### Author Topic: What resistor for 3mm LED?  (Read 4036 times)

#### steveyraff

##### What resistor for 3mm LED?
« on: February 19, 2017, 11:31:15 AM »
What the title says. I usually use a 2.2k resistor for pedals with 5mm LED's. I built one with a 3mm and it seems a bit dim. What is the best value to use for this size of LED?

Thanks,
Steve.

#### LiLFX

##### Re: What resistor for 3mm LED?
« Reply #1 on: February 19, 2017, 11:52:38 AM »
What the title says. I usually use a 2.2k resistor for pedals with 5mm LED's. I built one with a 3mm and it seems a bit dim. What is the best value to use for this size of LED?

Thanks,
Steve.

Depends on the forward voltage and current spec for the LED. Measure the voltage drop across the led...let's say it's 3 volts. Take your supply voltage and subtract the voltage drop (forward voltage) of the LED. Divide that number by the forward current spec for the LED. This gives you the smallest possible resistor that you can use with this LED. Adjust larger to limit current (brightness).

Just for practice. If your supply voltage is 9v, the forward voltage is 3v, and the forward current is 20ma. (9-3)/.02 = 300R

#### J0K3RX

##### Re: What resistor for 3mm LED?
« Reply #2 on: February 19, 2017, 03:13:09 PM »
Downlaod Electronics Assistant.. It's free!
It has a LED series resistor calculator as well as a bunch of other useful calculators!

Or, you can just use the web based version.
http://www.electronics2000.co.uk/calc/led-series-resistor-calculator.php
Doesn't matter what you did to get it... If it sounds good, then it is good!

#### merlinb

##### Re: What resistor for 3mm LED?
« Reply #3 on: February 19, 2017, 03:17:38 PM »
Calculators are fairly meaningless as LED brightness is independent of voltage, and current specs tell you next to nothing about how the LED *looks*. They can tell you the minimum resistance before the LED burns out, but that is normally way beyond what you actually need. There is no correct value for a certain 'size' of LED. Diffusion, efficiency, viewing angle, mounting position, all matter. Only way to know what looks best is to try a few different values.
« Last Edit: February 19, 2017, 03:20:58 PM by merlinb »

#### davent

##### Re: What resistor for 3mm LED?
« Reply #4 on: February 19, 2017, 09:05:26 PM »
I do it by sight, i've scads of unsorted LEDs of various sizes, colours and from various suppliers. I plug them in and test 'em as i need them. Slider adds a 10k in series. From bits and pieces lying around waiting to be used.

dave

"If you always do what you always did- you always get what you always got." - Unknown

#### bluebunny

##### Re: What resistor for 3mm LED?
« Reply #5 on: February 20, 2017, 02:39:27 AM »
I'm with Merlin and Dave - try it out.  It's the quickest breadboard circuit you'll throw together.  I got one of these tiny things from Tayda which is perfect for this:

And yeah, be "careful" of the online calculators.  As Merlin says, they tell you a minimum - which will likely be bright enough to kill small children.
• SUPPORTER
Ohm's Law - much like Coles Law, but with less cabbage...

#### antonis

##### Re: What resistor for 3mm LED?
« Reply #6 on: February 20, 2017, 06:40:06 AM »

and grab then a 1k5 resistor to do your job..
"I'm getting older while being taught all the time" Solon the Athenian..
"I don't mind  being taught all the time but I do mind a lot getting old" Antonis the Thessalonian..

#### GiovannyS10

##### Re: What resistor for 3mm LED?
« Reply #7 on: February 20, 2017, 09:41:31 PM »
And yeah, be "careful" of the online calculators.  As Merlin says, they tell you a minimum - which will likely be bright enough to kill small children.

LOOOOOOOOOOL!!! Man, i really laughed a lot.  But i am with you, one friend sent me some blue leds and i have to use 33K for this ones 3mm LOL! And here i buy some that i use only 1k Any questions? Only breadboard it!
That's all, Folks!

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-ARSE, Duck.

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