Author Topic: Yet another LED question.  (Read 1732 times)

jmdfd415

Yet another LED question.
« on: October 17, 2006, 07:47:40 PM »
Sorry for my stupidity, but I have searched around and I am still having trouble figuring out what value of resistors to use for my leds in my loop box I just made.  It will have 3 leds that I bought from pedalpartsplus.com  and the only info that I have about them is this:

5mm blue:
LED T-1 3/4 5mm clear lens blue in color, 5000mcd

5mm green:
LED T-1 3/4 5mm clear lens green in color, 10000mcd

5mm red:
LED T-1 3/4 5mm clear lens red in color, 5000mcd

Do you guys have any suggestions on what resistors I should use for these?  I will be running the pedal off of a dc jack instead of a battery.

choklitlove

Re: Yet another LED question.
« Reply #1 on: October 17, 2006, 07:50:21 PM »
it's a preference thing.  test different values to see which level of brightness you like.  most people use 1k to 4.7k.  one of those two will probably suit your needs.
my band.                    my DIY page.                    my solo music.

jmdfd415

Re: Yet another LED question.
« Reply #2 on: October 17, 2006, 09:41:00 PM »
Thanks I will try out a 4.7k and see what happens.

Rattlehead

Re: Yet another LED question.
« Reply #3 on: October 17, 2006, 09:45:49 PM »
sorry to interfere but i have a question about LED too.
what resistor should i use for a green LED,630 mcd?

 :)

R.G.

Re: Yet another LED question.
« Reply #4 on: October 17, 2006, 11:03:59 PM »
LEDs are devices which convert current to light. The LED is a Light Emitting Diode, so it is also a diode. It conducts current easily one direction, more poorly the other. Only forward current is converted to light.

An LED's forward voltage, like most other diodes, must be at least some threshold voltage before it will conduct at all. For instance, no LED that I know of will conduct with less than 1V across it. When you increase the voltage above the threshold voltage, the LED begins conducting a lot. It conducts so well that if you have no way to limit the current, the LED will burn out.

LEDs have some maximum forward current that they can stand. For the common T 1 3/4 5mm LED, this is often 20ma, based on the internal heat buildup from the forward voltage drop and the current. An LED can be used at any current from zero up to the max. It can be used over the max only in brief pulses, with time off between pulses so that the LED can cool down between them.

So in an effect, you want to set the LED up with some way to limit its current. This is usually done by a resistor.

If you are powering an LED from 9V, the LED needs somewhere between 1.2V and 3.0V of threshold voltage to turn on at all, and less than its max current. To calculate what resistor you need, you subtract the LED forward voltage from your  9V power supply, then divide whatever voltage is left over by the current you want to limit the LED to. That answer is your resistance value.

So if you have a 9V power supply, a 1.5V LED, and you want 10ma, the resistor to put in series with the LED is R = (9-1.5)/0.01 = 750ohms. If you have a 2.0V LED and want to run it at 5ma the resistor is (9-2)/0.005) = 1400 ohms.

Different LEDs have different forward voltages. Some are as low as 1.2V, some as much as 4V. Most are 1.5 to 2.5V. The way to tell is to either read the datasheet or test the LED with a 1K resistor and a battery, measuring the voltage across the LED.

An LED converts current to light roughly proportionately to the current. That is, the more current, the brighter the LED. But some LEDs convert current to light more efficiently than others. Some red LEDs can only put out perhaps 1millicandela per milliampere. There exist LEDs that can put out 1000millicandelas per milliampere. But their voltage may be about the same. One is just more efficient at converting current to light.

The millicandela rating does not have any bearing on what voltage the LED has or the current (and hence the resistor to be used) should be. You simply must find out from the manufacturer (a) the forward voltage and (b) the max current. Then you have to determine what supply voltage you will use and compute the resistor from it.

And please, no weeny javascript calculators. Those will permanently prevent you from understanding what it's about.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

skiraly017

Re: Yet another LED question.
« Reply #5 on: October 18, 2006, 02:00:06 PM »
And please, no weeny javascript calculators. Those will permanently prevent you from understanding what it's about.

I just keep trying different resistors until they visually match. Does this keep me out of the Weeny Club?

 :icon_mrgreen:
"Why do things that happen to stupid people keep happening to me?" - Homer Simpson

The Tone God

Re: Yet another LED question.
« Reply #6 on: October 18, 2006, 02:15:52 PM »
It is a good idea if you want to play around with different colour bright LEDs to figure out what intensity you like. Then as you order LEDs you can calculate what value resistor you need for your preferred intensity. You can check the LED's datasheet for a nice little graphic of intensity vs. current to help you out with your calculations if needed.

V = I x R is your friend. :)

Andrew

R.G.

Re: Yet another LED question.
« Reply #7 on: October 18, 2006, 04:12:38 PM »
Quote
I just keep trying different resistors until they visually match. Does this keep me out of the Weeny Club?
Definitely. If you can do it without a net connection or a canned calculator, you're not a javascript weeny.  :icon_biggrin:

Like they say in woodworking, measure twice, cut three times.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?