Cutting Amps With High Wattage?

Discussion in 'The Projects Forum' started by Big Expectations, Nov 14, 2012.

  1. Big Expectations

    Thread Starter New Member

    Nov 14, 2012
    For my engineering class I am creating a kit that can be used to teach/demonstrate magnetism to children. My idea involves cutting the current going through the circuit so the electromagnet loses around half it's power. Sadly I'm no good with electronics and don't know what to do.

    I figure I'll be using:
    ~10 ft of wire
    two 9v batteries wired in series

    So the idea is the battery -> on/off switch -> a selector switch (DPST) ->one path goes straight to the wrapping wire and the other goes through a resistor before going to the wrapping wire and connecting back to the battery.

    My issues are that from my calculations I'm working with around 78.5 watts and no resistors can handle that from my understanding. How do I go about lowering the amperage if I can't use resistors due to watts?
  2. wayneh


    Sep 9, 2010
    78 watts from 2 9V batteries? Surely these are not little "transistor" batteries?

    Anyway, just put a lightbulb in series, in parallel with a switch that will short across the bulb, allowing full current. The bulb will dissipate the heat easily and give a nice visual feedback.

    And there are resistors that could do the job, but they're not cheap.

    You could build a simple variable current supply with a single power transistor, if that's interesting.
  3. tshuck

    Well-Known Member

    Oct 18, 2012
    I like wayneh's solution, but as a bit of caution, understand that a light bulb is not Ohmic and it's resistance will change with the change in heat made from lighting the filament, so don't count on a constant current/field/strength/power...