Help modifying a DC motor control circuit

[ilium007]

Hi guys (real electronic engineers).... I am looking at purchasing a DC motor control kit that uses a linear 100k pot (VR1 in the circuit screenshot below) to control an opamp (IC1a in the circuit screenshot below).

https://quasarelectronics.co.uk/Item/3166v2-bidirectional-dc-motor-speed-controller-5-32vdc-5a

I am going to use this for my Dad's model railway as a train speed controller - the potentiometer will do the FWD / REV, but I will need to be able to accurately set the centre position / off position.

I am going to replace the 100k pot with a centre detent equivalent as I want a physical indication of when the train should be stopped but I'm not sure what will happen if the centre position of my new pot is not exactly at the centre position required by opamp IC1a.

From the circuit description:

Op amp IC1b is connected as a triangle wave generator and it provides the control signal for the voltage comparators. It runs at about 270Hz, as set by the 10nF (.01?F) capacitor and 470k? resistor connected to pins 6 & 7. The peak-to-peak amplitude of the triangle wave is less than the difference between the two voltage references applied to pins 9 & 12.
Therefore it is impossible for both comparators to be turned on simultaneously; only one comparator can turn on at any time. Otherwise all four Mosfets would conduct, causing a short circuit that would destroy them.
The triangle waveform can be raised or lowered by speed potentiometer VR1 and op amp IC1a which operates as a voltage follower. Shifting the triangle wave up causes comparator IC1d to turn on (its output goes low); shifting the triangle wave down causes comparator IC1c to turn on (its output goes low). At other times, both comparator outputs are high

I want to know if I can modify this circuit so as to be able to adjust the 'centre point' of the 100k pot feeding IC1a - maybe by replacing either the 100k or 10k fixed resistors (with green boxes around them on circuit diagram attached) with another adjustable pot to then set the centre point.

What I need is to be sure the model train DC motor will be stationary at the centre detent position of whatever 100k linear pot I swap with the one in the kit.

Any help would be appreciated.

[fourtytwo42]

Well IMOP that is not suitable as a model train controller because even in the "center" position of the pot the output is a pwm signal of 50% duty cycle causing motor heating etc. In most true model train controllers in the stopped position there is no voltage applied to the track.

Regarding your other question unfortunately the "center" position will drift depending upon the precision of the components, so even if you were to add presets to help calibrate it these would require periodic re-trimming.

At the end of the day you need a circuit designed to be a model train controller, search on the internet there are tons

[ebclr]

Just change the 100K resistor to 75k, and put a trimpot of 47k in series, on this trimpot you can adjust the center of the potentiometer to match the point you want with +-* 25%

Your circuit works, and at center will not have 50% duty cycle, as mentioned from the previous post, in fact, will be 0%, because at center no one of the comparators will work, and depending of the DC offset will decide with comparator will work

[fourtytwo42]

Your circuit works, and at center will not have 50% duty cycle, as mentioned from the previous post

OK I thought IC1B was an oscillator ?

[Benta]

Your circuit works, and at center will not have 50% duty cycle, as mentioned from the previous post

OK I thought IC1B was an oscillator ?

It is...

[ebclr]

It's not only an oscillator, it's an oscillator who generates a triangle wave and an adder of the offset generated by the potentiometer, this triangle wave will pass to the 2 comparators, and only one will generate PWM pulses on output according to the DC offset of the triangle wave, who set the right side of the FET bridge

[ilium007]

Thanks for the replies. I think I will look at using a Pololu DC motor driver and control with Teensy and a rotary encoder. I don't want to have to be calibrating the centre point of the pot all the time.