Question:

What resistor should I use?

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I am trying to build a circuit that has a DC motor attached to it. The motor draws .5 amps at 5volts. I'm going to have 12 volts going into it. How do I figure out which resistor to use to get the supply down to .5amps at 5 volts?

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  1. A resistor would drop 12-5 = 7 volts.

    7 volts at 0.5 amp is 14 ohms.

    But power dissipation is 3.5 watts, so you need a 5 or 10 watt resistor.

    A linear regular like a 7805 with a small heat sink will also work.

    A switching regulator like a LM2698 will work even better as you will only be drawing 0.25 amps from the battery.

    http://www.national.com/pf/LM/LM2698.htm...

    You may also be able to buy an assembled switching regulator for $10 or so.

    .


  2. Don't use a resistor; use a 5V voltage regulator instead (a few bucks at Radio Shack or any other electronics store).

  3. Either a resistor or a linear regulator will be quite inefficient, 12 - 5 = 7 volts drop times 5 amps is 35watts of power going to heat.

    Also the current draw of a motor will vary with load, so with a resistor you would have very poor speed regulation and torque. However, 7volts drop at 5A would take a 7/5  = 1.4Ω resistor rated at least 35watts a 1.5Ω would probably be close enough and probably easier to get. In fact Radio Shack has a 1Ω 10W, 2 for $1.99, if you bought 6, made up two strings of three in series and connected the two series strings in parallel you'd have 1.5ohms and a a nominal 60W rating. for $6, it will still work very poorly, but if you really want to use a resistor it is an option. (but double check the power rating, the description at http://www.radioshack.com/product/index.... says 10W, but the resistor in the picture is marked 2W it might jest be a generic picture, or the description might be wrong)

    Also the simple IC regulators you could get at Radio Shack can not handle 5A, their data sheets may well have example circuits for boosting the output be that will be a more complicated construction project and it would still be a power wasting linear regulator.

    A switching regulator or PWM (pulse width modulation) motor control would be much better way to go. Or possibly better yet would be to just get a 12V motor, unless there is something special about your 5V motor, it might be easier and cheaper to change to motor than to find a 5A + switching regulator. Here is one regulator that is only 2.5A (not enough) and it is $29.95 http://www.roboticsconnection.com/p-38-5... I'm sure a little more searching will find one with sufficient current, but I don't know that you'll find one a lot cheaper. Just for comparison, here is one 14.4V (pretty close to 12) 2.7A (more power than your 5A @ 5V) Motor for less than $11; http://www.trossenrobotics.com/store/p/5... I don't know that it will be suitable for you application, but it might be.

    [EDIT] Oh, **** I missed the decimal point, that was 0.5A not 5A well, never mind, most of the above makes no sense in that case. But still speed regulation and torque will be very poor with a resistor.  As the power loss will be much less at 0.5A a cheap 3 terminal linear regulator might work OK, but your stall torque may be rather low. The $30 2.5A regulator should work, giving considerable overload capacity.

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