Resister values

I am converting a 3-D cell mag lite to LED's. The reflector is bad so I ordered 12 5mm reflectors to fasten on the lens. I ordered 12 8000 mcd, 3.3 volt, 20ma LED's. The resister value for each is 60 ohms at 4.5 volts. What value resisters do I put on all 12? How do I know what wattage each LED needs? /ubbthreads/images/graemlins/bowdown.gif

please get some information at
www.ledsupply.com
there are some good information /ubbthreads/images/graemlins/smile.gif

Thank you, I got the same thing, 60 ohms. I found a dozen 62's today. Had to get 1/4 watt.

Oops, looked at them and the were 1/2 Watt. I asked for 1/8. Since there is no reflector I had a lot of room. I got 12 little white 5mm reflectors and sandwiched them between 2 clear plastic disks. I drilled the rear one for the reflectors and held the two together with 3 4-40 screws. The front one is the lens. I plugged the reflectors in the rear one. I wired the 8000 mcd's up with a 62 ohm resister on each LED. This is a 3 cell Mag lite. They are very bright and cast a wide beam with no sqwiggles or lines, just a smooth light. Now if I could just find silvered reflectors! A lot of light goes to the rear through the plastic.
My question is, why can't I just use one 62 ohm resister for the 12 LED's instead of one for each? Does anyone know where to get these little reflectors with a sil /ubbthreads/images/graemlins/naughty.gifvered surface?

[ QUOTE ]
miner said:
My question is, why can't I just use one 62 ohm resister for the 12 LED's instead of one for each?

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You can, but the 20 mA that goes through that one resistor will be all the current there is, it'll have to divide itself between those 12 LEDs, which will be very dim.

You can drop the value to 5 ohms (60/12) to bring the total current back up, but there's no guarantee it will divide anything like evenly.

Individual resistors are probably the best call.

Doug Owen

Its best to use individual resistors, that allows each LED to find its Vf, and should a LED burn out - the current would stay the same for the others, if you use a single resistor for many and one LED fails then the current would go up on the others and a run-away condition could start

I got some of these to use as little reflectors.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=66954&item=3831660118&rd=1

Thanks guys, I thought of the runaway condition and can see it happen with too small of resistances too.
RD, those are some nice reflectors, might have to send for some.