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3 colour portable LED Light Box wiring - Please Help Newbie Answered

I am building a portable LED Lightbox with a few rows of LEDs amounting to about 400 different coloured LEDs (red, green, white). Each row will be connected parallel as I want to use normal batteries to power it and not have too many wires. Ideally it should use AAA batteries and 1 switch to switch it on.

Each row will consist of 15 - 30 lights each. Each row will be the same colour (eg. 1 row red, 1 row green, 1 row white).

My question is:1
(1) Should I connect all the lights together and hook it up to the same battery pack? OR
(2) I should connect just the lights on each row together and wire all the positive of each row to the battery pack. Will it cause a short circuit since there will be many wires to just 1 battery pack.
(3) I have tried and each LED can be powered by a 3V battery, but is 3V enough to power 400 LEDs in parallel?
(4) If it is not enough, I could power a few rows together meaning I will have a few battery holders, but can all the battery holders be joined to just 1 switch or I need many switches as well?




Best Answer 8 years ago

You won't be able to run 400 leds off a couple of AAA batteries; just not feasable.

Each led ideally draws about 20 mA of current - x400 = 8000mA  = 8 amps. @ average 2.5 volts is 20 watts.  AAAs just can't do it.

SO; you will want to run off a higher energy source - wall adapter, beefier batteries, etc.

You need resistors to control current to the leds

You should run as many series leds as possible (with your power supply) with a resistor on each series string.

use an led calculator like:   http://led.linear1.org/led.wiz

Input your numbers:  source is your battery pack:  More is better = more efficient part count.  For a huge array of 400 leds, you want at least 12 volts DC.

Diode forward voltage varies based on your led colour:  red and amber are low, ~2-2.5, white and blue are higher ~4.  CHECK THE DATASHEET SO YOU DON'T BURN THEM OUT!

Forward current is 15-25mA  Aim low to be safe if you're unsure.
Total number of leds:  You have to calculate each group of colours separately because they use different voltages.  

12 -- 2.5 -- 20 -- 132 = 33 x 4 series leds with 100 ohm resistor.   etc

Thanks for your detailed explanation.

The guy who sold me the LEDs did recommend a 12V battery but it is quite big (like a mini car battery) and it will be quite difficult to bring around and the Lightbox which acts like a signage needs to be carried around or even raised above the head sometimes. Moreover if I get that, if it runs out halfway through the event, there will be nowhere to buy it. That is the main reason why I am thinking of getting normal batteries.

How about if I put AAA or AA (or even C or D) batteries in a series. I could make it 4.5V or 6V using one of those AAx4 batteries holder. I will not wire all the 400 LEDs together. I am thinking if I can wire maybe 20 to 30 LEDs together and have many of those 'strings of LEDs' and in the end wire them all up to the battery. Will that work using normal batteries?

Also I found 9V batteries that is rectangle. Maybe that could work?

The reason I do not want to wire in series is because if one bulb fails, the whole string in that series will not work.

Many thanks for your advise again.

**the guy who sold them...***

he sold you resistors to go with each individual led - and it will be very inefficient to run each one parallel, because every resistor will have to burn off 10 of the 12 volts as heat so that the led is not damaged.  Instead, use the calculator and get the right values for a more efficient drive.

Oh no, he sold me some resistors but he said to wire it to the front of each parallel. Meaning each 'string' of LED only have 1 resistor on the first one that is to be connected to the battery.

Exactly, so if its designed for 12 volts
the led gets 2-4 volts, and the resistor wastes the rest.

Put series strings of leds to properly use the voltage, and the resistor is just there to use up the rest.  (4x 2.5v = 10 volts, the resistor only needs to get rid of 2 volts, not 9.5) 
9.5v @ .02a = .19watts PER RESISTOR, by extension, per led, wasted.
2v @ .02a = .04 watts wasted per series string of 4 leds...

extreme example:
You could run an led on a thousand volt power supply, but you'd have to burn off 998 volts in a resistor; at 20ma (.02A) would be almost 20 watts burned off for one led.  You could run ~400 leds on a 1000 volt source, in series, and have no wasted power.

As said, you have to consider total power;  No matter how you slice it, watts are watts, and you only have so much power to go around:

8 amps at 2.5 volts is ~20 watts usage.

a 'good' AA battery is 2000mAh at 1.5v multiply and you get 3 watt hours AT BEST per battery.  The batteries also have a maximum current draw, and for smaller batteries it's not that much.  6 AA's will have 18 watt-hours total, so your 20 watt draw would last just under an hour.

I would still recommend a golf cart battery in a backpack, or as lemonie suggests hooking up to a lithium cordless drill battery pack(s) for enough juice.

Again, for efficiency's sake, run at LEAST 12 volts, preferably more, 18 or 24 volts would be ideal for an array this size.  As such, you'd CHEW THROUGH batteries quickly, thus, get big batteries.  D-cells are bare minimum for capacity reasons in a scenario like this.

One thought: Make the sign animated (kipkay shows instructions to make one) that lights up DIFFERENT PORTIONS of the sign up at different times, so that the power draw is averaging much less overall.

Oh My!! The Kipkay Ultimate Sports Fans Sign is the one I am looking for!! But his uses a relay thingy which I think I will skip coz I am already confused enough as it is... HAHA!! 

Since his uses 9V batteries, I might try my luck with the 9V since it is easily available. In this case I might just wire a few letters to one battery pack and the other words to another battery pack and have 2 switches to switch them on at the same time. Would like to wire these 2 batt pack to 1 switch but I guess that would also be complicated. :op

Really thanks for your help and suggestion. :o)

Actually I am going to make the attached. A bit embarrased to show it and will take it down tmr. :op

HyungJunBoard 3.jpg

Don't take it down!  That's a fantastic design!

If you are trying to choose between AAA batteries and 9 volt, consider that 9 volt batteries are 6 AAAA batteries.  I highly recommend getting D-cell holders and working with those.  You will get WAY more bang for your buck that way.

Thanks!! :o)

Erm... Do you mean I should use D-cell holders with a series of 6xD batteries to make it into 9V?? That way I should be able to power everything up with 1 battery pack? :op

Sure would, and you'd get hours of use out of a single set of batteries = try and get rechargables...


Many thanks for your help on this. I will go for the 6xD-cell. Thanks!! Thanks!! Thanks!!

400 is far too many for AAA. Frollard has it right.
If you want portable, consider a big pack of (something) like an electric-drill pack.
Come back with a big power supply and ask either of us again.