Instructables

wiring a lot of LEDs, don't know the math. please help

I'm trying to figure out the best way to wire up 187 LEDs on a panel. They will probably be 3.4~3.8V, I'n not sure how the math works to determine what kind of power I'll need or what the best way to wire them would be. Help would be greatly appreciated. Thanks in advance!

ok, well.. are you trying to find resistors? because if yes, then heres a great site. www.ledcalc.com
Patrik7 years ago
Do they all need to be lit at the same time? What sort of LEDs are they? Different color LEDs will require a different voltage, and brighter LEDs will need more current.
aintMichael (author)  Patrik7 years ago
they're all the same and yes all lit at the same time. Like I said, they'll probably all be 3.4V. I'm thinking I'd have to do them in groups, maybe a bunch in series then each bunch in parallel, but I've never done that many at once so I'm just trying to figure out the best way.
So blue or white LEDs? Red or green typically only take around 2V.

If we assume 20mA for each LED, you'd need 3.7A at 3.4V, if you put them all in parallel. Not sure where 'Muff gets the 300mA @ 5V from.

You definitely don't want to put them all in parallel though. I'd suggest putting a bunch of LEDs in series with one resistor, then putting a bunch of these in parallel.

For example, if we want to feed with a fairly standard supply voltage of 24V, we can afford to put 7 LEDs in series (7 x 3.4 = 23.8V). Plugging this into the calculator FrenchCrawler mentioned gives you a 10 Ohm resistor. Now just put 27 of these in parallel, for a total of 189 LEDs.

End result: 540mA at 24V, using 27 resistors of 10 Ohm.
Patrik Patrik7 years ago
PS: If you need exactly 187 LEDs, Just make one of the series circuits with only 5 LEDs, and a 390 Ohm resistor.
aintMichael (author)  Patrik7 years ago
I'm actually using UV LEDs which are 3.4V. Do you know if there's any kind of heat issue, or efficiency issues running that many groups in parallel?
The heat dissipation is pretty straightforward:

3.4V x 20mA x 187 = 12.7W

or if you want to include the resistors in my setup above:

24V x 20mA x 27 = 13W

Efficient though LEDs are, most of this power will actually be emitted as heat, not light (typical luminous efficiency for LEDs is around 10%; incandescent bulbs are only around 2-3%).

Depending on the physical design of your circuit (are you trying to cram 187 LEDs in as small a space as possible?), a heat output of 12-13W may or may not be a problem.
aintMichael (author)  Patrik7 years ago
Actually it's one every square inch over an 11 x 17 area. So I didn't think that heat would be an issue, just thought I'd ask since, like I said, I've never hooked up that many before. I really appreciate the help. Thanks
Roughly (very roughly) speaking, if you're not worried about heat dissipation for a single LED in a 1" square, you shouldn't be too worried about 187 of those in 187 square inch.

I'd be more worried about the amount of UV light this is going to give off. You're not going to get a sunburn, but you probably want to avoid looking at this array with the naked eye, because you might do some serious damage without even realizing the LEDs are on...

Danger of Eye Damage From Visible Light Emitting Diodes
aintMichael (author)  Patrik7 years ago
yeah, the whole system is enclosed, and I'm planning on a "safety" switch that only allows it to be turned on when it's closed. That being said, I'll probably wear sunglasses while I'm building it. :)
May want to build in a red warning LED, just in case... Hm... enclosed 11x17" UV light - building an exposure unit for UV sensitive emulsion, perhaps? I wonder if you could get a more uniform coverage by using a diffuser from a broken LCD screen. Don't know how well that transmits UV light though.
aintMichael (author)  Patrik7 years ago
Shhhh.. it's a secret. JK :) I've been looking at diffusers, and a lot of them have great features like "100% UV protection!" I'm actually looking at a few different things for that, so I'll have to see what works once I build the box. Thanks for all the help!
If you wait a couple of days, you'll see my newest Instructable that'll show about 100 LEDs powered up. However, if you need to know now, I'd say use a wall wart that puts out at least 300 mA @ 4-5V
aintMichael (author)  T3h_Muffinator7 years ago
I was thinking I'd need a more than that, maybe when I see your intructable I'll get a better idea. Can't wait.
Hmmm you're right... perhaps 400mA is a more accurate current value....
I CANT WAIT MUFF!!!!!!!!!
aintMichael (author)  FrenchCrawler7 years ago
that site is awesome, unfortunately it will only do 32 LEDs
Cool, thanks that is useful.