L.E.D’S
Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=23464
Printed Date: May 05, 2025 at 2:13 AM
Topic: L.E.D’S
Posted By: Legrones
Subject: L.E.D’S
Date Posted: December 27, 2003 at 11:08 PM
hello there i am looking to use some L.E.D'S so i was told i need a resistor with it is ther a special kind or not ?
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Dale / Cory
Replies:
Posted By: Teken
Date Posted: December 28, 2003 at 1:26 AM
470 ohm @ 1/4 watt will do . . .
Regards
EVIL Teken . . .
Posted By: xetmes
Date Posted: December 28, 2003 at 8:00 AM
I would say 470 as well since you provided no information about the LED...
Posted By: Teken
Date Posted: December 28, 2003 at 12:13 PM
Just in case people want to get technical about all this. You would want to know the following information about the *LED*, *power supply*, and *operating temperatures* in which the LED will be used in.
First you need to know the LED voltage drop. It is safe enough to assume a 1.7 volts for non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and low-current red, and 2 volts for orange and yellow, and 2.1 volts for green.
Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types. Assume 4.6 volts for 430 nM bright blue types such as Everbright and Radio Shack.
Design for 12 milliamps for the 3.4 volt types and 10 milliamps for the 430 NM blue.
Next step is to subtract the LED voltage from the supply voltage. This gives you the voltage that must be dropped by the dropping resistor.
Example: 1.70 volt LED with a 12 volt supply voltage. Subtracting these gives 10.3 volts to be dropped by the dropping resistor.
The next step is to divide the dropped voltage by the LED current to get the value of the dropping resistor. If you divide volts by amps, you get the resistor value in ohms. If you divide volts by milliamps, you get the resistor value in kilo-ohms or k.
Example: 12 volt supply, 1.70 volt LED, 20 milliamps. Divide 10.30 by .020. This gives 515 ohms. The nearest standard resistor value is 520, ohms.
Worst case a 550-600 ohms would also be fine.
My original reply only factored a 12 volt supply @ 30mA which in the real world is fine, because the LED is assumed to tbe flashing and not lit steady.
But technically, and for long term use 30 mA is too high, dependant upon ambient temperatures, the ideal resistor would have been a 550-600 ohm resistor. This assumes it is lit constantly, and not flashing.
12 volts supply, 1.70 LED, 20mA Max Current = 515 or 520 ohm resistor.
Hope that clear up any confusion for those who want to add in a few LED's...
Regards
EVIL Teken . . .
Posted By: jakita
Date Posted: December 28, 2003 at 12:44 PM
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