power problem (newbie here!)


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  1. #1
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    Apr 2009
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    Default power problem (newbie here!)

    Hi all,

    I'm making a very simple clock using a PIC16F870, with 4x 7-segment displays and latching drivers (HCF4511BE) to keep them nice and bright.

    In testing the circuit with simple static digits, I get a problem when I connect more than two of the displays. I've worked out that it's a power issue as the 5 volt regulator (L7 805CV) I'm using is getting very hot.

    What it the best way to isolate the logic from the display circuitry?

    thanks for your time!

  2. #2
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    Smile

    Hi clocks,

    Welcome to the forum. Do you have a schematic of your project that you can share with us so that we can get an idea of what your circuit consists of? Do you have current limiting resistors between the 4511's and the displays?

    BobK
    Last edited by BobK; - 30th April 2009 at 00:55. Reason: Grammar

  3. #3
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    Default heres the schematic...

    thanks for the interest nd quick response!

    below the schematic for a single display (with R220 resistors in place)

    the same is repeated four times using Portc.4-7 to control the latches.

    and the code:

    B var portc.0
    C var portc.1
    D var portc.2
    A var portc.3
    SEG4 var portc.4
    SEG3 var portc.5
    SEG2 var portc.6
    SEG1 var portc.7


    trisc = %00000000

    adcon1 = 7


    test:
    low seg4
    gosub number1
    high seg4
    pause 1000

    low seg3
    gosub number2
    high seg3
    pause 1000

    low seg2
    gosub number3
    high seg2
    pause 1000

    low seg1
    gosub number4
    high seg1
    pause 1000
    goto test



    BLANK:
    HIGH D
    LOW C
    HIGH B
    LOW A
    return


    NUMBER0:
    LOW D
    LOW C
    LOW B
    LOW A
    return

    NUMBER1:
    LOW D
    LOW C
    LOW B
    HIGH A
    return

    NUMBER2:
    LOW D
    LOW C
    HIGH B
    LOW A
    return

    NUMBER3:
    LOW D
    LOW C
    HIGH B
    HIGH A
    return

    NUMBER4:
    LOW D
    HIGH C
    LOW B
    LOW A
    return



    end
    Attached Images Attached Images  
    Last edited by clocks; - 30th April 2009 at 01:20.

  4. #4
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    Default ...and

    (sorry, it may not be clear that I have tied all the BCDA driver inputs together)

  5. #5
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    Hi Clocks,

    Any particular reason you are using LEDs instead of an LCD? Have you seen the data sheet for the 4511? You can do all 4 displays with 1-4511 and the SEG 1-4 pins would then go to transistors to drive the ground pins on the displays. Microchip has an application note showing how to make a clock using just the displays connected directly to the PIC. You might want to check it out.

    I have a clock product that I make that uses the Maxim DS1337 clock chip that has 2 alarm outputs. I don't have a fixed display on the product but when I need to check to make sure all is well with it, I plug a serial display onto the board. 4 PIC pins to the clock chip and 1 pin to the display.

    There have been quite a few clock programs on this forum over the past few years. You might want to do a search for them and also look at the Microchip site (www.microchip.com) for the app note on the clock.

    I must be honest with you though, I have never built a clock with LED displays but I can provide you with some guidance and if I can't help you there are many great people on the forum here that probably can.

    HTH,

    BobK

  6. #6
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    Default

    How hot is hot? 7 leds running ~15mA each = 105mA, worst case scenario you multiple it by the number of display you have + few mA for your PIC.

    Then you multiply it with the voltage across your voltage regulator (Vo-Vi), and you have the power dissipation for your voltage regulator.

    Assuming you're running it from 12 volt, things can heat up fast indeed. Heat sink will help, but not always necessary.
    Steve

    It's not a bug, it's a random feature.
    There's no problem, only learning opportunities.

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