Monitoring voltage and current ?

[timmers]

Hi,

All the comments are valid, but in my experience the voltage accross the motor, even with smoothing caps in the potential divider, will allways be near HT. The only way to measure the voltage ACCROSS the motor is to have two dividers, one on each terminal of the motor then motor voltage is the difference. But bear in mind the voltage accross a PWM driven motor is pretty much the supply voltage plus the back emf effects.

I suspect you are wanting to monitor the motor drive, and to take meaningful readings I would dispense with the voltage reading and use the current monitoring. To isolate the circuit from the PIC, use an opto coupler.

Tim.

[Nicmus]

I have to fully agree with timmers on monitoring only the current. With a little experimenting you can define your normal operating limits and take whatever action when things go wrong.

I’m a little unclear on the idea of monitoring voltages on both sides of the motor. If I understand it correctly you will need to add one more voltage divider on the lower side of the motor. That means your resistors should be almost the same as the other divider to accommodate the full voltage when Q1 is OFF. In this case when Q1 turns ON you will only read a 1/30 portion of the voltage drop on Q1 and R1 which is even less than your voltage drop on R1 only and which will force you to use one more Op-Amp.

I see no problem with a possible less than 1V error by approximating the voltage drop on Q1+R1 but this is one of the questions for iugmoh.

Regards,

Nick

[iugmoh]

oky In brief Iam working on thesis which title is "Parameters Identification of Permenant Magnet DC Motor" , in this thesis I built the mosfet driver to control the speed of motor , the motor is series wind motor but I convert it to separately excited motor by applying constant voltage to field winding , Motor specification is 110VDC , 3A , 200 watts,1500rpm , the motor aslo will be used as an educational kit which I will add all terminals to control motor and monitor voltages and currents and its speed so the student can use it with PIC or DAQ.

In my work I will make some simulations on voltage and current of motor and it's speed I will use DAQ first for flexibility with computer then after that I will use microcontroller PIC.