Led power help

[Bobthecooldad]

I need to wire  10,000 Led that can be reprogrammed to show words and an image.
I had the idea to join all the positives with a 330k resistor on each positive leg (massive Daisy chain) and to the positive straight to the battery. I was thinking of running it on an arduino uno powering a MOSFET to power the LED on the negative side. What sort of size MOSFET would I need as an online calculator said the whole thing could draw around 400watts what seems very high.. The ardunio will run off a battery and the led are going to run off a PC power supply and would I need a 5v rectifier.

[DmitryL]

I need to wire  10,000 Led that can be reprogrammed to show words and an image.
[..]

Errr.. 10,000 LEDs ?
Assuming that the voltage drop on 1 white/blue LED is 3.5V and constant current is about 20mA, every LED will be consuming 0.07W.
If you light up all of them, it will give you 0.07*10000 = 700W of consumed power at 100% duty cycle. Not bad, if you have 1KW PSU
If duty cycle is 10% it still gives 70Wt to dissipate.
you can fry eggs on your LED display

[Bobthecooldad]

the plan that i have been asked (work project) there is going to be a wood board and it needs to be filled with leds so the digital ones are not really going to work.
it needs to be fully changeable so i thought have the arduino on a blink timer and it to run 2 mosfets what then turns on only the selected connected ones that then read words

the idea on the circuit is that the mosfets connect bridge the negative only turning on certain led at a time meaning the issue of it being re programmable each time is gone as you just connect diffrent leds negative

[kizzap]

Firstly. AHHHH!!!!!!!!!!

Secondly: AHHHHHHHHHHHHH!!!!!!!!!!!! 

This is not the way you want to A) design a LED matrix, and B) Current limit a LED matrix.

Firstly you specify that you want a display that you can show words and images. Now were you talking a single fixed word, that you put the LEDs in the shape of the word, or are you more talking like a matrix, similar to the scrolling LED displays that you sometimes see in shop windows?

If you are talking about the second option, then you are probably best looking into some kind of LED matrix controller. They will make the display easier to control and take care of the current limiting stuff for you if you design it well.

[calzap]

Appears you want to make a 100 x 100 LED display (or similar resolution).  There are a number of these available, including ones that offer various sizes of LEDs,  a color range, and computer interface.  Is there something unique about your project that prevents use of pre-made displays?  You mentioned a wood board in which the LEDs would presumably be embedded.  Any reason holes can't be bored in the wood and a pre-made LED display mounted behind it?  Embedding 10K LED in holes in a board is going to take a lot of work.  Hopefully, you  have some automation to help with the process.

Mike in California

[Bobthecooldad]

it needs to be a still image it was becasue it needs to be massive that was why i did it as a chain otherwise i am going to have to wire individual ones

[Bobthecooldad]

the 100x100 display i ignored simply because it is going to be the size of at least 30 inch diagonally so i would need to program each one individually

[mariush]

Ok, driving 10k leds directly from one arduino is a bit of a dream. 

My advice would be to separate the whole panel into smaller identical blocks. For example, you can create 8 x 8 led blocks and use 169 such blocks to create your 104 by 104 led display (10816 leds).

To reduce the number of wires, you can use shift registers on each such block to take data in using just two or three wires.  If you also add a small microcontroller on each block or a smarter led driver, you could also control brightness of each led. 

Keep in mind the power consumption and what leds you're going to use. If your design is going to be in a window or somewhere in the sun, you may have to use more powerful leds to actually see something. You may not get by with 10-20mA leds. 

If you connect leds directly to the outputs of a shift register, you have to be careful about the current.  A shift register would only be able to handle about 70 mA so if you plan on connecting 8  20mA leds on a shift register, you're going to have problems. You can counteract this by using chips like ULN2803A between the shift register and the leds. Each darlington pair (there's 8 of them in the uln2803a) can handle up to 500mA but you do have to be careful about how hot the chip would get... you can't just connect 8 x 500mA leds to it because the chip is not 100% efficient so it would overheat fast.... but 8 x 100mA leds turning on and off would work fine.

But even with 20mA leds, if you're going to have 10k leds, you're going to have a power draw of 10000 x 0.02A = 200A .. that's A LOT of current. Think how many 5v power supplies you would need to power that panel, and how each of those power supplies will work when leds turn on and off, most power supplies don't like such high fluctuations in power usage.

It would be smart to use small dc-dc converters on each block of leds to convert a higher voltage to the voltage you're going to work with (3.3-5v, depending on led choice and drivers/shift registers/darlington arrays).  An 8 x 8 block of 20mA leds would need about  64 x 20mA + about 20mA for shift registers and uln2803a chips so about 1.3A of power when all leds are full on.  You can buy dc-dc converters from ebay that use lm2576 or similar chips and convert 12v to 3.3-5v at about 85-90% efficiency and then you could use 2 or 3 cheaper regular computer power supplies to power that huge panel. Less power losses.

Also keep in mind the HEAT generated. If you use 5v and have 3v leds, there's going to be some power wasted in the resistors used to limit the current going into each led.

For example, if you power a 3v led with 20mA from a 5v power supply, then you're going to have a 100 ohm resistor limiting the current which will waste about 40mW of power as heat.  Now if you multiply this by 10k, you're going to have 400,000 mW  or about 400 watts wasted as heat just in the resistors (not to mention the leds themselves).  In time, it adds up... you will heat up the room quite fast.

With 3.3v input and the same led, you're only going to waste about 6mW in a 15 ohm resistor, so 10k leds turned on will waste 60 watts, That's why it would make sense to get the voltage as close as possible to the voltage actually needed by the leds, or use proper led drivers.

[LukeW]

Say you want 104 x 104 LEDs... divide that up into a 13x13 grid of 8x8 modules.
Each 64-led module would have one chip on it (eg. MAX7219 or similar).

Each row of modules could have one Arduino, so you would need 13 Arduinos (or microcontroller chips), assuming that one full row (13 modules) is a good number to connect to one Arduino. This would depend on the update rate, SPI speed etc. You would need 13 pins to handle the 13 CS pins, with an SPI bus to every module on that row, but a standard Arduino has enough pins to handle this, excluding the SPI pins and the serial pins which you presumably use to feed data into the system.
Something like a MAX7219 handles the current regulation for you, so you don't need resistors, except for one current-set resistor per chip.

It seems doable.

[calzap]

Go to tindie.com and search on matrix or LED.  You'll find a bunch of inexpensive LED matrix controllers with varying specs. 

Mike in California