How to calculate resistance needed for voltage drop

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Discussion Overview

The discussion revolves around calculating the resistance needed to drop the voltage from a 19V DC power supply to 12V for a pump motor rated at 3.5A. Participants explore different methods for achieving this voltage drop, including using a resistor and a Zener diode, while considering the implications of varying load conditions on the pump.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculates that a 2Ω resistor rated for 25W is needed to drop the voltage by 7V at 3.5A, suggesting that their calculations are correct.
  • Another participant questions whether the power supply can maintain its voltage under load, indicating that the output voltage may sag as current is drawn.
  • Some participants propose using a Zener diode as an alternative method for voltage regulation, noting the need to select a diode that can handle the current requirements.
  • A later reply clarifies that the Zener diode should be wired in parallel with the motor, and discusses the meaning of terms in the context of a Zener regulator circuit.
  • There is mention of considering a regulator circuit if the pump load varies, which could affect the voltage supplied to the pump.

Areas of Agreement / Disagreement

Participants generally agree on the need for a voltage drop and the calculations for using a resistor, but there is no consensus on the best method to achieve this, as some suggest using a Zener diode while others consider a regulator circuit.

Contextual Notes

Participants express uncertainty about the design of the Zener regulator and the implications of varying load conditions on the voltage supplied to the pump. There are unresolved questions regarding the specifics of wiring and component selection.

taeagan
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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.

Thanks!
 
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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.

Welcome to Physics Forums.
 
The power supply reads that it is rated for DC output 120W or 6.32A.
 
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.
 
Cant we use a ZENER DIODE ?
 
Er.Electrical said:
Cant we use a ZENER DIODE ?
Sure can.
zenereg.gif

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.
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/zenereg.html#c1"
 
Last edited by a moderator:
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?
 
Last edited by a moderator:

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