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using 12 volt battery with a 7.2 volt drill

Discussion in 'Electronic Basics' started by Mike, Aug 3, 2006.

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  1. Mike

    Mike Guest

    Can I use a 12 volt battery with a 7.2 volt drill. these are for basic
    robotics we are trying to get into. Also would a 12 volt battery power
    a 14.4 or 18 volt drill.
  2. Eeyore

    Eeyore Guest

    Google Groupie alert !

    WTF makes you think you *could* ?

  3. jasen

    jasen Guest

    with PWM limiting the current that should be OK. without it may not last long.
    can't get any 12V drills cheaply ?

    14.4 would probably be OK, 18 probably not.

  4. Tom Biasi

    Tom Biasi Guest

    Yes you can but what would be the results?
    DC motors of this type are quite tolerant of overvoltage and would spin a
    little faster and get a little hotter, (more than just a little under load).
    Providing less voltage will reduce the RPMs and torque.
    Since you are beginning robotics, I would like to suggest that you get used
    to following specifications.
  5. Chris

    Chris Guest

    Hi, Mike. I guess you could use a 12V battery for a 7.2V motor, if you
    drop the extra volts. The simplest way to do this would be to get a
    string of five 1N5402 diodes, and put them in series between the
    battery and the motor like this (view in fixed font or M$ Notepad):

    | 5 X 1N5402
    | .-->|-->|-->|-->|-->|--.
    | | |
    | +| / \
    | --- ( M )
    | 12V - \_/
    | | |
    | | |
    | '----------------------'
    (created by AACircuit v1.28.6 beta 04/19/05

    This is OK for a small motor that's drawing less than 3 amps full load.
    Of course, you'd be wasting 5/12ths of your battery power as useless
    heat. And the diodes aren't going to regulate the motor voltage all
    that well.

    Speed and power of DC motors is dependent on input voltage. So, your
    14.4V and 18V motors will run, although the 18V one will be slow and

    Better to think 'er over, and get the right parts for the job. There
    are plenty of DC motors available for robotics that are made for 12V

    Good luck
  6. Drop the voltage with a bunch of heavy diodes in series. I know of someone
    who operates 7 volt drills off a car battery. He uses the drills in
    connection with a mobile logging operation.

  7. Rich Grise

    Rich Grise Guest

    If it's for a robot, you're going to be using PWM control anyway, right?

    Just make sure that your maximum duty cycle <= (7.2/12).

    Is it really a 12V battery, or is a lead-acid? In that case a nominally
    12V battery usually has 13.2 volts or so open-circuit.

    Good Luck!
  8. Hi,

    could you possibly explain why you prefer diodes here to using a voltage
    divider? I've googled a bit and I think I understand that it's because
    the load is significant and with a voltage divider the voltage drop is
    proportional to current

    ie increase the load => increased current => increased voltage drop
    => thereby limiting the voltage available for the load

    Is my understanding along the right lines?


  9. jasen

    jasen Guest

    could you possibly explain why you prefer diodes here to using a voltage

    silicon diodes will drop about 0.6 to 0.8 V each over a pretty wide
    range of load currents and temperatures, resistors will give a drop
    that's proportional to the current flowing through them.
  10. Alan B

    Alan B Guest

    Yes, your reasoning is correct. The diode scheme - let's say the logger
    uses seven diodes to run a 3A 7V drill from a 12V battery - will be wasting
    ~2W per diode. So why not use a voltage divider with a minimum 14W source
    resistor? The problem is that unless your voltage divider itself draws at
    least 10x the current as the parallel load, the varying load will
    significantly vary the voltage across the motor. In the case of a 3A load,
    you've got to have a divider that draws at least 30A. Wowsers! Since
    diodes are non-linear, they will drop the same voltage regardless of
    current, so you simply get diodes hefty enough to handle the maximum load.

    Note that I said the diodes will waste ~2W per diode: energy used that is
    not doing useful work. That's one answer for the OP's question - be
    cautious when mis-matching source and load, as apart from any other
    concerns, you are wasting energy.
  11. OK - understood. Thank-you to you and to Alan B for your responses.


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