Current calculations

Ok so I’m still pretty knew to aurdino boads and LEDs. This is only my second project. I’ve got approximately 600ish LEDs I think. I didn’t keep track and I’m basing my numbers off what’s left in the packs of LEDs but I also know I broke several while I was working on this project. You can see it here:

Ok so with all the LED calculators I found online says that it sound be pulling 12,000mA. In the video I’m powering it with 3 C batteries. I’m trying to figure out other battery options that will last the longest. However, when I connected a multimeter to it, the most it pulls is 0.50 amps which is 500mA correct? I don’t understand how this would be possible… So I reach out to all the people smarter than me!!! Am I doing something wrong or is there information I’m lacking? My daughter is wearing this dress in Disney world and I’d like it to last about 5 hours at least. I’m thinking of lithium polymer. Dumb it down for me please. You’ll probably loose me with a bunch of equations. Thanks for your help!!!

You'll need to describe how the LEDs are wired and what resistors are used. It would be good if you can post a schematic diagram (photo of hand drawn is fine).

All the lights are wired in a parallel. Each light has its own resister. Warm white lights and red have 68ohm and bright white has 200ohm.

I forgot to ask for the LED specs and how many of each type.

However, you can proceed as follows to more accurately estimate the current draw.

Assuming fresh batteries, and that the bright white LEDs are 3.2 V, the current each draws from a 4.5V battery pack will be about (4.5-3.2)V/200 Ohms = 0.0065A or 6.5 mA, less than what must have been assumed by the online calculator.

For the red LEDs, the current drawn would be about (4.5-1.9)/68 = 38 mA, which is higher than recommended.

Since they are all in parallel, the currents add. If the batteries are not fresh the voltage will be lower and the current less.

It looks like "warm white" LEDs are on all the time, red LEDs are controlled by the Lilypads and are on from time to time, and "bright white" LEDs blink infrequently. On the video, they look yellow, red, and blue, respectively. It looks like there about 400 "warm white" LEDs, and they're on all the time, while the rest are off most of the time.

It looks like you have a multimeter. Can you measure and report the voltage across a few of the resistors that are in series with warm white LEDs? That'll tell us the current that they carry. It would also be helpful to know the voltage across the batteries.

Here are some other questions about the warm white LEDs:

  • Do you know what size wire you're using to connect them? Do you know what it's made of?
  • How are the LEDs connected to the battery? Is there a single wire from each combination of a resistor and LED to the positive terminal, and another wire from each led to the negative terminal, or something like that? Or, are the LEDs connected to each other, and do a few wires connect many LEDs to the battery terminals? If that questions seems complicated, lets ask instead: Are there hundreds of wires between the batteries and the warm white LEDs, or are there a few of them? I looked at your earlier videos, and I can't tell. I'll note here that the biggest "conductive thread" that Adafruit seems to carry has a resistance of ten ohms per foot - high enough to limit the current to 0.5A with just one foot of wire, if the batteries are alkalines in series.
  • Does anything get hot while the LEDs are on? I worry a bit about this one. Since a child will be stuck in this dress, it's important to make sure that nothing gets particularly hot. Whatever power technology you select, you will probably want to do a dry run with the costume illuminated for the full length of time that she'll wear it, and check it frequently for overheating. I'd make sure to gingerly put my hands on the batteries themselves from time to time. If we don't find the answer to this, I'd keep a bucket of water and a spare adult handy during the test.
  • What kind of batteries are you using? I expect that they're standard alkaline "C" cells that could be bought at a grocery store.
  • When you made this measurement, were the batteries fresh?
  • When you measure the current, how do you do it? Are you confident that your measuring technique is right, and in particular, that it captures all the current that the batteries are supplying?
  • Are the Lilypads powered by the same batteries that serve the warm white LEDs?
  • And finally, is the brightness of the LEDs, as they are today, satisfactory to you?

I'll echo jremington's concern about the current through the red LEDs. It sounds high for an ordinary LED, and it sounds quite high for a Lilypad output pin. It would be a pity if the Lilypads started to burn out at showtime.

You warned us about asking too many questions. It would seem that I paid no attention to that.

