Resistors for each branch?

[**DONOTDELETE**]

Hello,
This is my first post, though I've lurked for some time. I'm messing around with multi-led arrays. When you run LEDs in parallel, do you need a separate resistor for each branch? I read on another forum (which I can't find now ) that if you have just one resistor for the array, you risk blowing the whole array if one led fails, because the same total current is divided amoung fewer branches. Doesn't make sense to me: wouldn't you just drop the total current?
How do you figure total current, anyway? Just multiply LED current by number of branches?
I would much rather save space with just one LED if I could get away with it. I'm trying to fit a lot of LED's and associated circuitry into a small space.
Thanks,
Mcd

 

[papasan]

i've done it in the past with just one resistor for the bundle of LEDs, i'm not too worried about one failing and causing the rest to be over-driven, just doesn't seem that likely to me...on the other hand i've had a couple of electrical engineers tell me that using discreet resistors is more efficient (still not sure why)...

of course, the best way to do it is run the LEDs in series and not use a resistor, but this requires a closely matched power supply, so not usually viable...

to figure out the current, add the mA together and use that to figure your resistor (i.e. 3x20=60mA @ 3.6v for 3 LEDs)...

 

[StuU]

If you run the leds in series, use a dropping resistor in the series.

For a 9V battery use two leds and a 100ohm resistor in series. The leds take up 3.5V each and the resistor drops the last 2V.

And yes, I try to use separate dropping resistors when running in parallel. This is because of variations between the leds whereby an led that tends to run at higher amperage will have a shorter lifespan.

 

[papasan]

oh yeah, i've also heard that if you only use one resistor for LEDs in parallel they will all get 'locked' to the LED drawing the least amount of current...

 

[Marked]

Resistors are rated in watts like 1/4 1/2 watt. Don't go over that rateing and you will be ok.