Can anyone help with BACK EMF

pklein · Joined: 14 Nov 2007 Posts: 7

Hi,

I am looking to measure the back emf on a dc motor. Im not sure how to go about the circuit. My drive circuit is simply through a TIP 120 darlington transistor. Im not too clued up on electronics. My motor is run off a 5v power supply.

Do I simply connect a wire from one of the motor sides to an adc channel? Or is it more complicated.

I saw an example where someone connected from both sides of the motor to 2 adc channels and measured the difference.

Any help is much appreciated.

ferkar · Joined: 14 Jul 2007 Posts: 38

i think you should connect a suitable emitter resistor to take a feedback then manuplate it as your intention.

hope helpfull..

RLScott · Joined: 10 Jul 2007 Posts: 465

As ferkar said, add an emitter resistor to your darlington driver. This will tell you how much current is passing through the motor. Multiply this current by the static motor resistance to get the voltage drop across the pure resistance of the motor coils. If the motor is turning, this I x R voltage will be something less than the applied voltage. The difference between the applied voltage and the I x R voltage is the back emf.

So how does an emitter resistor help you measure the motor current? If you know what the resistance is and if you know the voltage across that resistor, then the motor current is E / R (emitter voltage divided by the emitter resistor). You will have trouble trying to measure the emitter voltage directly with the A/D in your PIC. That is because you need a range of several volts to measure with any precision. But if you size the emitter resistor large enough to produce several volts, then your darlington will no longer be turned on by base drive you are supplying. Also this will reduce the voltage to the motor considerably. You will probably have to design a DC amplifier to bring the emitter voltage up to a level where you can measure it with a PIC. Size the emitter resistor so that it produces only 0.2 volts with maximum motor current. Then amplify the 0.2 volts up to 5 volts to read it.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting