Led amplifier help

Discussion in 'The Projects Forum' started by si1986, Oct 29, 2011.

  1. si1986

    Thread Starter New Member

    Oct 29, 2011
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    Hi everyone, I am in need of a little help please.

    Im looking to build a low res LED screen for a stage set up that is powered by these http://www.usledsupply.com/shop/rgb-32-spi-dmx-decoder.html

    The problem I have is that this board only allows for 300mA output per RGB pixel (100mA per led) and this really isnt enough as I need each pixel to be fairly bright maybe 30 or 60 LEDs (10 or 20 R,G & B)

    Is there a simple way I can use the output from the above decoder to control a simple MOSFET circuit to amplify the output. I have in mind something similar to this http://picprojects.org.uk/projects/bigmosfetrgb but I only need the MOSFET power part of the circuit. I would obviously have a MOSFET curcuit for each of the 96 outputs.

    Am I correct in thinking I could build a curcuit for each output that has its own power supply that powers the LEDs through a MOSFET and the output from the decoder controls the mosfet (as such)

    A little help would be great as you can probably tell.

    Many thanks
    Simon
     
  2. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    First I think posting the specs to the actual LED's you are using would be a great help in recommending a solution to your problem.

    Plus this info from your link:
    Apparently they already have a way of amplifying the power for higher current LED's....
     
  3. si1986

    Thread Starter New Member

    Oct 29, 2011
    6
    0
    I don't really know what LEDs I will use yet I just want to amplify the output so I can draw more current from each channel 1amp would be fine. All I'm after is a simple circuit suggestion that will allow me to run more LEDs off each channel.
    The problem is when you have 96 channels to cover it starts to become very expensive to buy amps for every channel.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    If you can provide a schematic for the unit you will be using to drive the LED's, it would make it simpler to offer a solution......

    As for the mosfet circuit, it seems easy enough to duplicate, but we would still need a schematic for the driver unit so the Mosfet circuit can be developed....
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Looking at the images posted on the site, this one:

    [​IMG]

    ...indicates that the outputs are for a common anode, then three current sinks for the cathodes.

    There is not a common ground supplied, so you would need to add one from somewhere, the most likely place being from the supply negative.

    The current sinks for the cathodes are most likely open-collector or open-drain.

    If you're going to use transistors, you will need one PNP and one NPN per color per LED to invert the logic twice - along with suitable base current limiting resistors, and base return resistors.

    If you're going to use MOSFETs, you'll need to use both a P-ch to drive the N-ch gate, and an N-channel MOSFET.

    Without knowing what your supply voltage is going to be, it'll be difficult to make decent recommendations.
     
  6. si1986

    Thread Starter New Member

    Oct 29, 2011
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    I'll use a 12v supply
     
  7. si1986

    Thread Starter New Member

    Oct 29, 2011
    6
    0
    Would you recommend transistors or MOSFETs? I gather I can draw more with the fets. Is there any way you could draw me a little circuit drawing to help me build these amps?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    OK, back up a minute.

    Are you planning on using RGB LEDs, or separate Red, Green, and Blue LEDs?
    It's an important question, as you CAN operate individual LEDs in series, but you CANNOT operate RGB LEDs in series.

    If you are using RGB LEDs, and are operating from a 12v supply, that means you will be dropping ~10v across each current limiting resistor for the red, and ~9v across each current limiting resistor for the blue and the green. You will require three current limiting resistors PER EACH RGB LED.

    Also, it is highly doubtful that you will be able to control the individual intensity of the LEDs. If you were thinking that you could project a video image on this low-res "screen", I'm afraid that you will be very disappointed with the result.
     
  9. si1986

    Thread Starter New Member

    Oct 29, 2011
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    I'll be using separate LEDs not rgb LEDs.

    I don't understand? That is what the decoder board does. I control each led via 1 channel of dmx via software I already have. I won't be projecting video but moving colourful images. Why are you think I won't be abe to control the intensity?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    OK, I'm not familiar with that particular board.

    Do you already have a decoder board like that one, and it works as you want it to (besides not being able to control enough LEDs?)
     
  11. si1986

    Thread Starter New Member

    Oct 29, 2011
    6
    0
    I don't have one yet. I was waiting to see if its possible to amp the channels first.

    I have a small rgb circuit kit I bought years ago it's very simple but seems to make sence and I cant see why it wouldn't work with the decoder we're discussing. The LEDs have a constant positive supply then the negative from each led goes the the middle pin of the MOSFET. Another fet pin goes to earth and the other pin to the chip via a 1k resistor. from what I gather the LEDs have their own circuit an the decoder circuit is just controlling the gets (sort of like a relay controlling a high current circuit from a low current circuit)
     
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