cva6633

First of all, sorry for asking this as it has most likely been asked a bunch of times.

I bought a bunch of white LEDs on ebay, and they came with resistors.

Now im running them from 12v with the 500 ohm resistors that they came with (one resistor per LED) and they run at about 16-18 mA. Is it normal for the resistors to get quite warm?

I measured the voltage across them at about 8.3 to 8.8 volts (it varies) and i calculated the power at about .168 watts, is it normal for a 1/4w resistor to get warm running at a little more then half its capacity?

I know this is a noob question, but i just wanna know if im doing this right.

Thanks

bananasiong

Hi,
That's normal for them to get warm. Most power is dissipated by the resistor, which is wasted. If you can redesign by connecting a few LEDs in series with a lower resistor to remain the current, more power will be spent by the LEDs but not wasted by the resistors.

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Sceadwian

Yes. At full power you'd likely lose your fingerprints if you touched them in free air. You might want to re-think running a single LED with a current limit resistor, the resistor wastes most of it as that heat. Several LEDs in series (2 or 3) with a much lower value resistor will waste little to no heat. If you know the voltage you're running the LEDs at and you know the voltage drop you can calculate (ohms law) the equivalent resistance of each LED and then tack on the current limit resistor to fill out whatever out of place voltage needs to be dropped to get the current you're looking for. Simulation software such as LTSpice are useful for coming up with these numbers without a lot of hand calculating but itself has a learning curve.

The white LEDs I have drop 3.1 volts at 20ma. Use four of those in series and that's 12.4 volts, that's a little room to work with. Add a 1-20 ohm resistor with it just to give the device some basic static resistance and you'll be 3 times more efficient, and the light output will only be a little bit less.

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cva6633

Thanks a lot.

I wanted to use one resistor per LED just because i have a bunch of them as they came with the LEDs. I guess resistors aren't expensive.

I was planning to make some DJ lights. I was gonna put about 20-30 LEDs together to make a spot light and didnt want the thing to heat up too much. Im working on a white one first, them im gonna do a couple other colours.

I will try the four LEDs in series when i get a chance to see how much it dims them down.

audioguru

Your LEDs are about 3.5V each. Four in series need at least 14V. Your 12V supply won't drive them properly.

Three in series need 10.5V and will light brightly from a 12.0V supply if they have a 62 ohm current-limiting resistor.

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Uncle $crooge

cva6633

I found a bunch of 3.3 ohm resistors and put one in series with 4 LEDs and they ran at about 12-13 mA. They are still quite bright so i think ill run them like that.

When i have time ill bread board the 30-40 LEDs and see how bright they are together.

I guess the LEDs are weired because they run anywhere between 2.8 to 3 volts.

Thanks again for all the help.

audioguru

LEDs have a wide range of forward voltage. It depends on how bright the moon was and the day of the week when they are made. Never buy LEDs made on a Monday or Friday when the moon was full. ;D

I bought a factory sealed bag of red LEDs and their forward voltages are almost identical. In parallel their currents are nearly the same, like LED flashlights (torches).

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sniper007

Measured Voltages at 20 mA:

	Deep red	660 nm	Gallium-Aluminum-Arsenide (AlGaAs)	   1.8 Volt
	Red	        630 nm  Gallium-Indium-Phosphide (AlGaInP)	   1.9 Volt
	Orange  	610 nm	Gallium-Arseen-Fosfide (GaAsP)	           2.0 Volt
	Amber	        592 nm	Gallium-Arseen-Fosfide (GaAsP)	           2.0 Volt
	Yellow	        585 nm	Gallium-Arseen-Fosfide (GaAsP)             2.0 Volt
	Yellow/Groen	565 nm	Gallium-Nitride (GaN)	                   2.1 Volt
	Green	        525 nm	Gallium-Fosfide (GaP)	                   2.2 Volt
	Turquoise       505 nm	Zink-Selenide (ZnSe)	                   3.0 Volt
	Blue	   470...430 nm	Zink-Selenide (ZnSe)                       3.3 to 3.5 Volt
	Violet	        420 nm	Indium-Gallium-Nitride (InGaN)	           3.2 Volt
	Ultra Violet	400...380 nm	Diamant (C)	                   3.5 Volt
        White	400...380 nm	Silicium-Carbide (SiC) +Phosphor           3.3 Volt

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