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Wiring a Dust Buster to a car's power...questions about when amps are drawn and how.

Discussion in 'Electronic Basics' started by Thomas G. Marshall, Mar 25, 2007.

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  1. I have a dust buster (hand held vac) that is considered the 7ish volt
    version and whose internal battery is charged with a 11 volt (and some small
    number of milliamps) AC->DC converter.

    Ok, two fundamental questions:

    1. If I wire the vac (complete with its battery) to the car's battery (via
    the aux "cig lighter" port), is it possible for the car to somehow overwhelm
    the vac's battery and fry it? I know that frying is possible when charging
    up a car battery with a charger left on manual for too long, so I figure
    that maybe batteries are considered shorts in the circuit somehow and will
    draw every amp you give it (???) That would mean that the vac battery would
    be fried?

    2. If I remove the vac battery and just connect the 12V car line to the
    motor, will the motor draw just the number of amps it needs? (Ignoring the
    difference in voltage, which I'm assuming will only speed up a DC motor).

  2. Yes. You need at least a series resistance.
    Incorrect. You will smoke the motor.
  3. Guest

    Or 3.)
    You could get some cell phone cigarette plug charger to replace the
    wall-wart charger and use it to just charge the duster buster
  4. Wouldn't a resister just heat up and waste power? I'm missing some
    fundamentals here...see below.

    Hmmmm........I better ask this a different way.

    When does an item draw "just what it needs" and when is current force fed to
    it? A lightbulb in an a/c circuit if rated for 100W will draw 100W and no
    more regardless of how much current is available . Why? And yet a motor in
    my car will fry up?
  5. Gareth

    Gareth Guest

    Things usually draw the correct current when connected to a power supply
    of the correct voltage. You said in your original post that this hand
    held vac was "7ish volt". A car battery is 12ish volts, which is nearly
    twice as much.

    It may work if the manufacturers have been very cautious with the motor
    rating, or if you only run it for very short periods of time and allow
    the motor to cool down before running it again, but I would NOT
    recommend it.

  6. No. The resistor should limit the current while not operating to a trickle
    charge to the battery.
    If the voltages match you are fine. If you apply 11,000 volts to your TV it
    will never work again.

    Voltage is pressure. Current is flow.

    If the water pressure is too high the tap will blow off the wall. If the
    possible water flow is unlimited, the tap will allow what it needs.
  7. ehsjr

    ehsjr Guest

    Yes - which is exactly what you would need it to do.

    Your motor is rated at ~7 volts. If you give it the
    voltage it was designed for, it will take "just what
    it needs". You are going to ~double the voltage when
    you run it from the car's electrical system. That is
    why it will burn up - unless you reduce the power
    to it with a resistor (which will waste power as
    you noted) or by some other means.

    A light bulb rated for 100 watts on a 120 volt AC
    circuit would blow if you fed it with 240 volts -
    it would not take "just what it needs" to produce
    100 watts.

  8. Rich Grise

    Rich Grise Guest

    Is this a learning project? If all you want is a 12V car-vac, you could
    get something like this:

    To modify your existing one, we'd have to know a lot more about the
    specific unit that you have.

    Good Luck!
  9. What I was talking about was the current: It will take "just what it needs"
    if the voltage is the correctly matched, right? So there is no way to match
    the voltage but push too much current into it, is there?

    The analogy of pressure and flow are helping me here.
  10. Yes. If the voltage is correct, it will draw as much as it needs.
  11. ehsjr

    ehsjr Guest

    Right. When you talk about it taking just what it
    needs that automatically brings current to mind.
    Give it the right voltage, and there is no way
    to push too much current into it.

  12. Ah ok, I'm re-reading your reply before mine. You were pointing out that if
    you, say, double to the voltage to a 100 watt device, that it wouldn't
    simply draw half the current it needs to keep the total consumption at 100W.
  13. Unless it was designed to, no, it would draw double the current and four
    times the power. Bye bye gadget.
  14. ehsjr

    ehsjr Guest

    Exactly. In fact, the power consumption would go
    way up. (Well, briefly, because the device would likely
    burn out and then power consumption would go to zero)

    It is time for math, and ohms law.
    E = IR E is voltage, I is current and R is resistance.

    Consider a circuit with a ~12 volt battery (your car) and
    a ~6 volt load device - your dustbuster. We don't have
    more specs, so for the sake of the example, let's
    assume that the dusbuster takes 1 amp when it is connected
    to the proper voltage. E=IR, so 6 = 1*R, meaning R = 6.
    Now, when you plug the dustbuster into the cigarette
    lighter E changes from 6 volts to 12 volts. R - the
    resistance of the dustbuster, does not change. So on
    a 12 volt supply, 12 = I*6, meaning I = 2.
    So doubling the voltage means you doubled the current.

    Power (watts) = E*I. The dustbuster in the example
    drew 1 amp at 6 volts, meaning it consumed 6 watts.
    But at 12 volts, it draws 2 amps, meaning it consumes
    24 watts - *four* times as much! That will cook
    the dustbuster.

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