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Quick LED question

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lee_uk

New Member
Hi everyone, Im a guitar player and ive recently been making a guitar boost/volume pedal, i came up with the idea of adding 32 LED's to the top of the unit, they are the blue ones, the idea is when i click the pedal i get a volume boost from the pedal plus all the LED's light up.
Ive linked all the LED's in series all the + together and all the - together and wired them straight onto 2xAA 1.5v alkaline batteries giving an output of 3v, via a DPDT toe switch, so the guitar signal is seperated from the LED side.

My quick question.. do i need to add a resistor to the circuit? will my LED's burn out if i don't? bearing in mind the LED's will only be lit while the boost is on.

Thanks

Lee.
 

kpatz

New Member
You should have a resistor to limit the current. Ideally, one per LED since they don't always all draw the exact same amount of current. Though blue and white LEDs seem to be more tolerant of no-resistor usage if the voltage is low enough. I've seen blue LEDs driven from 3 button cells without a resistor.

You might find the blue LEDs don't work well at 3V, you might have to add another battery to get 4.5V. Blue and white LEDs need more voltage than the more typical red/green/yellow/orange LEDs.
 
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Hero999

Banned
You can't power blue LEDs from a two AA cells without a boost converter.

What's their nominal voltage drop?

Probably 3.3V to 3.8V

Connecting them in series will only make matters work: you'll need a power supply voltage of >112V to power them all in series.
 

Berserk87

New Member
all the blue leds ive seen have a forward voltage of 3.2V - 3.8V, which means you might not be able to light them up on that 3V.

20mA would work fine, so 3V/.02 = 150 Ohm resisters attached to each led.
 

lee_uk

New Member
3v lights all the LED's quite brighty, with 4.5v they glow brighter but get warm after a few seconds, i have 32 of them all working on 3v, they are all the same type, can i get away with using 1 resistor? if so do you know what the value would be. and would 3v burn them out after time?
Thanks for all the help so far.

Lee.
 

audioguru

Well-Known Member
Most Helpful Member
The blue LEDs need 3.5V when their current is 20mA. But at 3.0V they light dimly when the alkaline battery is brand new (but not for long).
 

kpatz

New Member
3v lights all the LED's quite brighty, with 4.5v they glow brighter but get warm after a few seconds, i have 32 of them all working on 3v, they are all the same type, can i get away with using 1 resistor? if so do you know what the value would be. and would 3v burn them out after time?
Thanks for all the help so far.

Lee.
3V is on the low side for a typical blue LED. But maybe yours work fine at that low a voltage. Test it with a single LED though. Hook 1 LED up to the 3V batteries and measure the current. If it's within the recommended current for the LED (usually around 20mA, or less) you're good to go. But, before you continue, try with weak batteries that are only putting out 2.5V and make sure the LED isn't too dim.

If the current draw of one LED with fresh batteries is over 25-30 mA, then you'll need a resistor, probably a small one of 100 ohms or so. If you want to use 1 resistor for all 32 LEDs, you'll need a low value (maybe 10 ohms) and a high wattage (say, a 5 or 10 watt) resistor. Also, experiment to see the minimum amount of current that gives you the brightness you want, since you'll be multiplying that current by 32 when you hook all the LEDs up, and the more current you're drawing, the faster you'll drain the batteries.
 
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Hero999

Banned
If you want to use 1 resistor for all 32 LEDs, you'll need a low value (maybe 10 ohms) and a high wattage (say, a 5 or 10 watt) resistor.
I wouldn't recommend that because the LED with the lowest voltage will hog all the current.

I'd recommend three AA cells and a 51R resistor per LED.
 

audioguru

Well-Known Member
Most Helpful Member
32 LEDs at 20mA each draw a total current of 640mA.
Two brand new AA alkaline cells will drop to 2.6V in about 15 minutes then the LEDs will be off or be extremely dim. You will see them dim during the 15 minutes life of the battery.
 

Hero999

Banned
They won't draw 20mA each if connected to a 3V battery, even with no current limiting resistor.
 
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