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segment current when multiplexing — Parallax Forums

segment current when multiplexing

jonwjonw Posts: 67
edited 2006-11-25 15:39 in General Discussion
I have a max 7219 display driver and a stamp bs2 with a counting program.
·I want to measure the segment current.· I have a digital scope and a DDM. Will I get a meaningfull reading of the current at this multiplex frequency?
·
······ Also I have the BOE but would like to build· my counter on to a·PCB using the 24 pin bs2 module only,· what parts do I need for the stamp?·
········· Is it· only·· 1· 7805
··························2·· DB-9
···················I want to·built it with the 9 pin DB-9· but use· a USB·cable sometimes,···can I do this······· Jonw·

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-11-23 16:18
    from Page8 of the datasheet:

    INTENSITY CONTROL AND INTERDIGIT BLANKING

    The MAX7219/7221 allow display brightness to be controlled with an external resistor (RSET) connected between V+ and ISET.· The peak current sourced from the segment drivers is nominally 100 times the current entering ISET.· This resistor can either be fixed or variable to allow brightness adjustment from the front panel.· Its minimum value should be 9.53K, which typically sets the segment current at 40mA.· Display brightness can also be controlled digitally by using the intensity register.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2006-11-23 20:09
    As for a PC Board, Monday a complete PCB Layout will be posted for a Digital Alarm Clock, which uses the MAX7219 and four 7-Segment Displays.· It’s actually two boards, one control board and one display board.· There is also the BS2, DS1804 and LM386 and a backup battery.· You might find it useful.

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    Chris Savage
    Parallax Tech Support
  • jonwjonw Posts: 67
    edited 2006-11-24 04:03
    Yes·· But can· I get a meaningfull measurement with a meter?

    ············ jon
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-11-24 16:48
    As I see it -- No.· It's "strobing" too fast.·

    You could use a low-value (<< 1ohm) resistor in series with a segment, as a shunt, and·using an oscilloscope, view the voltage-drop across it (if this is important to you.)

    What's the gag?
  • metron9metron9 Posts: 1,100
    edited 2006-11-25 15:39
    If you know

    the forward voltage of each segment
    the resistance
    the max7219 output voltage
    the frequency and duty cycle of the output

    could you just calculate the average current and maximum current?

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    Think Inside the box first and if that doesn't work..
    Re-arrange what's inside the box then...
    Think outside the BOX!
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