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# Can I get some help on what resistors and how many to use to hook up 20 3mm leds? Answered

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## 12 Replies

Think of them as 3 separate strands because they use separate voltages. You don't need to run them all in series, because it's hard to make 36v with batteries.

I HIGHLY advise against 9v batteries, they are terrible in nearly every way. Cost, total storage, the damn connectors, everything about them.

Pick up a 3 or 4 AAA, AA, C, or D battery holder which outputs either 4.5 or 6v.

"I'm wanting to use 8 3mm blue leds that are 3.4v a piece, 8 3mm yellow that are 2.1v a piece, and 4 red that are 2.1v a piece."

I'll set this up as if you are using a 3series holder, 4.5v:

Source voltage: 4.5v
8x blue 3.4v @ 16mA
8 yellow 2.1 @ 16mA
4 red 2.1 @ 16mA

use this website to do the calculations: http://led.linear1.org/led.wiz

Blue:
Solution 0: 1 x 8 array uses 8 LEDs exactly
+----|>|---/\/\/---- R = 82 ohms
+----|>|---/\/\/---- R = 82 ohms
+----|>|---/\/\/---- R = 82 ohms
+----|>|---/\/\/---- R = 82 ohms
+----|>|---/\/\/---- R = 82 ohms
+----|>|---/\/\/---- R = 82 ohms
+----|>|---/\/\/---- R = 82 ohms
+----|>|---/\/\/---- R = 82 ohms

The wizard says: In solution 0:
each 82 ohm resistor dissipates 20.992 mW
the wizard thinks ¼W resistors are fine for your application

Yellow:
Solution 0: 2 x 4 array uses 8 LEDs exactly
+----|>|----|>|---/\/\/---- R = 22 ohms
+----|>|----|>|---/\/\/---- R = 22 ohms
+----|>|----|>|---/\/\/---- R = 22 ohms
+----|>|----|>|---/\/\/---- R = 22 ohms

The wizard says: In solution 0:
each 22 ohm resistor dissipates 5.632 mW
the wizard thinks ¼W resistors are fine for your application

Red:
Solution 0: 2 x 2 array uses 4 LEDs exactly
+----|>|----|>|---/\/\/---- R = 22 ohms
+----|>|----|>|---/\/\/---- R = 22 ohms

The wizard says: In solution 0:
each 22 ohm resistor dissipates 5.632 mW
the wizard thinks ¼W resistors are fine for your application

3dub6 (author)2012-01-07

I had planned to run them off 3 different battery supplies, so I could run 8 leds with just 4.5v or 6v is that what your saying? Do I need to put a resistor on each bulb?

frollard (author)2012-01-07

the drawings show what you need to do. in the red case, you put 2 leds in series with 1 resistor

the leds are -|>|-
the resistors are -/\/\/- in the pictures

the blues have one resistor per led

the yellows are like the reds, 2 leds, one resistor.

don't worry about cost, they're worth a few cents each. Many ebay led retailers will include free resistors when you buy the leds, just tell them the ohmage you want.

3dub6 (author)2012-01-07

The leds I ordered doesn't come with resistors, but I'm ordering a big bundle of resistors, what I need to know is if I hook it up the way ur saying is all of the leds ran off of one battery source, or is it a battery source per led color, and also what kind of batteries would u use for this, and is this wired in series or parallel

frollard (author)2012-01-07

nope, same voltage on each, that's what the resistor is there for, so each strand which has a different voltage requirement gets the same power to each.

So, imagine every line of my answer above, with 1 or 2 leds with a resistor on each, and ALL the + connections are connected to battery +, ALL the - connections are battery -.

LEDS are polarized, they only go one way, that's what the arrow means.

Resistors can go either way.

3dub6 (author)2012-01-07

Ok I'm going to make a box of some sort to put all these resistors in, if I run a wire from positive side of my battery and split it off to each resistor an run from each resistor to the positive side of the corresponding leds, then run a wire from the negative of each led to the negative side of the battery would that work, or is there a simpler way to do this. Sorry for asking so much, but just want it to work rite and thanks for the help so far

frollard (author)2012-01-08

That's the correct way! Since it seems sorta complex, this is why many circuits are made on pcb, circuit boards.

3dub6 (author)2012-01-08

Thanks a lot

frollard (author)2012-01-07

and each 'string' in the solutions above has the + connections connected together to battery + and the right side -'s connections together (all in parallel) to negative.

iceng (author)2012-01-06

well we need to know what kind of LEDs and what battery source DC voltage.
Or do you want us guess yelow LED RUNNING uh an 7VDC battery ?

A

3dub6 (author)2012-01-06

I think I may have figured it out. I'm wanting to use 8 3mm blue leds that are 3.4v a piece, 8 3mm yellow that are 2.1v a piece, and 4 red that are 2.1v a piece. If I did this rite 8 blue is 27.2 volts so I'm going to use four 9v batteries that makes 36v . I done some research 36v-3.4v=32.6÷.16A=203.75 so I need 2 100 ohm resistors and a 7.5 resistor, yellow is 16.8v so im using 2 9v batteries 18v-2.1v=15.9÷.16A=99.375 so I need a 100 ohm resistor for that and the red is 8.4v so I would use 1 9v battery 9v-2.1v=6.9÷.08A=86.25 and I would use 100 ohm resistor

frollard (author)2012-01-06

I want a 7v battery!