24 LED's 4.8V

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FNinjaP90

FNinjaP90

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Jan 8, 2003
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24 LED\'s 4.8V

Hello, I want to put 24 LEDs together in parallel powered by a 4.8V nimh source. It HAS to be 4.8nimh. Some have said that I needed to put a 39ohm resistor on EVERY led to lower it down to 3.6V. Is it ok if I just put one resistor at the base of the circuit and treat the 24LED's like one giant LED? If putting one at every LED gives a little better runtime/performance/lifetime I don't care. I just wanted to know if its OK to just have one resistor.
 
Re: 24 LED\'s 4.8V

As long as the LED's are well matched, you can use one resistor. Check the LEDs forward voltage @ 30ma or so for uniformity.

It will have to be a much lower resistance and higher wattage.

I build 1 watt UV lithium-ion rechargables with 8 5-mm LED's in parallel and it hasn't caused any life problems.
 
Re: 24 LED\'s 4.8V

It is advisable to use a separate resistor for each LED as it will help eliminate the destruction (by overdriving) of any LED with a lower Vf.

Given that quality white LED's will cost $1 - $2 each, and your resistors should cost about $0.02 each, this is a worthwhile investment.
 
Re: 24 LED\'s 4.8V

I made something along this line. I cheated by using one resistor for evert four LEDs.

This drestically reduced the number of interconnections and still privide fairly good protection.

By setting the current for each LED at 30mA gives a total current for each LED branch of 120mA. If one of the LEDs goes out then you still only have 40mA per LED. Even if two LEDs go out you are still only 60mA per LED and a lot of people normally overdrive their 5mm LEDs at 60-80mA.

And of course this is with FULL batteries. As the batteries discharge the current goes down increasing the safety margin.

I would recommend a resistor for every 4-6 LEDs if you are not planning on overdriving them too much.

wwglen
 
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