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

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5 years ago
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)  frollard5 years ago
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?
5 years ago
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)  frollard5 years ago
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
5 years ago
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)  frollard5 years ago
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
5 years ago
That's the correct way! Since it seems sorta complex, this is why many circuits are made on pcb, circuit boards.
3dub6 (author)  frollard5 years ago
Thanks a lot
5 years ago
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.
iceng5 years ago
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)  iceng5 years ago
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
5 years ago
I want a 7v battery!