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Variable voltage full scale ADC conditioning

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Hi, I'm making a motor controller and I would like to read the Back EMF from the motor. I have the drive circuit all designed and working but I am interested in reading the voltage on both of the motor terminals.

The main difficulty is that I want to condition the high voltage from the motor down to a 0v to 5v signal to feed into a PIC. This wouldn't be a problem if I was designing this circuit to work with a fixed motor voltage e.g. 12v or  24v but I want to make this motor controller very generic and able to drive a wide range of motors.

The motor controller chip I'm using allows maximum voltage of 80v so I would like to be able to have some signal conditioning before the ADC which would automatically scale the attenuation of the terminal voltages.

I do not know of a circuit that will do this.
I hope the EEV community can help me out with this one as currently I'm stumped.


Why do you want to "recover" the back EMP voltage/power ? I've thought of this myself before but to be honest after all of the transformations take place you may not have much left. Knowing the exact back EMF voltage will be hard as it will depend on the specific motor and drive voltage. I'd expect that the load put on it will also have an effect on the votlage. I've measured up to 400V on two 24V 11A fan motors if that is any guide. Usually back EMF is completely eliminated with a reversed diode so you also need to check that having a reversed voltage on the motor will not be an issue (it may have a breaking effect ? that will just eliminate any gain in efficiency ?).

Of course the input voltage will be oposite polarity. I suppose you could use an inverter SMPS but you still need to make sure the back EMF does not go to harmful levels

I wish to use the back-emf as the feedback for a PI(D) controller. To allow me to maintain a constant speed under varying torque.

I can already read the back-emf produced on my scope and it varies between 0v and ~23v on my 24volt motor.

My H-Bridge and drive system allows me to get the back-emf just fine.

I'll try to include pictures from my scope but at the minute I cannot get it working.


There are ICs called digital potentiometers, which work like a digitally programmable variable resistor, and digitally programmable Variable Gain Amplifiers (VGAs). Then you can use one of the two for letting the MCU program set the measure gain.

BTW is it a DC of BLDC motor controller?

Its a DC motor.


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