How to make a cluster of 26000mcd Led's

[PrebKlok]

How to make a cluster of 26000mcd Led\'s

I have just bought 50 x 26000mcd LED's on Ebay. I would like to make a cluster of 6 or 7 LED's, with a CR123A battery as energy source. How would I do that smartest. The LED's are rated to a voltage drop at 3.3V-3.5V and a current at 25 mA.

 

[OddOne]

Re: How to make a cluster of 26000mcd Led\'s

You'll need to provide more voltage than just a single 123 to drive them efficiently, or provide a step-up regulator to boost the batt voltage to what will run the LEDs at their desired current.

You could easily drive a cluster of seven in parallel off a Madmax 350 and push ~70mA into each, which should be awesome if the LEDs are able to handle that kind of overdrive. /ubbthreads/images/graemlins/grin.gif

oO

 

[PrebKlok]

Re: How to make a cluster of 26000mcd Led\'s

Hmmm... I read somewhere that you shold NOT put LED's in parallel, since the voltage drop is not exatly the same over different LED's => the LED's with lowest voltage drop would eat all the current.

 

[OddOne]

Re: How to make a cluster of 26000mcd Led\'s

Yes, that is quite true. However, I've found that if I get LEDs from the same batch there's not enough difference to be a problem. For example, I got a bunch of Nichia NPSW500s that were of the same manufacturing batch and the variation in Vf was only a few milivolts at its widest when I fed each 30mA during individual testing. I subsequently paralleled twelve strings of three in series each and ran them off a 12VDC regulator with no resistors and had no problems at all.

If you test your LEDs and their Vf is within a percent or less then you shouldn't have any problems paralleling them.

oO

 

[Doug Owen]

Re: How to make a cluster of 26000mcd Led\'s

[ QUOTE ]
PrebKlok said:
Hmmm... I read somewhere that you shold NOT put LED's in parallel, since the voltage drop is not exatly the same over different LED's => the LED's with lowest voltage drop would eat all the current.

[/ QUOTE ]

Sorta true. The lowest Vf will get *more* current, but not all since it has dynamic resistance. That is Vf changes with current. About 15 ohms worth for typical 5 mm parts. This means that you pick up almost .5 Volts more for an additional 30 mA. So mismatches, as long as they're reasonable are not really a problem. Only a few mA 'moves around'. You can buy some extra insurance and add a bit more resistance, say 10 ohms, in series with each LED.

Give it a go, you've little to loose....

Doug Owen