Go Down

Topic: What resistor do I use? (Read 226 times) previous topic - next topic


What resistor do I use?

I have a 24v motor that needs 9A  and a relay with an output of 10A 24V.
What reesistor can I use to get it down to 9A.
I am not being lazy i generally do not understand how to get from A=V/Ω
to the needed ohms.

Thanks for the help.


Oct 21, 2019, 08:44 pm Last Edit: Oct 21, 2019, 08:46 pm by DVDdoug
You don't have to add a resistor. 

24V/9A is 2.67 Ohms.    That's the resistance of the motor when it's drawing 9A.    That's probably  the startup/stall current and it will probably "draw" less current (have higher resistance) with no load on the motor (after it get's going).

You wont find a resistance spec for a motor, only voltage and maximum current.

Of course, your power supply has to be capable of supplying the necessary current.

The voltage & current contact ratings on a relay are the maximum.   i.e.   It's OK to use a 120V / 10A relay to switch 12V at 1 Amp.  (The coil voltage & current are "exact" specs.)

The current rating on a power supply is the maximum.   If you have a 12V 10A power supply, you can connect a regular little LED (with the usual current limiting resistor) and you'll just get a few milliamps.   

LEDs need a resistor because they are non-linear.    (Their resistance goes down as the voltage goes up.)




I am assuming your load is not constant so the resistor will constantly have to keep changing.  The simplest solution is to either use a Buck Converter, that will reduce your loses by about 60% which will be dissipated as heat in your resistor.
They also make motor controllers for a few dollars available on eBay.

Disregard this statement: "LEDs need a resistor because they are non-linear.    (Their resistance goes down as the voltage goes up)." LEDs are current devices not voltage devices as many assume hence the reason for the resistor is to limit the current.  They have a VF rating which is the  Forward voltage across the LED.  This value changes with current but in many instances it is actually more stable then a zener when used with a relative constant current.  I have used them as cheep low voltage zeners. They also have a current rating which must not be exceeded, many have a maximum of 20 milliamps but they can go to many amps.

The motor should be rated in watts, With that and the other information connect to a ohm's law calculator (http://www.ohmslawcalculator.com/ohms-law-calculator and you can calculate what you need.  Motors have a rating called locked rotor current, that is the current it will draw upon initial start or salled.

If my assumptions are incorrect you need a 2.66667 ohm resistor capable of dissipating 216 watts. Using a buck converter at 90% you will dissipate about 22 watts. That should reduce the amperage requirement from your 24V power supply by about 8 amps.

Good Luck and have Fun,
This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!


Oct 28, 2019, 04:24 pm Last Edit: Oct 28, 2019, 07:28 pm by SteveMann
I have seen a repeated confusion about AMPS from new users.

A component or device needs current to operate.  LEDs, relays, motors, processors, and displays are examples of components and devices that require current (at a specific voltage) in order to operate.  The current requirement of the component or device is measured in Amps or milliAmps (and sometimes microAmps, and rarely nanoAmps).

Passive components like wires, switches and relay contacts may have a current rating, usually in Amps.  This is the not to exceed amount of current that that component may withstand while transporting current to the device that needs it.

Power supplies are rated in Amps.  This is what power is available.  As long as the sum total of the Amps required by your devices is lower than the rating of the power supply, you are good to go.

For example, this power supply (A.K.A. Wall Wart) is capable of delivering 1.5 Amps at 12 Volts DC.

Please do not ask for help by PM. I will not respond.
If you need help, post a question on the appropriate forum.


A general rule for power electronics is no resistors.

Excess current capability in a supply is of no consequence, it will just run cooler.

Excess voltage in a supply means power-conversion is needed, which means switch-mode, not resistors.
For instance PWM control of motors is exactly this - the inductance of the motor works with the PWM to
act as a power converter.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

Go Up