Stepping down voltage with resistors

[mngeow]

Okay so I have a 12V DC motor , but my supply is a 24V one. So I'll have to step down the voltage to 12V for the motor. So I was thinking of connecting some resistors in series with the motor to reduce the voltage.

But I'm not sure what value resistor to use. I tried to measure the resistance of the motor using a multimeter , then connect a resistor with the same resistance in series with it. So that each of them will receive 12V. But what I found out was that the resistor was getting some around 8.5V while the motor was getting roughly 16V. Am I doing something wrong here?

Thanks in advance

 

[Nigel Goodwin]

You can't use a resistor, as the current varies massively according to load.

 

[misterT]

The resistor only protects the motor so that the maximum allowed current through the motor is not exceeded. When the motor accelerates and gains speed, the current drops, and so does the voltage across the resistor. You can increase the value of the resistor to protect agains "over-voltage", but then you are limiting the available torque you can get.

Resistor really is a bad solution, but if you need a fast solution to protect your motor, use it while you find a better solution.

EDIT: The speed of the motor is directly proportional to the voltage and is limited by the mechanical construction. If your motor can handle the speed it gains at 16V, then there is no problem. Exceeding current rating is more dangerous because current through the motor creates heat and can burn the motor. Using a resistor limits the max torque you can get.

 

[alec_t]

....so what you need is a simple active current-limiter circuit. What is the maximum allowable current through your motor? Can you post its specification?

 

[misterT]

....so what you need is a simple active current-limiter circuit. What is the maximum allowable current through your motor? Can you post its specification?

The speed of the motor needs to be limited also. This means that the voltage needs to be limited. What is the application?

 

[Vizier87]

why don't you use PWM? A 555 timer can handle that.

 

[colin55]

Or you can use a string of 3 amp diodes.

 

[Diver300]

I tried to measure the resistance of the motor using a multimeter , then connect a resistor with the same resistance in series with it. So that each of them will receive 12V. But what I found out was that the resistor was getting some around 8.5V while the motor was getting roughly 16V. Am I doing something wrong here?

As the others have pointed out, using a resistor is probably not a good idea. However, if you don't need much start torque, for instance on a fan, you could use a resistor.

As you probably know, motors take more current when stalled than when running at full speed. The resistance of the motor doesn't change as they speed up. What changes is the voltage generated by the motor. That voltage opposes the supply voltage, leaving less voltage across the resistance, so less current.

You had only measured the resistance. When the motor spun, it must have been generating around 8 V, leaving 16 V difference between the supply voltage and the voltage it was generating. That 16 V was split evenly between the resistance of the motor and the resistor you had added.

From the numbers you gave, you might well get away with a larger value of resistor. If you told us the application or the value of the resistor, we might have more relevant suggestions.

 

[Ziddik]

Use 7812 regulator ic for 12V 1amp out

 

[MrAl]

Hi,

I would probably just use a voltage regulator too unless i needed speed regulation, but i dont think you want to get into all that unless you really need it.
Did anyone ask about what power the motor was yet? That would be good to know too.

 

[jones1000]

If u wanna use resistors so make a voltage divider circuit

 

[colin55]

Voltage divider circuits don't work for motors.

 

[Nigel Goodwin]

Voltage divider circuits don't work for motors.

VERY high wattage and VERY low resistances would

But it's a VERY, VERY bad solution, VERY, VERY inefficient, and with high risk of burns from the resistors.