Calculating voltage drop

[ScuzZ]

This is a two question post:

1.

I'm attempting to run a motor thats rated 12v@1.5Amps at half the amp rating to slow the speed of it down.

I have tried using a resistor to limit the current, but that also drops the voltage. Is there a way to keep the motor running at 12v but halve the amps?

2.

I have also built a voltage divider that measures 6v at the test point between the two resistors, but when I place the motor in circuit, the voltage drops to 3v. What causes this and how do you calculate it?

 

[ericgibbs]

hi scuz,

A common way to run the motor slower is pwm, pulse the motor with 12v square waves, such that the duty cycle is around 50%.
Its also usual to measure the back emf of the motor in order to adjust the pwm for the motor run at a constant speed.

If I understand correctly, you have two resistors in series with 12v across the resistors, so the centre point is at 6V , without the motor connected?

If so, as soon as you connect your motor in parallel with 'lower' resistor the voltage will drop at the centre point.

Eric

 

[ScuzZ]

I was considering PWM, but I didn't want to go to all that trouble. I thought there might have been an easier way to get the half amps to run.

Are you also trying to say that I should use the parallel resistor theory to increase the bottom resistor to compensate for the resistance of the motor?

 

[Nigel Goodwin]

Ohms law means you can't alter the current without altering the voltage - a simple resistor though is a poor method, as it limits power far too much. A voltage regulator would be a better choice, with PWM much preferred.

 

[AllVol]

If you have a 555 timer on hand, you can quickly Pulse Width Modulate the power for your motor. There are many circuits available to show you how.

 

[Sceadwian]

The 555 is the way to go for simple PWM. There is no 'simpler way' than PWM really. The nice thing is is once you do that it's possible to further modify the circuit to provide variable current from very low to very high values. Keep in mind, a PWM'd motor will still use full amperage, it will just only use it 50% of the time, creates a LOT of noise especially in higher current motors.