Voltage drop resistors for LEDs?

grolschie · May 10, 2009, 12:54:35 AM

Hiya.

I have some 3mm blue LEDs (2V, 15-20mA) for a pedal that I am building. What resistor values should I be using? I see some schematics use values as high as 10k-15k for LEDs, but when using 10k I get no light.

I used an online calculator and its seems that the correct value is 470ohm - which is clearly much smaller. I realise that I could just try much smaller (or even the recommended value). The bezels are quite fiddly to get at in the pedal, so I don't want to mess around too much with various resistors in this particular case.

Generally speaking, what is a good rule of thumb for LED resistor selection in pedals? I want to avoid popping, conserve power, and it doesn't need to be overly bright. Thanks in advance.

Cardboard Tube Samurai · May 10, 2009, 12:58:51 AM

Formula: Vs - VLED/i = required resistor

9v-2v = 7v
7v/0.015 = 466.66666666666666666666666666667r

470r is what you're after!

grolschie · May 10, 2009, 01:06:36 AM

Best I go with the calulated value then. Many thanks.

lerxst88 · May 10, 2009, 01:18:43 AM

yeah calculated value + a little will give you the brightest LED but if youre not sure of voltage drop or current draw of the LED, try using a 1K resistor.

MohiZ · May 10, 2009, 04:12:54 AM

I wonder if 15mA is a bit much compared to the consumption of the effect itself? It is pushing the limits of the LED. I used to use 470R resistors without any problems as well, but 1k or 2k2 resistors don't seem to make the LED any dimmer. I'd go with the biggest value that's still bright enough for you, so as to conserve power and lengthen the lifespan of the LED.

grolschie · May 10, 2009, 06:19:18 AM

Errr...... I tried the 470ohm resistor, no joy also..... because I had forgotten to wire the 3PDTs to the ground. Something so simple and I didn't see it.

Anyways, I soldered two wires in there and these blue LEDs are so bright! Any recommendations of a value that would tame them alot, but still work ok? Thanks and sorry about that.

Cardboard Tube Samurai · May 10, 2009, 06:32:38 AM

I think the last time I was having the same brightness troubles, I went through a whole range of resistor values and it wasn't until I got somewhere up around 4k7 that it was reducing the brightness to a bearable level. I really don't understand why people insist on "ultra-bright" LEDs in effects applications

grolschie · May 10, 2009, 06:34:36 AM

Thanks. I wanted blue, simply beause it wasn't green or red.

tranceracer · May 10, 2009, 02:54:05 PM

Quote from: grolschie on May 10, 2009, 06:19:18 AM
Errr...... I tried the 470ohm resistor, no joy also..... because I had forgotten to wire the 3PDTs to the ground. Something so simple and I didn't see it.

Anyways, I soldered two wires in there and these blue LEDs are so bright! Any recommendations of a value that would tame them alot, but still work ok? Thanks and sorry about that.

If you have a 100K pot laying around you can try using that to "dial-in" the brightness of the LED then take the reading of the POT when you've found the right setting. I've done this and it's saved me a lot of time swapping out resistors.

Andi · May 10, 2009, 03:09:04 PM

Quote from: Cardboard Tube Samurai on May 10, 2009, 06:32:38 AMI really don't understand why people insist on "ultra-bright" LEDs in effects applications

They can be handy for daytime outdoor stuff. Generally I'd agree though - I use ultrabrights but a fairly big resistor so you get plenty of brightness but not too high a consumption.

R O Tiree · May 10, 2009, 04:06:33 PM

Quote from: tranceracer on May 10, 2009, 02:54:05 PM
If you have a 100K pot laying around you can try using that to "dial-in" the brightness of the LED then take the reading of the POT when you've found the right setting. I've done this and it's saved me a lot of time swapping out resistors.

Better to use a 10k pot. The useful range of current limiting resistors for a 9V supply is in the 470R to 5k or so. Using a 100k pot puts you in the 0% to 5% band, as opposed to 0% - 50% band for a 10k pot. Much easier to dial it in?

tempus · May 10, 2009, 04:17:17 PM

QuoteI really don't understand why people insist on "ultra-bright" LEDs in effects applications

Well, if you can use a 4.7K resistor it'll draw a lot less current, which is nice in any case, but especially if you're powering them directly from uProcessors.

grolschie · May 10, 2009, 04:39:36 PM

Thanks everyone. I think I will try a 4.7k this evening.

Cardboard Tube Samurai · May 10, 2009, 04:59:01 PM

Quote from: tempus on May 10, 2009, 04:17:17 PM
QuoteI really don't understand why people insist on "ultra-bright" LEDs in effects applications

Well, if you can use a 4.7K resistor it'll draw a lot less current, which is nice in any case, but especially if you're powering them directly from uProcessors.

I'd still prefer to not be blinded when switching my pedals

grolschie · May 10, 2009, 05:01:57 PM

Would it still light up with a 10-15k?

biggy boy · May 10, 2009, 05:05:43 PM

Quote from: Cardboard Tube Samurai on May 10, 2009, 04:59:01 PM

I'd still prefer to not be blinded when switching my pedals

Hey maybe that's why guys like Slash wear sunglasses when they play

80k · May 11, 2009, 12:29:41 AM

I always use the highest I can to conserve power. With blue and white and violet LED's, I have been able to get good results with as high as 7.5K, I believe.

grolschie · May 11, 2009, 01:54:35 AM

Thanks y'all.

grolschie · May 11, 2009, 04:34:12 AM

Update: 4.7k's work nicely. Thank you all.

earthtonesaudio · May 11, 2009, 06:25:59 AM

Before you put the LED in the pedal, you can check out various resistors on the breadboard. Use the biggest value that still gives you enough light to save on power consumption. Less current means less audible popping, also.

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Voltage drop resistors for LEDs?