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LED Displays - Common Anode or Common Cathode?

Discussion in 'General Electronics Chat' started by mbu, Mar 9, 2007.

  1. mbu

    mbu New Member

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    Hi,

    What are the advantages/disadvantages of a common LED anode display and a common cathode LED display?

    Thanks...
     
  2. kchriste

    kchriste New Member Forum Supporter

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    There is really no advantage from using one or the other.
     
  3. Sceadwian

    Sceadwian Banned

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    You chose common anode or common cathode based on the LED drivers ability to source or sink current. Most modern chips are effectively symmetrical in their ability to source or sink current so it's not usually an issue. So it's really up to what you're going to drive that particular LED matrix with.
     
  4. dave

    Dave New Member

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  5. mbu

    mbu New Member

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    kchriste and Sceadwian,

    Thanks for the answers - I appreciate it very much!:)

    Mike
     
  6. Oznog

    Oznog Active Member

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    Actually much more often the driver likes common anode:
    NMOS generally drives much better than PMOS. More current capacity, lower rds-on for a given size and cost.

    Often you have a +12V source and a +5v driver sharing the same ground. That driver can put out 0V/+5v to drive an NMOS gate, but it can't drive a PMOS without additional circuitry.

    But of course it depends on your specific driver. There are certainly instances where common cathode will be called for.
     
  7. gramo

    gramo New Member

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    Just a few other things to take in consideration with segment displays..

    For multiplexing there are a couple of things to take into account,

    By hooking a single resistor in series with the common, you’re going to put every other LED in reverse bias, eg

    [​IMG]

    If any LED Segment is on (supplied with +5V for example) there will be a voltage potential above the 220ohm resistor, any other LED Segment not in use will now be in reverse bias (reducing the life of each segment)

    So the correct way to wire them up would be

    [​IMG]

    Now the reverse bias is much smaller and the segment will last much longer

    Just on multiplexing, NPN's are more efficient for switching common cathode, and PNP's for common anodes. Here’s an example of using an PNP and NPN for a common cathode segment display;

    [​IMG]
     
    Last edited: Mar 10, 2007

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