10 easy steps to LED resistor value identification

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David Moeller

Active Member
I started writing this for another thread, and I thought I should start a new one so that others could refer to it in the future. This should unravel the mysteries of the LED for a few and give a little more direction for their use.

I just wanted to add a note about LED's, they will work without a resistor, but it really is better for them if they have a resistor inline and they will last their intended life. A little bit about LED's, for those who don't know, LED stands for "Light Emitting Diode" what this means it that the function if it is to let current through one way and not the other. Think of it as a one-way valve, and as a byproduct, it gives off light. Of course these diodes are engineered specifically to give off light.

On the circuit level, they "read" as a switch. Think of it this way...

-Disclaimer- Now this is only an example for dramatization "don't do this at home, ever!!!!"
Take jumper cables to your car battery, put one on positive and one on negative; the other ends put on a standard 12v light automotive bulb, one on positive and one on negative - instant light!!! - For an LED, take the bulb out and replace it with a switch; put one on positive and one on negative; switch it on, "BAMM!!!" SHORT CIRCUIT...:cry

This is how the LED reads in a circuit, the resistors purpose is to act as a load so the circuit reads correctly, not a short circuit. "Well, that sounds fine and good, but what value of resistor do I use???" Well, you use the formula to calculate it (Great... math...:unsure)

:) 10 easy steps to LED resistor value identification:)

1) You take the voltage, 9v in this case.
2) Take the LED voltage drop (this means how much does it use up) easy, it
states it on the package!!! For white LED's probably 4v.
3) Subtract them!!! 9-4=5 Easy so far, right?
4) Take the LED current rating (it says this on the package too)
for White, to be safe we will say 20mA which is .020A but it can be
higher.
5) Divide the two 5 /.020 = 250
6) Resistors come in standard values so pick the nearest standard value
which is greater, for us it will be 270 ohms.
7) The resistor % just means how accurate it to the stated value, 5% or
10% is fine, what ever you can find.
8) The wattage is for how much power will go through it, so anything small
1/4w or 1/2w is fine.
9) HOOK it up! It does not matter what side the resistor is on. It
It does matter what side of the power the LED is hooked up to, the long
lead on the LED is ALWAYS positive. (unless someone cuts it!)
10) Enjoy your lighting!!!:thumbsup

I hope this helps explain a little more about this lighting product, and makes it easier to install your LED's for your projects.

Dave
 
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TridCloudwalker

Sr Member
RPF PREMIUM MEMBER
How about power? I'm working on a PL Enterprise that when I'm done will have 100 - 200 LEDs in it. I'll be powering it with a wall wart, but how do I figure out the best voltage box to buy?
 

Scapey

Sr Member
[Pointed out a typo that could cause confusion]

Cheers dude, makes more sense now :)
 
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David Moeller

Active Member
TriCloudwalker, If you want to run a large quantity of LED's as mentioned, I would suggest using an LED driver. The current need for the driver will be stated in the specs, then you just find an appropriate "Wall Wart" though you will probably need a larger capacity one for your needs. If you look for surplus electronics you can get some that are of the "switching" type that are small and lightweight used for computers and such. Yes, it costs some money to do this, but it will be cheaper in the long run than having to change LED's out!

I'll have to find out which driver it is, but I'll pm you the link...

Reverend Scapegoat, White LED's specs are going to be all over the board, they are becoming more efficient with less current - You really have to see the specs from the LED you are going to purchase. If you have the Brand and model number then you can look them up online and find a PDF. If you order from a manufacture or distributer, then the specs will be posted in the catalog / book.

If you find some that are from Ebay etc... then you will just have to guess. Most standard LED's current rating is 20mA but I have some high power ones that are around 600mA and they are blinding.

If you are questioning about the current 20mA = .020mA, Yes, you are correct, I'll fix my post, it is .020A, my bad, "but it works in the formula!!!":thumbsup

Good luck on everyones lighting projects....
Dave
 

TridCloudwalker

Sr Member
RPF PREMIUM MEMBER
TriCloudwalker, If you want to run a large quantity of LED's as mentioned, I would suggest using an LED driver. The current need for the driver will be stated in the specs, then you just find an appropriate "Wall Wart" though you will probably need a larger capacity one for your needs.
My brother works at a sign shop and has these two laying around for the taking. Would one of these work, or is this overkill? The 100 - 200 LEDs will be for a PL Enterprise with DLM's Federation flasher/strobe circuit. No fancy torpedo or warp effects.


 

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