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# LED question (Hey! they use electricity!)

#1
03-25-03, 05:41 AM
exeter_acres
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LED question (Hey! they use electricity!)

Not sure if this is the best place to ask this...
But I am wiring up a small project with 4 LED lights..
they are each 3.7v and I will run them from a 12v supply..
I know I need a 470ohm resistor but my question is this..
can I use 1 resistor and wire the LED's in series, or do I need to use a seperate resistor for each LED...?

I know resistors are cheap, but it is going in a small space and want to make it as clean as possible..

Any ideas??
Curtis

#2
03-25-03, 05:56 AM
Member
Join Date: Sep 2002
Location: welland ontario
Posts: 7,932
You can buy 12 volt LEDs.

#3
03-25-03, 06:18 AM
RickJ6956
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The preferred method is parallel - each LED gets its own resistor - but you can wire them in series with a single resistor. You may have to move up to a higher wattage resistor (1/2 instead of 1/4?).

I'd also start with a potentiometer to see what R value works best. 470 might not be right for series wiring.

I wired a string of mini LED's as "Christmas lights" around a small decorative lighthouse. I wound up using 6 red and 6 green in series with a 1/2-watt 1.2k resistor and a 9v battery. The battery lasted until about an hour after the party was over.

#4
03-25-03, 10:51 AM
grignon
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I think your best bet is to use a separate R for each LED.
The total I available is ~18mA (8.3V/470ohms)-with one R in series, if all the LEDs are perfectly matched for forward V drop(==3.7V) they'll share the I equally and just shine dimmer than 1R/1LED.
If they are not matched, you may well get high variability in the brightness of the individual LEDs-perhaps to the point where one or more don't radiate at all.
What kind of LEDs are these?
3.7V and 18mA are pretty high values for indicator types. There are standard high efficiency reds available that shine brightly at 1.8V/<1mA.
As far as space, cut the anode lead (+) of the LED short and solder a 1/4W R with one short lead to the stub. This will take very little more room than the LED itself.

#5
03-25-03, 02:24 PM
exeter_acres
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thanks for the info... I think a seperate for each is the best way..

What kind of LEDs are these?
They are very high intensity for lighting effects..

I'm a geek and they are going inside my computer!

Here are the specs
Blue 5mm LED:
Voltage norm 3.0v- max 3.3V
brightness 3000-4000 mcd

Ooops, they are not 3.7 but 3.0..

SORRY!

Thanks
Curtis