Thomas N wrote:
>
> I am a bit confused.  Let's say I have:
>
> +5V---0/ 0--------(0)-------/\/\/\------------|
               |                         |
                ---------|<--------------

> +5V   switch    motor    Sense Resistor   Ground
                        diode

> Let's say the switch is closed and the motor is running, then suddenly I
> open the switch, should the voltage drop across the sense resitor be at 0
> mA? If not, where is the current comming from?
>
> I am planing to use PWM (open and close the switch) at around 10Khz.  How to
> I measure the average current flowing thru the motor while it is running
> accurately?


First add the diode if you are using PWM on a
motor that has inductance (that's all of them!).

Thomas, search the net for "Jones on steppers"
he has a very nice tutorial that covers motor
controlling, and the "constant current" chapter
applies to all motors, not just stepper motors.
Well worth reading if you plan of getting good
performance from any motor.
-Roman

PS. Start your PWM experiment at a SLOW speed
first. This will be easier to control and
measure. Once you speed it up to 10kHz you start
to get quirky things like effects from switching
speed limits of the transistor and diode, and
switching surge losses get much higher. It might
work great at 500Hz but cook parts at 10kHz. :o)

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