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Old 11-12-2019, 10:36 PM   #1
flyerrich
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USA LED lights determining resistors

How do I determine what resistor size to use. Is their a sheet showing that info? I have some LED's that I want to use but I know they would be too bright. I am looking for info to use as a guide to get different results using maybe a 3 or 9 volt DC power supply.
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Old 11-13-2019, 05:44 AM   #2
gregc
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try using a potentiometer to adjust the LED to the desired brightness (liminosity) at the voltage you plan on operating then at and measuring the potentiometer resistance. (start with the pot in the middle and don't let it go too bright, or put a 150 Ohm resistor in series to limit the current)

different batches and colors of LEDs from the same manufacturer may require different value resistors.
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Old 11-13-2019, 06:14 AM   #3
Dennis461
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For LED's with no specifications, I go for 20 milliamps max.

18V / .020 = 900 ohms.

I bought a bunch of LED's with resistor attached from China for HO scale buildings and they started burning out after a few months of operation. Found they had a 600 ohm resistor installed, not enough ohms IMHO.

p.s. OHM upside down looks like WHO
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Old 11-13-2019, 03:40 PM   #4
traction fan
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West 3-9 volts

Quote:
Originally Posted by flyerrich View Post
How do I determine what resistor size to use. Is their a sheet showing that info? I have some LED's that I want to use but I know they would be too bright. I am looking for info to use as a guide to get different results using maybe a 3 or 9 volt DC power supply.
Thanks
flyerrich;

I do quick tests on LEDs using a 3 volt watch battery and no resistor. I don't recommend this as a permanent setup though, even if you kept the supply voltage at 3 volts. If you're going to use nine volts then 100 ohms of resistance should work. I've used as little as 67 ohms on a single LED. However, you would have to up the resistance a lot, before the LED would not light up at all. I would suggest you get half a dozen resistors in the general range of 100 ohms each. Connect them in series and try a nine volt battery feeding the LED through all six resistors with a total resistance of 600 ohms. If that brightness looks good to you you could adopt that size resistor for the rest of your LEDs. If the LED doesn't light, or is too dim, go down one resistor and try the LED with 500 ohms.

good luck, have fun

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Old 11-13-2019, 04:00 PM   #5
Lemonhawk
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I generally use 1k resistors when connecting LED's to 12 volts. Since LED's are current devices, you really need a resistor of some sort, but using a resistor to control brightness is not the most effective brightness control. Pulse Width Modulation is much more effective but a lot more complicated.
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Old 11-13-2019, 04:15 PM   #6
T-Man
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A white LED can be 3 to 3,5 volts. Yellow or green are around two. Red is in the middle.

AT 9 volts white would need 300 ohms ohms and yellow and green need 350, Red would need less than 350 ohms but will appear bright when next to yellow and green with the same resistance. 350 ohms is an odd number so go with 370 ohms for yellow and green at nine volts and 330 ohms for red.
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Old 11-13-2019, 09:36 PM   #7
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To calculate the resistor needed, ( supply voltage minus LED voltage )/current

9 volt supply, white 3 volt 20ma LED (9v-3v)/.02A = 300ohm

If you put LEDs in series add up the voltages, if in parallel add the current.

"[Or just push the voltage] up until it explodes then back it off 10% or so," Wade, guy in the machine shop.
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