Hi,

This is a part of the popular DC motor controller MC2100:

I'm trying to decode the function of some components. For example, I think R32 is a shunt resistor. But, what could be the purpose of R24, R26, R27, C16, R25, R23, C13, and C14? It looks like too many components around the motor. R24 and R26 go to the micro pins, but what are they measuring?

 

The R dividers to the micro pins are to scale the voltages
that are higher than the UP can handle to its common mode
range, pins are probably an A/D or comparator. Look at the
manual for the 2100 to find out what pins are for. Caps of
course to eliminate noise so A/D looks at a clean signal.

Regards, Dana.

 

They could be to allow the A/D of the micro to measure the back EMF of the motor in the PWM off period so as to get an idea of the motor speed.

 

The Voltage across C16 with no signal from the mosfet Q2, is 0.49V provided by R22, R27, this is probably fed into the micro for current monitoring, as the motor spins and gets a load the voltage across R32 increases, and so your voltage across C16, . Ideally we need the full picture!

 

Those resistors a such large values, maybe they are used to protect the micro from spikes from the motor and the mosfet when they switch. I really don't know. Why use a floating 5 volt supply ? I am not an analog guy by any means and this circuit proves it.

 

R32 on the MC2100 is a current sense resistor. Goes to pin4 of the μP.
Max.

 

They could be to allow the A/D of the micro to measure the back EMF of the motor in the PWM off period so as to get an idea of the motor speed.

Measuring speed with Back EMF method is reliable? More reliable than using a magnet and switch for low speeds?

 

Measuring speed with Back EMF method is reliable? More reliable than using a magnet and switch for low speeds?

Using the back EMF is a reliable method of measuring speed, BUT it is not as simple as many wish it was. Filtering out brush noise and getting the sample timing right are important requirements.

 

The Picmicro Mechatronics hardware/firmware example has examples for voltage detect and the slot opto method for an example of pulse sensor.
http://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en012134
and Mechatronics https://www.microchip.com/DevelopmentTools/ProductDetails/PartNo/dm163029
See project 5 for program for slot opto.
Max.

 

Measuring speed with Back EMF method is reliable? More reliable than using a magnet and switch for low speeds?

Using the back EMF may be faster than using a one pulse per revolution sensor. AND it may indeed be more reliable, at least with some motors.

 

If you are going down to any kind of low rpm, I would recommend using a minimum of 4 pulse/rev, or 10/rev if possible.
I use a slot opto, If using the magnet/reed switch type debounce is an issue that usually has to be addressed.
Max.

 

If you are going down to any kind of low rpm, I would recommend using a minimum of 4 pulse/rev, or 10/rev if possible.
I use a slot opto, If using the magnet/reed switch type debounce is an issue that usually has to be addressed.
Max.

I would love using a slot opto in the machines that have a reed switch. But at the same time, I want to take advantage of the physical structure of the device, in this case, of the treadmill. Some of them, specially the home versions, always bring a reed and a magnet attached in the wheel.

 

I want to take advantage of the physical structure of the device, in this case, of the treadmill. Some of them, specially the home versions, always bring a reed and a magnet attached in the wheel.

So you would need to take debounce into account.
Max.