Power Supplies, resistors, and ohm's law

In summary, the conversation is about using a 10amp 12v power supply with a 12v 3amp motor. It is explained that the power supply is capable of supplying 10 amps, but the motor will only draw 3 amps at 12 volts. It is also mentioned that the current drawn by the motor may vary depending on its RPM. The conversation also discusses the possibility of using resistors or other methods to limit the current in case of a start-up surge or stall of the motor. It is recommended to keep the question and answer simple, and to refer to sources for further information on voltage and current sources.
  • #1
StephenDoty
265
0
Hey

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?

Thanks
Stephen Doty
 
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  • #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!
 
  • #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?
 
  • #4
StephenDoty said:
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?

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.
 
  • #5
StephenDoty said:
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?

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:

1. What is a power supply and how does it work?

A power supply is an electrical device that provides the necessary power for electronic circuits to function. It converts high voltage alternating current (AC) from the electrical outlet into low voltage direct current (DC) that is safe for electronic devices to use. The power supply typically consists of a transformer, rectifier, and regulator to convert and control the voltage and current.

2. What are resistors and why are they important?

Resistors are electronic components that resist the flow of electrical current. They are commonly used in circuits to control the amount of current or voltage, and to protect other components from damage. Resistors are important because they allow us to control and manipulate electrical signals in a circuit, enabling us to design and build various electronic devices.

3. What is Ohm's Law and how is it used?

Ohm's Law is a fundamental principle in electronics that describes the relationship between voltage, current, and resistance. It states that the current through a conductor is directly proportional to the voltage across it, and inversely proportional to the resistance of the conductor. This law is used to calculate the values of voltage, current, or resistance in a circuit, and to understand how changing one of these variables affects the others.

4. How do I calculate the resistance of a resistor?

The resistance of a resistor can be calculated using Ohm's Law: R = V/I, where R is resistance in ohms, V is voltage in volts, and I is current in amps. You can also use a multimeter to measure the resistance of a resistor. The value of resistance is indicated by the color-coded bands on the resistor, or can be looked up using a resistor color code chart.

5. What are the different types of power supplies and resistors?

The most common types of power supplies are linear power supplies, switching power supplies, and uninterruptible power supplies. Resistors come in different types such as carbon composition, metal film, and wirewound, and can also vary in their resistance values, power ratings, and tolerance levels. There are also specialized resistors such as potentiometers, thermistors, and varistors that serve specific purposes in electronic circuits.

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