Calculating voltage drop - Electronic Circuits Projects Diagrams Free

ScuzZ · 16th February 2007, 07:47 AM

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 · 16th February 2007, 08:07 AM

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 · 16th February 2007, 08:43 AM

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** · 16th February 2007, 09:32 AM

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 · 16th February 2007, 10:25 AM

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 · 17th February 2007, 02:27 AM

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.

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16th February 2007, 07:47 AM	(permalink)
ScuzZ	Calculating voltage drop 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?

16th February 2007, 08:07 AM	(permalink)
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

16th February 2007, 08:43 AM	(permalink)
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? Last edited by ScuzZ; 16th February 2007 at 09:19 AM.

16th February 2007, 09:32 AM	(permalink)
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. __________________ PIC programmer software, and PIC Tutorials at: http://www.winpicprog.co.uk

16th February 2007, 10:25 AM	(permalink)
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. __________________ All those who believe in psycho kinesis, raise my hand.

17th February 2007, 02:27 AM	(permalink)
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. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."