm

You will need more than 2V and the 1K is a good start. For example 9V and 1K will likely give you about 7mA.

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Re: Finding proper voltage for LED

by adafruit on Tue Nov 09, 2010 11:21 pm

in gneral:
IR are 1.8V
red/orange LEDs are 2V
yellow/light green are 2.2V
blue/deep green/violet/white are 3.4V

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Re: Finding proper voltage for LED

by stinkbutt on Wed Nov 10, 2010 3:30 am

Wow, I never put that together before, but the voltage drop is pretty much equal to the energy in a single photon of the appropriate color of the LED, at least when it's energy is expressed in electron-volts.
Red M&M, Blue M&M: They all wind up the same color

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Re: Finding proper voltage for LED

by Entropy on Wed Nov 10, 2010 3:21 pm

Yup, the color is determined by the bandgap energy.

As others have said, just measure the voltage across the LED when driven with a higher than normal current limiting resistor.

LED I/V curves are very nonlinear, so the difference in Vf between 1 mA or so and 20 mA will be a VERY small amount.

In terms of determining current handling rating - almost all 5mm LEDs are rated for 20 mA continuous current. If you want to be safe, drive it at 10. If it fails at 10, it probably belonged in the trash to begin with. :)
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Re: Finding proper voltage for LED

by zener on Wed Nov 10, 2010 3:28 pm

pstemari beat you to it:

pstemari wrote:
Re: LED sorting
by pstemari » Sun Jul 11, 2010 9:58 pm

Usually (white/blue/UV LEDs being the exception) the voltage drop is just the quantum energy of the wavelength, i.e. hc/lambda, or or roughly 1240 divided by the wavelength in nanometers, +/- resistive losses, weirdness in the connections, etc.

Basically, when an electron drops through the band gap, the energy it loses is emitted as a photon.



Vf = 1240/nm , Genious!

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