Sorry I fell asleep :slight_smile: I don’t mind questions as long as I understand them lol
Wiring is 24 gauge speaker wire copper and aluminum.
Each led is connected to each other. So there are wires connecting each resistor together and each opposite end of the resistor connects to led. I was careful too to face all the resistors the same direction even though I read that it doesn’t matter. So very few wires connect to terminal.
I’ve had it on for 20-30 minutes so far with nothing getting hot. When I decide on final battery source I will do a longer test on my child mannequin for 4-5 hours. I doubt the costume will be on for that long at one time but just in-case I’d rather test it well.
Batteres are 3 c alkaline bought from grocery store in a series battery holder to get 4.5v they are not new same ones I’ve been testing the dress with as I go along. I didn’t have any fresh batteries here so I hooked up a freshly charged power bank I has 2200mah. I know the most it can output is 800ma but I tested it with this as well and the highest the current goes is .65 and it pulled at 4.8v
I watched a lot of videos to make sure I’m measuring right. I’ve it black lead in common plug and red lead with 10Adc plug and set to 10A. Ground is hooked up straight to battery ground and I’ve got red lead touching positive battery wire and black lead touching positive wire going into dress.
The one pack of batteries supplies power to the whole dress, soft white lights, lilypads, and EL Wire. I’m very happy with the brightness nothing over powers anything. I just wanted to make sure my calculations are correct so I can pick out a good size battery and use the battery calculator I found online to see how long the battery will last. I’m thinking of a Lipo with a built in chip as well as hooking it to a power bank modual for the extra protection and easy charging. Oh don’t know if I said but all the lights are 3mm if that matters. Let me know if I missed anything lol

I hadn't looked at the video, because I have a slow internet connection at home.

Beautiful dress, and fantastic work!

If the lilypad is using PWM, then steady state current calculations don't mean much, because the LEDs are off a lot of the time. The average current draw as measured by the multimeter in series with the battery pack lead, is as good an indication as any.

If the average current draw is 500 mA, alkaline C batteries should last several hours (for the Energizer E93, about 10 hours, see )

Thank for the help guys :) I guess I'm just really second guessing the multimeter since I know each light is 20mA so I didn't know how that changed the more lights added to the equation. After its been on for a few minutes the soft white constand on dims for second when the red swirl lights go. So I wasn't sure if that was because of the type of batteries I was using or if I just need to except that will happen. They dim just enough for me to notice, I don't know if anyone else would even notice. But when the red stop swirling they tad brighter again...

OK. Please forgive me if this next bit is too elementary. Your technique for measuring current seems to be reasonable. However, you don't explicitly say that you disconnected all the LED wires from the power supply before connecting the meter between the power supply and all of the wires that serve LEDs. Two things could have gone wrong: if you didn't disconnect the supply from the LED wires, then most of the current goes around the meter, through the existing connection, and the meter can't see most of the current. If you did disconnect, but didn't connect all the LED wires to the meter, then you're only measuring part of the current that drives the LEDs. Again, I apologize if that's too elementary. It's hard to get a handle on your skill level - it seems to be more than adequate for this project, but you claim to be a neophyte.

I don't see voltage measurements across some of the warm white LED resistors. By my calculations, each of those LEDs would draw close to 20 milliamps - (4.5V - 3.1V) / 68 Ohms = 0.0205 Amps, or 20.5 milliamps, with a theoretically perfect battery; with a real battery, it'll be something less. So, the roughly 400 of them that I estimate should draw about 8 amps from an ideal power supply. I'm presuming a voltage drop of 3.1V across the warm white LEDs. I can't see the resistors clearly in the photo, but they look to be consistent with 68 Ohms, which would have color bands blue, grey, black. If the resistors were something larger than 68 ohms, that would explain a lower-than-expected current reading, at least in part.

I'm presuming that the warm white LEDs aren't driven by a Lilypad, but that their resistors are directly connected to the power supply.

24 guage wire is small, but it's resistance is only about 0.14 Ohms per meter in aluminum, and about 0.83 in copper. Worst case, that corresponds to about 0.11 volts for a round-trip of a meter and a half, at a half amp. Not much; that's not it. It's also not making much heat.

