limiting amps from battery

Discussion in 'General Electronics Chat' started by bert77, Nov 16, 2014.

  1. bert77

    Thread Starter New Member

    Nov 14, 2014
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    hi this is a general question about resistance of a wire...

    If there is a 24v battery that can supply 500 amps (cca) and is rated a 80 ah, what is the best way to limit the flow to say 20 amps in a short circuit without affecting the voltage too much?

    if the (short) circuit is protected by a 20a fusewire it will of course blow, but if the fusewire is extended long enough it will become a heating element and i guess increase the resistance due to heat.

    my question is, if the short 20a fuse is kept cool by means of a very large/efficient heat sink to stop it blowing, would it throttle the flow of amps as required to 20?

    alternatively, would a soda crystal solution (salt tank with large electrodes) always be the best way to limit dc current?

    bert
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    Um, no. The increased resistance IS NOT due to heat.

    The resistance increased due to the increase in the length of the wire. The additional length is additional material. The pass of the electrons just got longer, maybe even much longer. So the electrons are going to keep bouncing inside the wire for longer time because they have to travel longer distance because you increased the length of wire. All that bouncing around and friction is actually transfer of energy from electrons to the rest of the wire, which in turn is heat.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    cooling the fuse might raise the current rating, but ti will still blow around 20 amps, not throttle current, open up, thats the job of fuses.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    You could use an electronic current limiter circuit. But note that under short-circuit conditions the limiter will be dissipating about 20A * 12V = 240W. Will the circuit need to tolerate a short-circuit for any significant length of time (?) as that has a large factor on the heat sink requirements for the limiter.
     
  5. #12

    Expert

    Nov 30, 2010
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    6,744
    378 feet of 12 gauge wire will provide 0.6 ohms of resistance and qualify as large enough to not catch on fire, as long as you don't keep it wound on a spool. (Give it some air flow.) Still, the usual way to limit current to 20 amps is to build a device that already has 0.6 ohms of resistance. Your basic premise seems foreign to us. Why waste 240 watts of power in a large resistor instead of inflicting that power on the job you really want to do?

    What is the job you really want to do?
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I would use a reset-able circuit breaker like the ones in my airplane. They come in various current ratings, and can be reset as soon as they cool down...
     
  7. bert77

    Thread Starter New Member

    Nov 14, 2014
    5
    0
    good points,
    the application for the current is spray arc mig welding. when the wire touches the metal, there is a short, and then a resistance as the current pass through the plasma generated, (assuming arc is maintained by correct wire feed speed), otherwise the shorts repeat as the arc collapses and the circuit closes again, (hence the sizzle/crack sounds as this repeats).
    welding machines of this sort have voltage stabilisers and current limiters, but am trying to make my passive setup using 2x 12v car batteries work a bit better with thin steel.

    ideally 18v is needed across the arc so i need to lose 6v aswell.

    i managed so far to get a .3 ohm resistance in the circuit with about 3m of 1mm copper, but not sure my multimeter is very accurate with fractions of an ohm.

    thanks for any further advice
    b.
     
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