# Powering 40 leds on 9V or less



## R0y4L91 (Nov 4, 2009)

Hi there, 

I have to apologise now in case I say something stupid. Id like to know if there is anyway I could light up around 40 LEDs using 9V battery or a smaller battery. From my experience I have wired 44 LEDs using the 9V battery on parallel circuit. (1 resistor for 1 LED). I would like to know would it be possible/ok to use 1 resistor per ~44 LEDs on series circuit or even parallel. Or what would be the best way of wiring this amount of LEDs?


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## Gunner12 (Nov 4, 2009)

What is the goal of this project?
How much do you want to drive each LED?
What kind of LEDs are these? I'm guessing 5mm or 3mm LEDs from a small 9v rectangular battery.

Depending on the Vf of the LED, you could probably have 14 strings of 3 LEDs in series, or other combinations. If the Vf of the LEDs are too different from each other, then the LED string of the lowest Vf will get the most current and potentially burn out.

:welcome:


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## R0y4L91 (Nov 4, 2009)

Sorry I didnt give enough information. Yeah they are 5mm blue ultra bright LEDs. They use about 3.2V. In my previous project, I was soldering a resistor per LED and I thought it was a bit too much and I was wondering if there was easier way of doing it. What you mean "drive"? 

But i'd like to see if it would be possible to use less than 9V...probably 4 AAA batteries or something like that


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## Gunner12 (Nov 4, 2009)

I meant how much current do you want each LED to see.

How bright does each LED need to be? Would a single high power LED work better?

It's definitely possible to drive the LEDs with a lower voltage source.


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## lolzertank (Nov 4, 2009)

Ideally, you would use 4xAA or a Li-ion battery. In that case I would wire them all in parallel with a resistor to each LED.

If you really want to use a 9V battery (low capacity), you can put pairs of LEDs in series and use half as many resistors.


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## mvyrmnd (Nov 4, 2009)

R0y4L91 said:


> Sorry I didnt give enough information. Yeah they are 5mm blue ultra bright LEDs. They use about 3.2V. In my previous project, I was soldering a resistor per LED and I thought it was a bit too much and I was wondering if there was easier way of doing it. What you mean "drive"?
> 
> But i'd like to see if it would be possible to use less than 9V...probably 4 AAA batteries or something like that


http://led.linear1.org/led.wiz

Enter all the required specifications, and it will design the circuit for you!

If it comes back with no answer, it's not possible...


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## J_C (Nov 6, 2009)

Ultimately the fewer resistors you want to use, the higher the input voltage will have to be so you can put more of them in series.

While it is correct what others noted about some series of LED being brighter with specimens in the series that have a lower forward voltage, you could go through the tedious process of binning the LEDs yourself so you can sort them to have a similar total series sum of forward voltages.

For example, take a fixed output 3.0V supply and connecting a multimeter in series on current measurement mode, measure the current through each LED. If you have a supply even a bit lower than 3.0V even better as you stand less chance of damage to LEDs with especially low forward voltage but most should survive 3.0V fine.

Having gotten readings on each, put them in series so the sum of the current reading of all in the series is as close as possible to the other series.

The result would be running parallel strands of 3 LED in series off the 9V battery with no resistors at all, but as battery voltage drops you will suffer more light reduction and be unable to as fully drain the battery since it will be below the summed forward voltage of each series at a point where there's insufficient light output (depending on the project goal).

Ultimately it seems the ideal is a custom designed boost circuit to power all the LEDs in one series strand, but it will be more costly and possibly not much smaller than just using a bunch of 1/8W resistors.


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## alpg88 (Nov 6, 2009)

i build a light with 45 leds driven by 6v, all in parallel, and 1 ohm sand resistor, i know connecting that many in parallel isn't the best thing to do, but it worked fine, thou, i had to buld 10 sub c 5s2p pack that i put into 6v lantern batt body, alkaline (even thou that big) dropped voltage too much.

if you working with 9v best bet is to wire 2 leds in series with resistor,


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## J_C (Nov 6, 2009)

alpg88 said:


> i build a light with 45 leds driven by 6v, all in parallel, and 1 ohm sand resistor, i know connecting that many in parallel isn't the best thing to do, but it worked fine, thou, i had to buld 10 sub c 5s2p pack that i put into 6v lantern batt body, alkaline (even thou that big) dropped voltage too much.
> 
> if you working with 9v best bet is to wire 2 leds in series with resistor,



What kind of LEDs did you use? Isn't that roughly ~85mA per LED at full/peak battery pack voltage, 60mA at nominal 6V?


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## alpg88 (Nov 6, 2009)

i used 10mm 140000mcd 20ma rated at 3.6v leds, i didn't calculate resostor value, i hooked up led cluster to battery, and used pot and dmm to get 3.6v than i mesured what value pot was adjusted to, and replaced it with big square sand resistior.

i try to use this way as much as i can, cuz calculating doesn't take into accout voltage drop of the battery.


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## TorchBoy (Nov 7, 2009)

And your method doesn't take into account Vf variation from spec across your batch of LEDs, alpg88.


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## alpg88 (Nov 7, 2009)

TorchBoy said:


> And your method doesn't take into account Vf variation from spec across your batch of LEDs, alpg88.



no, it doesn't.


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