LED schematics

JR_301

I went to led.linear1.org/led.wiz and was looking at the layout given to my specifications, and it was 15 LEDs, but im making an LED bar, and the schematics showed that i needed 120ohms resistors per 3 LEDs, but I dont want to have to run a line for 5 different segments, what type of resistor would I use for all 15 instead of one for each each 3. (would I just add 120X5 to get my answer, or does it not work that way?)

ill be using a 12v battery.

Franklin

The resistor is to limit the current through the LEDs so you figure how much current you need (from the data sheet for the LED) and the voltage and calculate the resistance with R = E/I

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- Stephen

JR_301

Franklin said...
The resistor is to limit the current through the LEDs so you figure how much current you need (from the data sheet for the LED) and the voltage and calculate the resistance with R = E/I

I dont have any data on the LEDs I was able to scrap some off of some solar yard lights my parents had given me, (no labels) the solar lights used 1 AA battery (1.2v 400mAH rechargeable NiCd) with a circuit board, silicon diodes (2), a capacitor and thats it.

Post Edited (JR_301) : 6/22/2009 3:12:32 AM GMT

JR_301

I think I understand now..... that I probably in order to do this need less resistance for the 15 LEDs..... I just need to figure out the correct voltage per LED, which is a 5mm LED and the general voltage for a 5mm LED is 3.3..... which yeah seems odd and I dont think I fully understand how a 1.5v battery could output 3.3v......... hmmmm maybe im missing something.

Post Edited (JR_301) : 6/22/2009 3:35:16 AM GMT

Zoot

The "voltage" for the LED is the voltage DROP -- everything beyond that is current, which is limited by the resistor.

A regular red led might have a voltage drop of anywhere from 1.7 to 2.3 volts, for example. So if you put 5v in one side, you get 3.3v out the other. The resistor just limits the CURRENT so the LED doesn't burn out (and/or to limit the current being used by a microcontroller pin so it doesn't burn out).

So you can calculate current for a series string of LEDs easily enough, BUT you must also take into account voltage drop. If the drop is 1.7v, you can't power three LEDs from 3.3v (the total drop would be 5.1v, more than you started with). With 12v, you can perhaps do 4 or 5 LEDs.

This calculator may help:

www.theledlight.com/resistancecalculator.html

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When the going gets weird, the weird turn pro. -- HST

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1uffakind.com/robots/resistorLadder.php

JR_301

Zoot said...
The "voltage" for the LED is the voltage DROP -- everything beyond that is current, which is limited by the resistor.

A regular red led might have a voltage drop of anywhere from 1.7 to 2.3 volts, for example. So if you put 5v in one side, you get 3.3v out the other. The resistor just limits the CURRENT so the LED doesn't burn out (and/or to limit the current being used by a microcontroller pin so it doesn't burn out).

So you can calculate current for a series string of LEDs easily enough, BUT you must also take into account voltage drop. If the drop is 1.7v, you can't power three LEDs from 3.3v (the total drop would be 5.1v, more than you started with). With 12v, you can perhaps do 4 or 5 LEDs.

This calculator may help:

www.theledlight.com/resistancecalculator.html

WOW! that was a breath of fresh air, I was so confused on that, THANK YOU!

allanlane5

They do make DIP packages of resistors which can cut the number of discrete resistors you need to install.