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Adjustable 24V DC power

Discussion in 'Electronic Repair' started by [email protected], Jul 10, 2007.

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  1. Guest

    Hi all,

    I need some help here.
    I want to verify a voltage "trip" point on a board that operates on
    battery supplied 24vdc.
    The maximum draw is about 600MA

    The board should change to "recharge required" at 22VDC

    My thought is to put some sort of resistor in line on the main battery
    feed so I can slowly reduce the voltage to a point where it trips the
    recharge indicator. A voltmeter after the resistor would allow me to
    monitor the voltage level where the recharge indicator is activated.

    I am not an electronics guru (obviously) so something "off the shelf"
    would be great.
    Maybe some type of adjustable resistor? Any ideas?

  2. DaveM

    DaveM Guest

    The easiest way to do that would be to beg or borrow an adjustable power supply.
    Substitute the battery with the power supply.
    Set the power supply to the nominal battery voltage, connect the voltmeter to
    the power supply output, then slowly adjust the power supply output voltage
    until the recharge indicator turns on.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    "In theory, there isn't any difference between theory and practice. In
    practice, there is." - Yogi Berra
  3. Zebra

    Zebra Guest

    How about using two 12V car bulb (current drain can be control by selection
    of watts) and connected them in series (making up a 24V load). Then connect
    it at the battery and wait for the voltage drop.
  4. Guest

    Unfortunately, I don't have anyone to beg or borrow one from. I hoped
    to just add the "adjuster" set up to an extra battery pack for future
    testing use. Is there anything out there that i can put in line? If
    anyone knows the proper terminology for what I need, I can try and
    search for one.

    Thanks again!
  5. Arfa Daily

    Arfa Daily Guest

    If the draw is really around 600mA, then you're not going to do it with a
    pot. As Dave suggested, the best way to do it would be with a variable PSU
    subbed for the battery, but failing that, you could use 1 ohm resistors
    inserted in series with the battery. Each one added in series will cause a
    drop of about 0.6v (not quite true as the amount dropped depends on the
    current draw, and as the volts drop, the draw will probably also drop as
    well, but near enough to do what you want to do). Each resistor should be a
    1 watt type, which will still get pretty warm. If you wanted to use a higher
    wattage, that's fine. If it does what you want, and you want to be able to
    use it again in the future, you could just wire the resistors to a single
    pole rotary switch. If you want to go a bit 'finer' with each step, you
    could use 0.47 ohm resistors to give about 0.3v per step, or 0.22 ohm for
    half that again. Obviously, these are all approximate figures, but right
    ball park for what you want, and not of any consequence how accurate anyway,
    if you are going to have a voltmeter slung across the test. If that sounds
    like what you need, and you don't know how to wire such a switch, mail me

  6. Franc Zabkar

    Franc Zabkar Guest

    Make up a bank of series cells, eg 14 or 15 AA alkalines, or C or D
    cells if you can justify the cost. Just do your testing quickly.
    Otherwise try to disconnect the load section of your circuit and keep
    the sensing section operating.

    - Franc Zabkar
  7. whit3rd

    whit3rd Guest

    It's easy to run an adjustable voltage regulator (LM317) for this
    purpose; two fixed resistors and one adjustable resistor, the LM317,
    and a small capacitor (probably not required, but it's recommended)
    will make a 18-23V regulator. Just power the regulator from the
    battery, wiggle the pot 'til it measures 22VDC output, and draw
    a red line there. Or do it right, put a 'too low' line at 21.3V and a
    'too high' line at 22.6V.

    Radio Shack should have all the parts.

    The regulator can be inserted between a fully charged battery and
    the unit-under-test. It's going to need some heatsinking on the
    regulator (get an aluminum box for the project, and bolt the
    regulator chip through the convenient hole).
  8. Zebra

    Zebra Guest

  9. Bill Jeffrey

    Bill Jeffrey Guest

    Power it up with 16 "C" or "D" batteries in series (16 x 1.5 = 24
    volts), and wait for the voltage to drop. Buy the cheapie batteries
    (zinc carbon, often labeled "super duty" or something equally
    misleading), since they are less expensive and discharge quicker than

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