The internal resistance of an alkaline C cell is 0.15 to 0.3 ohms; three in series would be about half an ohm to an ohm. That's much more than we can expect from the wire. At half an amp, that yields half a volt to a volt. Worst case, the voltage drops as low as maybe 3.4 volts for a new set of batteries. if the typical warm white LED voltage is 3.1 volts, the current will be (3.4 - 3.1) / 68 = 1.7 amps; at 3.3 volts, the current is close to half an amp. A few voltage readings across some of the 68 ohm resistors in series with the warm white LEDs will go a long way toward verifying this notion, as would a voltage reading across the power supply while the LEDs are illuminated.

Three C cells in series can deliver between five and ten amps to a short circuit, based on Eveready's data for their standard alkaline cell. They'll deliver considerably less to LEDs with a forward voltage drop above 3 volts, as in this arrangement.

Note that a red LED typically has a voltage of about 2 volts. With a 68 ohm resistor, that comes to about 37 milliamps. I can't speak to the Lilypad particularly, but the Arduino Uno likes to keep its output current at 20 milliamps per pin or below. It looks like all the red LEDs are on for a brief time. That may be overstressing the Lilypad, and could lead to early failure. You may want to consider increasing the resistors for the red LEDs to 150 ohms.

I think that the hottest thing in the costume would be the batteries, which would generate maybe a half watt. Not much; the wearer might welcome it in winter. Still, it's worth testing.

Finally, I'll echo the sentiment of jremington: it's indeed a very nice piece of work.

Hi and yes i did disconnect the all the LEDs and only one battery line goes into the dress. I manually counted the soft white lights and i broke more than I though. But the numbers still don't add up. 323 Soft white lights, 78 red lights and on the lilypad the max per pin is 40, and 45 bright white lights. So an correct total of 446. 323 of which is constantly on. I still consider myself a new be, I do not know proper terms for formulas but I read a lot, pick up quickly and jump in just start figuring things out. Plus the EL wire is constantly on but I know it takes VERY little current. But that still in addition to all the LEDs. I think I'm just going to accept that is pulling low current. LOL not much else I can do. I've reached out to those smarter. I think I'm going to use 1 12000 mAh lipo battery its flat and and lighter than the 3cs. It has a protection board on the battery myself and i'm going to connect it to a USB power bank module for the added protection board plus it will be easy to charge as well. It should last a long time... Once I hook it up i'll do a 5 hour test on it. Unless anyone has any reasons not to.

My immediate thought is that you may not be using adequate thickness of (copper) wire for the power and/or ground return that's shared by 100's of LEDs. I know that the sort of wire/thread used for wearables is a lot higher resistance compared to plain copper wiring for instace. You could look for voltage drops across the wiring with the multimeter, or check that the expected voltage drops appears across LEDs+resistors.

It sounds like you've measured the current accurately. At a half amp, the total power is around 2 watts. I don't see anything getting hot at that level. Doing the 5 hour test is probably overkill, but, hey, we overdo it when it comes to our kids all the time, don't we?

If you change the battery technology, you'll want to do another current measurement. If it's markedly different, then maybe the dress rehearsal is a good idea. I'd still like a voltage measurement across some of the warm white LED resistors.

Nice work.

Nice work! I can't add to what the others have said about current draw, it has been well covered. But I do want to caution you to charge the LiPO battery off-dress and on a non-flammable surface!

Amazing! You've done a great job 8)

Paulcet: Nice work! I can't add to what the others have said about current draw, it has been well covered. But I do want to caution you to charge the LiPO battery off-dress and on a non-flammable surface!

LOL I planned on it :) But I realize other may not have though of it. I was going to basically cut the end of a USB cord and hard wire it in to the dress that way I can plug into the power bank but completely remove the bank for charging in a lipo safety bag. But be for I sauder the the USB wire together with the others I'll do more measurements to be safe. I real appreciate everyone's comments and help. If I get time I'll see about voltage testing through out the dress:)

Ohhhh... About the red. I forgot to mention that there are 2 red lights per pin. One for each swirl. So I keep checking the resistor calculators online. And they all say different things. Each red light as their own 68ohm resistor, so do you still think I should increase the resistors on them?

The red LEDs can probably be wired in series pairs. Their voltage drop is generally less than 2v. Worth trying anyway.

As others have explained, don’t bother to disconnect anything to measure current draw to the LEDs, just measure the voltage across the resistor and calculate the current from that.