Wiring LEDs together

forget about PP3 9V batteries, they are unsuitable.

From the Duracell MN1604 datasheet:

Rated capacity 580mAH. At just 25mA load the volts drop to about 7.5V in ten hours, and you want over 10 times this current

martin

A great opportunity to do my 9V battery rant. Never design anything that needs a 9V battery. Use a DC/DC if you have to. OK, there are exceptions, notably if the current required is near the self-discharge of the 9V battery.

Long live the AA cell.

Back to the LED question, the experiment would be to power one series pair, then two, three, etc and watch the source impedance of the 9v battery become part of the equation.

Back to the rant, two AA cells are about the same size as one 9V battery. AA cells are about 2.5AH. Now granted, alkalines drop their voltage linearly, but as a figure of merit, do this comparison:

9x.58=5.22 3*2.5=7.5 so you get 40% more operating time. AA cells can easily be purchased for under 25 cents, while it is hard to get a 9V battery for less than a buck. So you get 40% more operating time at 1/4 the price.

chomp

ARGHH wimp republican stuff

checkout the F cell from GP batts GP1300FH

monster stuff, but can't be charged in one hour, the cables will melt

martin

Okay, so it sounds like everyone pretty much agrees that I shouldn't use 9V batteries. So... what would you all recommend? Should I just pick up some of those 4x AA battery holders and use those?

Anybody have any idea what the mAh on a standard AA would be like? I mean - will they be able to drive 50+ LEDs at the required voltage and mA?

Or... would it be better to go with something like the F cell from GP that martin recommended? I do need to keep costs as low as possible, but I'll do what's best.

Thanks a lot for your replies, guys!

Just found the F cell today, havent got a price, but it may be worth looking at 12V lead acid motor bike batteries. But you havent said how long you want the leds to stay on for, 1 hour,

10 hours etc.

zz need sleep, bye

martin

Hey all!

Thanks for the reply. These LEDs are for a theatre production. They have to stay on each night for about 1 minute and theres about 20 performances. If I can power them for 45 minutes on one battery, I'll be happy.

The LEDs are also installed inside hats, so there's not a lot of room to place a big lead acid battery. I'd like to stay smaller than around

8 cubic inches or so (they're top hats).

Thanks! Let me know if there's any other info that might be helpful. I really appreciate your help!

The Flashlight Manufacturers in China are doing this by paralleling 52 LEDs and driving them with three AA cells in series and not a single resistor anywhere (except, of course the internal resistance of the cells and the LEDs). But of course, they have control over the LED manufacturing process and can assure that all the LEDs have about the same forward voltage drop so that current hogging is minimized. I'm also amazed at the color temperature match they are able to achieve within each flashlight. Maybe this all comes about through automated testing and binning of raw LED manufacturing output, but for instance, a complete 32 LED flashlight with three AAA cells in beautiful anodized aluminum winds up costing the end user just $9.50 (probably less if you buy it at the monthly Electronic Flea Market in Cupertino CA - - See this vendor's website

. It hardly pays to mess around with LEDs in a do-it-yourself effort any more.

If you have an LED set that's not well controlled for forward voltage, then you're probably going to have to use resistors and higher voltage, probably losing some power in resistor dissipation. I'd go for the inexpensive finished flashlight instead. But if what you're planning involves a non-standard configuration, than I would still order a finished flashlight and just grab the matched LEDs out of it in order to avoid using resistors.

Good luck

Chuck

Thanks for the reply.

Unfortunately, this is kind of a custom rig, and we do require the use of LEDs that we purchased from a special retailer. They are super bright 12000 mcd white LEDs that run at a typical voltage of 3.4 V and no more than 30 mAh. We have 9 hats and each hat requires somewhere between 35 and 65 of these LEDs.

I have another question. I did some research and found these lithium

9V batteries at Radios Shack, which advertise 2000 mAh. So I picked one up, and hooked it up to the configuration that I described in my original post (54 LEDs, 27 pairs w/ 82 ohm resistors on each pair). Only about 10 of the LEDs were at their full intensity!

I was baffled by this, because I figured, if the LEDs were pulling 800 mA, the 9V battery should be able to supply it in full capacity, right? It made sense that my normal alkaline batteries dimmed because they pushed under 600 mA, but these lithium ones were pushing the full load in theory. Am I totally misunderstood on the concept of electricity? Can anyone explain this to me? It seems like I'm providing the LEDs with more than enough power, but they're still dimmed - I'm sure it just comes down to the fact that I don't know what I'm doing - but I'm going to keep trying anyways - with your help of course!

Thanks again for your help!

