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Wiring ForumWiring HardwareTangible computing › Figuring out power requirements

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Figuring out power requirements (Read 5103 times)
01/04/07 at 18:50:26

roy_pardi   Offline
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Hi!

This is only partly a Wiring question: I have a new project I am working on that will use 48 LumiLEDs driven by the MAX7219 chip. I am trying to figure out what the power reqs. are and am sort of out of my depth. LumiLEDs are not regular LEDs an draw more current but I can't determine if there is an issue with the MAX7210 or the Wiring board handling that current.

I've assembled all the tech specs here: http://roypardi.com/collider/tech.htm

If anyone can x-ray the specs... and determine if I am going to be ok with this, it would be a great help.

thanks!

--Roy
 
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Reply #1 - 01/05/07 at 05:17:54

barragan   Offline
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Dear Roy a few questions, Lumileds have a special power supply that comes with them, are you using that one? I can see you are going to connect them individually, which means each of them will ask for 350mA, The forward voltage of the LED is 3.42V so the limiting resistor for that if you are going to need if you have a 6V power supply will be of about 7.3 Ohm.
When all leds are on (at full brightness) you'll need a total of 48*0.350A = 16.8A at 6V which is a very strong power supply, I recommend for you to look for a Linear power suppply from Digikey, (PowerOne) are good. Make sure it is LINEAR, a switched one (they are a bit cheaper) will introduce a lot of noise into your circuit.

 
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Reply #2 - 01/05/07 at 06:20:51

Alan_Kilian   Offline
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If you can get your links to the PDFs working I'll take a lok for you.

I suspect that the 7219 won't be able to drive them at their full current, but that's
just a guess not having read the data sheet.

I have a guy who can help you get them working at full-power, and I'll talk
to him Friday evening.
 
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Reply #3 - 01/06/07 at 04:10:45

roy_pardi   Offline
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thanks for the replies! (and sorry about the broken links.. Sad - all  fixed now)


-about the LumiLED power supply - I wasn't aware they had or needed a special power supply. They seem to do ok running off my variable bench supply. The are awesome bright!

-I follow about the total current needed, thanks for working it through. I don't think all the lights will be on at once but a large number of them certainly could be. (this is a sort of "a-life" -like particle accelerator idea where light sprites will interact with each other)

-about the MAX7219 not driving them at full current, I suspected that after reading the data sheet 10x... I seem to keep hitting this issue: how to drive a lot of higher current lamps with a limited number of pins. Any help on this would be much appreciated! (esp. since I learned yesterday that this piece was accepted to a show.. heh. Nothing like the pressure of failing in public to motivate!)

--Roy
 
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Reply #4 - 01/06/07 at 23:38:10

barragan   Offline
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One Idea to drive them could be through Optocouplers or even MOSFETS, those in general can handle higher loads than the microcontroller can handle by itself
here is a MOSFET driving a 12V lamp, http://wiring.org.co/reference/electronics/lamp.html
in this way the power of the load is independent of that of the microcontroller etc.
the MOSFET used in there is the one recommended here: http://wiring.org.co/reference/electronics/symbols.html#150
Both Optocouplers and MOSFET Switches behave as an LED for the Wiring board so you can turn it ON or OFF as you wish, and on the other side they will be turning ON or OFF another circuit which operates ata different voltage and current. I'll try to post some more information on this later.
 
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Reply #5 - 01/07/07 at 03:26:32

roy_pardi   Offline
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thanks - I used TIP120 (Darrlington) transistors to do this in my last piece and it worked great - but I only needed 16 lights so I had enough free pins. I tried to use the MAX7219 with that same TIP120 circuit so I could drive more lights but I couldn't get it to work.
 
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