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How to calculate resistance needed for voltage drop

  1. Aug 24, 2011 #1
    Hi. I'm hoping someone can help me out with a pretty simple physics question. My electrical circuit skills are pretty rusty, so please bear with me.

    I'm using a 19V DC laptop adapter to power a 12V DC pump motor. The pump is rated for 3.5A. I have an on/off switch between the power supply and the motor. I'm trying to figure out what kind of circuit I need to put in between to drop the voltage from 19V out of the battery to 12V at the motor.

    By my (likely overly simplified) calculations I think that I need a 2ohm resistor rated for 25W+. Is it as simple as just putting a resistor in between the + side of the battery and the motor? Do I need anything more complicated than that? I have to think that I'm wrong here.

  2. jcsd
  3. Aug 24, 2011 #2


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    Does you 19V DC adapter indicate at what current it can supply at that voltage or what its power rating is? Most of these wall warts output voltage can sag as you draw power from them.

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  4. Aug 24, 2011 #3
    The power supply reads that it is rated for DC output 120W or 6.32A.
  5. Aug 24, 2011 #4


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    Sounds like it's good enough for what you are doing. So if you want the voltage at your pump to be 12 volt, you'll need a drop of 7 volts. At the rated pump current of 3.5 amp you would need a resistance V/I=7volts/3.5amp=2Ω. The power dissipation would need to be I2R=(3.5amp)2(2Ω)=24.5watt. So your calculations are correct.

    One thing to consider is if the pump load varies (depending on what's being pumped), the pump current draw will vary, hence the voltage you supply the pump with will also vary. One could consider a regulator circuit if this is a problem.
  6. Aug 24, 2011 #5
    Cant we use a ZENER DIODE ???
  7. Aug 24, 2011 #6


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    Sure can.
    Now you'll have to pick a Zener diode that can handle the current requirements. Here a calculator that will get you in the ball park.
    Zener Regulator/Zener Regulator Design
  8. Aug 25, 2011 #7
    Thanks, guys. I'm definitely impressed with myself that I was able to get my brain to figure that out, as simple as it was.

    However, you lost me with the zener diode. I completely understand the concept that the voltage may vary with load on the pump, however, I have no idea how I would go about designing the zener regulator.

    In the diagram, is "unregulated power supply" my charger/adapter? Assuming so.

    What's Vz? Is that my pump? If so, does that mean that the diode and the load resistor are just wired in parallel with the pump?
  9. Aug 25, 2011 #8
    Last edited: Aug 25, 2011
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