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Driving the Cathod of a seven Seg display

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Discussion in 'General Electronics Discussion' started by marmitas, Nov 26, 2012.

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  1. Nov 26, 2012 #1
    marmitas

    marmitas

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    Oct 9, 2012
    I am making a 3 digit sign with a MAX7219. The display has a common cathode and each digi draws 800mA when all the segments are lit. My question is if the attached schematic is correct. And would also appreciate suggestions about an appropriate P channel MOSFET
     

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  2. Nov 26, 2012 #2
    Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The circuit looks good, assuming that the MAX7219 output is low active.

    The MOSFET, however, is an N-channnel type, not P-channel. You can use any N-channel FET provided the following conditions are fulfilled:

    1) IDs >= 800mA
    2) Vt <= ~3.5V (logic level MOSFET)

    In this circuit you could also use a bipolar NPN transistor.

    Harald
     
  3. Nov 27, 2012 #3
    KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    There is a problem with your schematic, and in any case, it's only a tiny part of the design, so it's impossible for us to tell much from it. Also, I don't think the MAX7219 is the best solution for what you want to do.

    The MAX7219 has current limiting built into the segment drivers. The maximum segment current is 40 mA, so with all segments ON (including the decimal point, if there is one), the MAX7219 can supply a maximum of 320 mA, not the 800 mA that you say the display needs.

    Are you aware that the MAX7219 is a multiplexed display driver, and if you're driving three displays, each display will only light at one third of its normal brightness?

    Your description so far is very brief. If you want some useful advice, you need to describe your project in a lot more detail, as well as giving the part number for the display (or a schematic of it, if it's made from discrete components).
     
    Last edited: Nov 27, 2012
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