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DAC Problems

Discussion in 'Electronic Basics' started by Immo Birnbaum, Jul 19, 2004.

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  1. Hello everybody,

    after spending two semesters listening to electronics lectures, I've
    recently started assembling some boards with simple circuits on them
    and I've also hooked some of these up to a microcontroller evaluation
    board based on a Motorola HC08. Recently I bought some parts for my
    next little project, but I just can't get the job done. I'd greatly
    appreciate if anyone around here could help me with that little
    problem.

    Here's what it's all about:
    I've got several old speed/traction gauges which were once used in the
    dashboard of a locomotive. Operation of these gauges is really simple:
    they have a + and a - connector, so just hook them up to a PSU, set a
    voltage and the gauge will show an according value. The gauge reaches
    "top speed" at approx. 3.4 Volts, measured with the multimeter
    parallel to the gauge's connectors. Here's a picture:

    http://immo.birnbaum.bei.t-online.de/t1.jpg

    At that "speed", the current is approx. 1.5 mA, gauge and multimeter
    connected in a row. See also:

    http://immo.birnbaum.bei.t-online.de/t2.jpg

    Great, I thought, this thing could be computer-controlled - it would
    make a great CPU load meter or something similar (of course replacing
    the km/h scale with something else). So I went out and bought a few
    DAC chips, type AD7524, an 8-bit DAC, and did the following:

    - connected D0 to D7 to the microcontroller board
    - connected WR and CS to GND, as those are low-active
    - connected VREF to the board's power supply, which is the output of a
    7805, with a potentiometer inbetween so that I can regulate the
    reference voltage.

    When I started testing the board, I had D0 to D7 connected to the
    HC08, but later just set all data inputs to high, so that VOUT is
    always =VREF. I'd later reconnect the data inputs to the
    microcontroller. So far, the entire thing worked. I set the
    potentionmeter so that the voltage above it was approx. 3.5 V.
    With all the data inputs high, the output voltage (OUT1 to GND)
    also was 3.5 V as long as I didn't connect the gauge to the DAC's
    output. As soon as I did that, the output voltage dropped to 0.9 V and
    the needle of the gauge didn't move any further than 40 . When
    reducing the resistance of the potentiometer, the needle would move a
    little further, but not too much.

    Can anyone tell me what's wrong here? I hooked up two of these gauges
    to the output of the 7805 with potentiometers inbetween to check if
    the 7805 could supply the two gauges at all, and it worked fine. Could
    it be that the DAC cannot supply the required current? Do I need an
    op-amp?

    Thanks in advance if anybody could help me with that problem.
    Immo Birnbaum

    P.S. I also tried the DAC's "voltage mode" which was described in one
    of the datasheets, which means connecting VREF to OUT1 and getting the
    output from VREF or something like that, the effect was the same.
     
  2. Jamie

    Jamie Guest

    sounds like your over loading the DAC.
    try using a simple emitter flower current booster transistor to
    drive the meter.
    P.S.
    you will need to acount for the apox .6 volt drop on the
    BE.
    something like a general switching transistor should do.
     
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