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4511 BCD and common anode 7-segment display

Discussion in 'General Electronics Discussion' started by NikitaB, Jul 28, 2011.

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  1. NikitaB

    NikitaB

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    Jul 28, 2011
    Hi all,

    I purchased 2 of the 4" Kingbright SA40-18SRWA 7-segment modules, and two 4511 BCDs to interface the displays to a LabJack. I discovered however, that the Kingbright displays are common anode, while the 4511 BCD is meant to drive a common cathode display...

    As I am fairly new to electronics, I am unsure of what I need to add (or if it is hopeless) in order to make the common anode displays compatible with the 4511 BCD. I read on the SparkFun comments for the MAX7219CNG driver that one could use a 16 bit hex inverter in order to make the chip compatible with a common anode display, but I'm not quite sure how to implement this. Ideas, suggestions, schematics, plans, websites, part numbers, anything helps!

    Thanks!
     
  2. poor mystic

    poor mystic

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    Apr 8, 2011
    Yes, if you were stuck on a desert island with only a crashed airplane for parts you could use an inverter but I recommend going back to the shop and getting the common cathode display the chip wants.
    The conversion of the o/p of the 4511 to suit your display will complicate and possibly uglify your project, which besides will end up costing more in circuit board real estate and wiring effort.
     
  3. NikitaB

    NikitaB

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    Jul 28, 2011
    Hmmm, alright. Unfortunately, the displays that I am using are 4" tall, and as I am no longer abroad (where I got the common anode displays) I have to go through an online store. But there just seem to be no large common cathode 7-segment displays out there.

    Even though an inverter would be archaic, I would still appreciate some schematics for that setup as I think that is what I will have to use.

    Thanks
     
  4. poor mystic

    poor mystic

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    Apr 8, 2011
    How much current does the display want to drive each segment please?
     
  5. NikitaB

    NikitaB

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    Jul 28, 2011
    Per the data sheet, the forward current per segment is 60mA. Here is a quick link to the data sheet as well: http://tinyurl.com/7-segment

    Thanks for the help
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    As per the datasheet, the 4511 isn't actually capable of driving these anyway as it is limited to 25mA.

    The datasheet does suggest a method to drive common anode displays -- it requires an additional transistor and resistor per segment.

    You could possibly use an inverting line driver in place of the base resistor and transistor. Alternatively, there are a number of other driver chips. One of the many ULNxxxx chips may be what you want. Some of them even have the base resistors incorporated.

    edit: Note that you will still require series resistors to limit the current to the segments.
     
  7. NikitaB

    NikitaB

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    Jul 28, 2011
    Quite true. So that is a double mistake on my part. I would like to use only a few digital out pins on the LabJack, so I don't think that the ULNxxxx will do the job, as I believe it will use ~8.

    What is the family of shift registers that can handle 60mA? I've been looking online, but can't find much besides the basic +/-30mA shift registers.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Digital output pins? Use the 4511 to drive the ULNxxxx.

    In this case the fact that you need a ULNxxxx or something to get the extra current means that a common anode display is a feature and not a bug.
     
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