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Thread: Power Supply For Modular LED Matrix

  1. #1
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    Power Supply For Modular LED Matrix

    Hi, I’m designing a modular led matrix pcb with tlc5940s chips which will be connected to a teensy 3.2 and a raspberry pi. To be honest, I don’t have a great understanding of how power works and I’m seeking advice on what type of power supply I should use to power both the LEDs and the raspberry pi.

    The teensy will be powered over usb from the pi and the tlc5940 chips will be powered from the teensy vin pin. I’m planning on eventually scaling the project to over a 1000 LEDs, so I’d like to know what type of power supply I could use to power both the Pi and a 1000 LEDs (all at once with full brightness) and how I might calculate what power supply I should use should I decide to power more than a 1000. These are the LEDs I’m planning on using.
    Last edited by TrueBlue; 09-01-2018 at 08:54 PM.

  2. #2
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    Well I am no expert but basically it's this:
    This LED datasheet says 20mA current nominally. This times 1000 LEDs is 20.000mA or 20A. On 3.3V you would get P=U*I = 3.3 * 20 = 66 Watt.
    These are not numbers to laugh at and can not be provided by a RasPi pin or Teensy pin or by USB(2).
    So you would need an external power supply at least. This is not even considering peak currents.

  3. #3
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    Thanks for the response NotoYota!

    So I’m currently working on a board with 32 LEDS. There will be 10 boards connected together, so 320 LEDs total. I’m planning on using the pi to send data to the teensy and since the pi is powered at 5 volts I’d like to power the LEDs at 5 volts as well. So based on the formula NotoYota provided, would 32 Amps be correct for powering the LEDs? 320 x 20 = 6.4A. 5 x 6.4 = 32A. If this is correct perhaps a 5v 35a power supply should be sufficient for the pi and LEDs?

    I’m still planning on having the teensy be powered by the pi (since I'd like to be able to program it while connected to the circuit) and the 20 TLC5940 chips be powered by the teensy. Here is the data sheet for the TLC5940 chip. Would I run into any problems powering the chips from the teensy? And would any problems occur if the power from the external supply and teensy are grounded together?

    I also understand that 2 capacitors should be added to the power supply for decoupling. How do I determine the values of the capacitors?

    Finally, since the data and power will be transferred through several circuit boards what type of connectors would you recommend? I was previously using JST SH connectors but I understand there may be more suited connectors/wires for handling higher power and data

    Thanks for the input!

  4. #4
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    Its 320 leds * 20 mA = 6400 mA = 6.4A
    You multiplied it again by 5, but when you multiply voltage by current you get power in watts.
    So: 5V * 6.4A = 32W

    So not as much as you thought.
    You were talking about 20 TLC5940? Why 20? How much leds will each TLC5940 have to drive?

    I am confused by your use of the term "powered by". You say the teensy powered by the pi and the TLC5940 powered by the teensy.

    I think you mean "controlled by"?
    Anyways, TLC 5940 I believe has pins for separate led power. If you use multiple power supplies you still should connect all the grounds together but you should keep your positive voltages strictly separated.

    For example touching a teensy 3.6 pin with 5v will destroy the Teensy. I have practical experience

  5. #5
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    Ah my mistake. I added a few watts to the calculated 32 for the pi and thought it may be better to have little extra just incase. Or is that a bad idea? I believe the pi would only need 1w. Is it possible to have the pi and LEDs hooked up to a power supply with only one output? Or would I need a power supply with 2 outputs?

    Also you mentioned 'peak currents' in your first post. Could you elaborate on how I may need to consider this?

    Each TlC5940 has 16 pins for 16 LEDs. So 20 TLCs x 16 LEDs = 320 LEDs.

    Yes, the teensy will be "controlled by" the pi but will also be powered from the pi over usb. Does that clarify it?

    I'd like to only use one power supply. I understand that that the different voltages should be kept seperate. And thanks for clarifying that they can be ground together.

    My question is, will the teensy be able to power that many TLC5940 chips. And what would be the limit before you would suggest powering the TLC5940 chips from an external power supply?

    Lmk if If you need more clarification about anything. Thanks!

  6. #6
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    You can get 5V 30A aluminum epoxy sealed powersupplies off fleabay, I use these with Teensy, the inputs are 8-40 volts, the output is 5V 30A
    Pay close attention to the output listed, alot of those powersupplies have different volatage/amp outputs, some are 12V 6A and 10A, then 5V has 4A,5A,10A,15A,20A,30A

    They also fit in the palm of your hand.


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  7. #7
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    Quote Originally Posted by TrueBlue View Post
    Ah my mistake. I added a few watts to the calculated 32 for the pi and thought it may be better to have little extra just incase. Or is that a bad idea?
    What I was referring to is that you were talking about 32A not 32W which is a big difference.
    But, indeed having a power supply that can deliver more current than you need is never a problem. Voltage should match though.

    Quote Originally Posted by TrueBlue View Post
    I believe the pi would only need 1w. Is it possible to have the pi and LEDs hooked up to a power supply with only one output? Or would I need a power supply with 2 outputs?
    A power supply with one output is not a problem. You can basically connect multiple wires to the power supply. So from the power supply you have wires going to the Pi, wires going to the Teensy and wires going to the TLCs.
    In this case everything will be wired parallel and the pi and the teensy draw their own currents.

    Before you start projects like this I suggest you learn this basics about power, voltage and especially current.

    https://www.khanacademy.org/science/...ircuits-part-3

    This video is specifically about calculation of power and current in parallel circuit but I suggest you watch the other videos in the series too.
    Absorb everything so you can do your own calculations. Also you can calculate the power supply you would need if every led was using peak currents at the same time.

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