Can a 3.3v LED be used with 2 AAA batteries without a resistor?
I want to make a torch with a 3.3v super-bright LED (used for case modding) with a 25mA forward current. The power source will be 2 AAA batteries (~1.5v alkaline) connected in series to make ~3v. Is a resistor required, and if so, what is the colour code/value?
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No resistor required if the forward voltage (Vf) is 3.3 volts and the battery voltage is 3V (2 AAA or 2 AA or 2 C or 2 D batteries in series) If you were to use a 9V, however, then you'd need one.
Aug 16, 2009. 1:50 AMawang8 (author)
says:
Thanks for the answer. That's exactly what I thought, but I had to made sure first (LEDs are like $4 each from local electronics store now.)
Thing is, they won't reach full brightness either on a 3V supply. You really NEED to drive them a little harder, from a bigger supply to reach rated current and brighteness.
True strike, but...
Often times, LEDs reach so close to full output at a slightly reduced voltage that it makes no functional difference. It really depends on what you're trying to do.
My main concern would be that the LED simply wouldn't light, although If published as Vf = 3.3V, the min/max spread for Vf is likely in the range of 3-3.6V. Design trade-off...more batteries and added regulation circuit vs lower brightness...
Often times, LEDs reach so close to full output at a slightly reduced voltage that it makes no functional difference. It really depends on what you're trying to do.
My main concern would be that the LED simply wouldn't light, although If published as Vf = 3.3V, the min/max spread for Vf is likely in the range of 3-3.6V. Design trade-off...more batteries and added regulation circuit vs lower brightness...
Aug 27, 2009. 11:36 PMawang8 (author)
says:
Actually, the min/max is 3.1/3.4 but with 3.0/3.1 theres not much difference, and I find it works fine (finished building torch).
Sep 22, 2009. 9:33 AMPocketBrain
says:
Yes, but light intensity will drop off rather quickly as your cell expends energy and the voltage (and current capacity) drops. Search for the Joule Thief, a dead-simple, easy-to-build circuit that can boost the output of a 1.5V battery to drive a 3.3V LED. Also, you can get standard-size (20-30mA) LEDs for a quarter nowadays. I get them locally at Marlin P. Jones (10 for .25 each, or .43 each for the big ones).
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