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Power Supplies, resistors, and ohm's law

  1. Jan 10, 2009 #1

    I have a 10amp 12v power supply and a 12v 3amp motor. Is there any way to add enough resistors to use the 10amp power supply with the 3 amp motor? If not is there any thing that I can put in line with the motor and power supply so the 10amps won't burn up my motor?

    Stephen Doty
  2. jcsd
  3. Jan 10, 2009 #2
    You can use it without any resistor. 12V 10 amp only mean the supply is capable to supply 10A, you can use only 3A out of 10A. It is the voltage that is important. You have a 12V motor, so you can hook directly onto the 12V 10A supply and it will only draw 3A and both the supply and the motor will be happy!!
  4. Jan 11, 2009 #3
    so even if I get a 40amp 0-30v power supply and set it to 12v, the motor will pull only 3 amp from the power supply even if it is on 40amps?
  5. Jan 11, 2009 #4
    You stated that you have a 12 volt 3 amp motor, but you didn’t mention at what RPM the motor draws 3 amps. The current that a motor's armature draws varies with armature RPM.

    The commonplace PMDC motors for instance, draws maximum current at start-up (armature motionless). As the armature begins to increase in RPM, a back emf voltage is generated that is proportional to the armature's angular velocity. This generated back emf voltage opposes the applied voltage thereby reducing the amount of current that will be drawn through the armature windings from the applied voltage source.

    The electric fuel pump on my vehicle draws a steady 3 amps at its high RPM operating RPM, but when 12 volts is initially applied (armature motionless), the instantaneous current draw briefly exceeds 12 amps. The fuse is only rated at 10 amps, but it's a slo-blo fuse. The fuel pump armature accelerates so quickly that the current very rapidly decreases to the steady 3 amps, which prevents the fuse from producing enough heat to blow the fuse. Even so, my vehicle’s battery and its electrical wiring are capable of handling this load.

    If your particular motor happens to also draw an instantaneous current that just briefly draws 10 amps or more, it may cause your power supply to crowbar and shut down. In this case, you may have to install a series resistor and a 3-position switch. The positions would be for: Off, Start, & Run. Upon start-up, the switch position places a current limiting resistor in series with the motor’s armature windings so as to limit the initial high start-up current. Once the motor has come up in speed (its current demand reduced), simply flip the switch to Run and the series resistor is eliminated and the motor runs directly from 12 volts with its normal torque and velocity for the given physical load.
  6. Jan 11, 2009 #5
    Yes!! Even the power supply has 40A capability, the motor has internal resistance that can only draw 3A at 12V. The amprage of the power supply tell you how much it can put out. Current drawn is governed by the load.

    You put a 1K resistor across a 10V power supply that can provide 10A, the resistor will only draw 10mA as long as the power supply put out 10V. This is just as simple as I=V/R!!!

    I am not counting on any of the start up current surge like the other responder said and anticipate what if the the motor get stalled because you have not mentioned any of that. If you have other more complicate situation, please specify it.

    You ask a simple question, I give you a simple answer!!!! If you get into it, you can always adjust the current limit of the supply, put a fuse etc. But that is out of the context and it can get complicate.
    Last edited: Jan 11, 2009
  7. Jan 11, 2009 #6
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