I think you mean "mA", not "mAh" -- the first (milliamp) is a measure of current, the second (milliamp-hour) is current integrated over (multipled by) time.

Do you have a multimeter? Have you measured the current going through each LED string? Depending on the battery itself, it may or may not have been able to deliver 800mA. A cheapy carbon-zinc 9V generally can't (without significant voltage drop), whereas a NiCad can -- I'm not sure about lithium cells.

Note that the battery being 2000mAh has nothing whatsoever to do with whether or not you can (readily) draw 800mA from it. What it *does* tell you is that, assuming you *do* draw 800mA from it, it'll last approximately

2000mAh/800mA=2.5 hours.

mAh != mA

A battery's milli-amp-hour rating is a measure of total energy, not the maximum current. For example, a 2000 mAh 9v battery might only be able to push 20mA, but for 100 hours (20 mA * 100 h = 2000 mAh) (for example). A "D" cell with the same capacity (2000mAh) might be able to push 2000 mA for 1 h (2000 mA * 1 h = 2000mAh).

What you want is *peak* current. Bigger batteries offer more peak current, with 9v being the least (no matter what the mAh is) and D cells being the most (of the corner-store type batteries). Mostly this is due to internal resistance; the resistance of the battery itself is acting like an additional resistor in series with your LEDs, reducing the current.

Thanks again for all of your replies!

That makes a lot of sense. I did indeed mean that each of the LEDs require 3.4 mA, not mAh. I think you're also right about the 9V not being able to provide enough "peak current" (look at how much I'm learning!).

After some more research I came up with this: What if I use a total of (16) AAA batteries in each hat? This would provide around 24V and I could wire 56 LEDs in series of 7 LEDs with a 6.8 ohm resistor with each series. This *should* use 240 mA. Assuming the AAAs give 1250 mAh (from Energizer's site), they should last 5 hours. This is all under the assumption that they will be able to provide at least 240 mA of peak current.

So, does that sound like a smart way of going about this project? I can get all of the batteries and holders for them for under $80, so the price isn't terrible. Also, how do I know what peak current a battery can supply? Can i be sure that the AAAs will provide at least

240 mA?

If I'm still being an ignorant idiot, just say so. I just have to get this done quick, right, and at a reasonable price - and all those LEDs have to light up real bright.

Thanks so much for your help!

I know nicads are very low impedance. NiMH is a bit higher, but still better than alkaline cells. You would have to check, but I recall a nicad AA cell is around 30mOhms. The advantage to nicads is that while the initial voltage is lower (1.25 versus 1.5V), a nicad hangs in around 1.2V until it is exhausted, then it dies quickly. So not only is the impedance lower, but the voltage will be better regulated. I don't think I ever pulled more than 200ma from AA cells, so I don't know how well your circuit will react, but give it a shot.

BTW, 30ma sounds high for continuous operation (i.e. non-pulsed).

5 or 10 alkiline cells. The better solution is 10 cells. Both are described below. First, the 5 cell: 5 alkaline AA 7.5 volts - enough for 28 strings of 2 LEDs at 6.8 plus a 24 ohm resistor per string. With 28 strings at ~29 mA per string, that's a total of ~816.6 mA. If you can't add an LED to get 56 LEDs, your last string would use a ~137 ohm resistor (made with a 1500 ohm in parallel with a 150 ohm). Energizer AA is rated at 2850 mAh under light load, and 1500 mAh at 500 mA discharge. Since you are almost double that, figure ~ 750 mAh and you need ~ 570 mAh for 45 minutes.

Give yourself a 50% margin - replace the cells after

10 performances, and you should be good for 20 performances with room to spare. If you consider the 5 cells together as 1 battery, it meets your "happy criterion", but why chance it? Buy 10 cells and be safe.

The 10 cell solution: If you have room for 10 cells, you can reduce the number of strings to 14, using 4 LEDs and a 40 ohm resistor per string. If your last string has to have 3 leds instead of 4, it will need a 160 ohm resistor. Current draw in that configuration will total 420 mA, and your cells are rated at about 1500 mAh with a 500 mA draw, so you're golden for 45 minutes of run time across the 20 performances.

The performance numbers for the Energizer are found here:

Ed

^^ 47

Should be 47

Ed

They'll last *way* less than an hour. As the discharge rate goes up, the mAh rate goes down. At 25 mA discharge, they'll provide 1250 mAh. But at 240 mA discharge, the mAh rating looks like it's about 500 mAh. And the voltage decreases to .8 in those ratings. With 7 LEDs in series, you can't really afford any drop in the cells. You have only .2 volts above the 23.8 the LEDs need - that's only .0125 volts per cell that you can afford to drop, and cell voltage drops rapidly.

