Hi I am looking into wiring LED's into a parallel circuit. I need to wire 400 red LED's for my project and really I dont know what I am doing. The are 5mm LED's 5000mcd 1.8-2.2V and 24mA max. I would like to use a rechargable battery but doesnt matter how big or small it is. Anyway if anyone has any advice on what voltage to use and what size resistor or resistors it would be greatly appreciated. Thank you very much.

 

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

Chapters 1 and 2 cover the basics you'll need.

 

If you have 10 rows of 40, and you use two 12v batteries in series for 24v you will need a 220ohm resistor for each row of 10.

You can use this wizard to get your wiring diagram:
http://led.linear1.org/led.wiz

Input:

 24 for source voltage (two 12v batteries wired in series)
  2 for forward voltage (Vf)
 20 for current (mA)
400 for number of leds.

It will give you a few options. Solution #2 (the last one) will give you the 10x40 layout

and be sure to read Bill_Marsdens LED Blog post:
http://forum.allaboutcircuits.com/blog.php?b=378

[ed]
Jeez Bill, you got in there as I was writing. So much for being fast.
[/ed]

 

if I used two 6V lantern batterys would that work? does anyone know the current output on one of those? I cant find where it says.

 

Define what you want to do a little more carefully, colors of LEDs, quantity, that sort of thing. You must have a resistor, or an equivalent.

Red is a good color, each LED will drop around 2.5 volts. They can vary depending on the specs, so that would be useful, but for now I'll assume 2.5V Vf (forward dropping voltage).

If you have 12V (2 6V batteries in series) then you can have 4 of these leds per chain, each with it's own resistor. If the LEDs take 20ma (0.02A) then it goes something like this...

4 LEDs at 2.5V drop 10V

12V - 10V = 2V

To calculate a resistor for 20ma then

R = 2V/0.02A, or 100Ω. A ¼W resistor will work fine.

This is an illustration from the article I recommended you read. If you want to make sense of it, then follow up and read the article.

 

Well I am putting these led's into a table. They are red and the specs on them are further up in a previous post. I understand most of what yall are saying but what I dont get is doesnt it matter how much current my source pushes out? like if i used 2 6V lantern batteries the wiring diagram from that like above says that 1340mA is pulled from the source? well does those 2 batteries put out that much or am I not grasping this concept the right way? thank you

 

doesnt it matter how much current my source pushes out? like if i used 2 6V lantern batteries the wiring diagram from that like above says that 1340mA is pulled from the source? well does those 2 batteries put out that much or am I not grasping this concept the right way?

You should look at the datasheet for the lantern batteries at the website of a battery manufacturer like Energizer. They don't make carbon-zinc or "heavy duty" carbon zinc batteries anymore but make alkaline batteries that are much better.

With a load of 1.34A the battery voltage will be only 8.8V in about half an hour. The LEDs will be dimming then after 15 or 20 minutes they will be turned off because the voltage will be too low.

EDIT:
The datasheet was made by a little schlool kid. He used the voltage of a single battery cell instead of the voltage of 4 cells that are in this battery. I corrected it.

 

The batteries provide according to voltage and load (resistance). The math formula Ohm's Law dictates the relationships here.

E=IR, or Volts = Current X Resistance

If you go through the article it shows how to match the current limiting resistor to the string of LEDs. If done correctly each string is 20ma (0.02A). The total number of strings then dictate the total amount of current.

Two 6V lantern batteries will make a voltage source of 12VDC (more is better, but that is the assumption in this case). 12V will drive 4 LEDs per string. You want a total 400 LEDs, that means 100 strings at 20ma each, for a total current of 2A. These batteries are not going to last long, sure you don't want a power supply that uses line voltage?

If you used 2 12V batteries it could be wired for 24V, this could drive 8 LEDs, for a total number of 50 strings, each at 20ma each, for a total of 1A from the battery.

No way around it, 400 LEDs is going to use a lot of current, and suck battery life quickly.

If you use lead acid batteries you have to be extremely careful not to discharge the batteries more than 50%, as it will permanently damage them.

A typical lantern battery as shown in AG's post uses 4 C cells, so you will get better results using D cells, but it is going to be expensive going that route.

 

ok well I need these led's to definitley last longer than like 2 hours. now I am either thinking about just using a outlet 110v supply but would prefer not to or I could use Li-Po batteries. Which one sounds better? If I bought like 2 11.1 V Li-Po batteries that run 1500mAh that would power all 400 for awhile right?

 

Two 1500mAh lithium batteries will light 400 LEDs (2A of current) for about half an hour on one full charge. You might see them dimming as the battery runs down. But the lithium batteries will not fully discharge because the LEDs would be turned off because the voltage is too low.

 

so pretty much get as high as an amp battery as possible to make them last longer.

 

allright well I will just look for the highest amps I can and buy 2 or 3 of them to get them to last as long as possible. mabye 4000mAh or more x3 will suffice.

 

Why do you need so much light power?

 

what do you mean? why do i need the leds to stay on for so long? well I am building a poker table and the led's need to last through at least 1 sitting on a single charge. so I will just buy probably 2 gel cell batteries that are 12v and run either 7000mAh or 12000mAh and that should work.

 

Light power is brightness. 400 LEDs will be too bright.
Are you in Nigeria, India or Pakistan where the electricity is turned off for most of every day (load shedding)? Why not use an electrical power supply instead of batteries?

 

no I would just prefer not to have a cord that needs to be plugged in in all honesty. I could do that I would just rather use batteries. How do you control the brightness of the led's? or are you just saying that that many together would be bright? or does it have to do with amperage or voltage im pushing?

 

See PWM.

Have you read this?

LEDs, 555s, Flashers, and Light Chasers

It covers the topic pretty well, plus has ideas for special effects. A µC might be better, but that is up to you.

If you really just have to use batteries buy a large bank of D cells, or several (more than 4) SLA (sealed lead acid) types. It will be expensive though, and you will have to work out a charging scheme for the reusables.

You can get battery holders from Radio Shack. I think 16 D cells would do it, wired for 24 volts. If you aren't using these for light you can reduce the current from 20ma, which will also let the batteries last longer.

 

oh ok well how much should I reduce the current to? in half? I want them to glow not necessarily be like flashlights or anything. I was going to order 4 gel cell batteries off of ebay that are 6V 12A per hour. That would do the trick I think. If I even need to use that many. if I used D cells how often would I have to replace the batteries though if I ran them for 4 hours a week? and how do you reduce the current?

 

It isn't 12 amps per hour. (Quite confusing, i know) Its usually rated in discharge over a 20hour time frame. However you shouldn't discharge those types of battery more than 50%. Its safer to divide that in half for real usage.

So you would have to wire them in series to get 12v and would keep the 12ah. In series wiring, you combine the Volts but keep the Amps.

Parallel wiring, you keep the Volts and combine the Amps.

If you build this system with 400 leds, you will want to dim them via PWM. Say...a 50% duty-cycle.. That will draw about half of what you were expecting. Your lights will be less bright, and last much longer.

 

Have you thought about using strip leds?
LED Type: 3528 SMD LED
Color: red
Power: 4.8W per meter
Voltage: 12V
Application: decoration
Package: 5 meters per roll