You are neither ignorant nor an idiot. To be ignorant, you need to ignore something you know - and this is not your area of expertise, so you are simply unfamiliar with it. And far from being an idiot, you are checking first to look for the pitfalls, rather than charging forward blindly, as an idiot might do.

Love to see a picture of one of the top hats when you're done!

Ed

Harbor Fright 18vdc , $10 on special but get the 1.8ah , not the smaller one .

White LEDs are .02 amps , for highest eff' . DONT RUN AT .03 amps .

LM431 and some parts to make a flyback to drive any voltage you need . Flybacks need low inductance , or the frequencey is too low . Start with less than .001 Henry , more than 10 microHen'

I just saw a nice MOSFET to drive it ..

only $.10 each . PMN45EN , Id=1.8 amps @ Vgs =2.5 v . about .05 Rds .

BTW , anyone into ARM s/w ?

Hi everyone, Thanks again for all of your very helpful replies!

I looked over everything you all said and did some thinking. I like Ed's (10) AA battery solution and I'm pretty sure we have the space allowance in the hats for them. However, before I commit to purchasing the battery holders, resistors, batteries, and such, I want to be sure that the AA batteries will be able to push 500 mA of consistent current - I can't have these guys going dim on me. I wasn't able to find the maximum peak current on any battery manufacturer's web sites. Do you all know for a fact that 10 AAs can push at least 500 mA?

In response to Werty's post, I checked out Harbor Freights web site, but the only 18vdc batteries I found were drill batteries. If these are what you were referring to, they definately won't work, just due to their size and weight. I've listed the exact specs of the LEDs we have below.

Emitted Colour : WHITE Size (mm) : 5mm T1 3/4 Lens Colour : Water Clear Peak Wave Length (nm) : N/A Forward Voltage (V) : 3.2 ~ 3.8 Typical Voltage (V) : 3.4 Reverse Current (uA) :

No no no no no! Death to the AAA battery. It is nearly as bad as the

9V battery in terms of economics. AA cells are your biggest bang for the buck. The maH capability of a battery is a function of volume. The AA and AAA cells are the same length, but the AA cell has a larger diameter. The volume increases as the square of the radius, so a little more radius means a lot more capacity.

The resistor represents wasted power, so I can see why you are going for more cells in series, but stick to AA cells. I still think you should try AA nicad cells. Since you have the LEDs on hand, it will be a simple experiment.

These Sanyo AA cells are only 19mOhm.

There are discharge curves on the datasheet. I'm not sure what the "lt" or "It" unit means, but I think the 1It discharge curve is at

1100ma. Often in batteries, you refer to the current as a function of C, the cell capacity. That is, If you charge 1100ma for 1 hour, you have a capacity of 1100maH. [Technically this is not correct as the efficiency of charging isn't 100%. You can see the charge conditions are 110ma for 16 hours, not 10hours, as would be the case for 100% efficiency.) Thus I'm guessing the 1It rate is 1100ma. The battery holds up for about 50minutes at this rate.

Hi everyone,

Today, I took some of your suggestions, and decided on trying out a (8) AA version for a hat requiring only 24 LEDs. I wired 8 series of 3 LEDs with the same 82 ohm resistors I already had, which theoretically allowed 25 mA through the LEDs (I compromised between the 30 mA I was using, and the 20 mA that was suggested). I used 8 AA batteries because I was able to buy a prebuilt 8 AA holder from Radio Shack. At 25 mA, in total it used only 200 mA.

Overall, I was pleased with the results. I posted a picture here:

The only thing that might become a problem is the weight of 8 AA batteries. It's a bit heavy and may hinder the actors movement with the hats. I'm going to run this by the costume folk and see what they think.

Now, assuming the costume department will be okay with the weight, would I be safe to use 8 AA batteries to run as many as 66 LEDs? That

*should* draw 550 mA at 25 mA per LED. Is this okay? Also, should I keep using resistors to keep the current at 25 mA? or should I stick with 20? or 30?

Thanks for all of your help again guys. I wouldn't be able to do it without you all.

I have another unrelated question. I hooked up my micrometer to the leads on the 8 AA battery holder to check how much current it was pushing. I set it to 10A and the display read somewhere around 1.200. Does that mean it pushes 1200 mA? The snaps on the battery holder also started smoking. Does that mean I was drawing too much current for the battery holder to handle? or the snaps? or something else? Also, I hooked up each of the micrometer leads to an end on one of the LEDs while it was turned on to try to check how much current it was actually getting. I assume I did something wrong here, because the display said 0.05. How would I go about checking this correctly? Or did I do something else wrong?

Thanks so much everyone!