Potentio replacing with DAC

I have this drive, that drives a BLDC motor, and it has a 10 k potentiometer which can be used to vary speed,
and i replaced that with DAC(the one on the left in schematic) and it worked fine for a couple of days, but one day opamp U1a heated up, and i replaced it and now the drive shows erratic behaviour,
the drive goes to max speed on 0 volts, and stops at 5, when the three pins of potentio are disconnected the signal pin shoes five volts, and when i connect my dac to it, regardless of dac's output, output voltage is 3.3 V, and only increases when DAC's output is greater than 3.3 Volts.

i though it was a current issue so i replaces the output stage with the one on the right so as to increase the current, but still nothing. What should i do?

BLDC motors contain all sorts of stuff, oscillators, power stages flux detectors. . . Just because there is an externally available pot to control the speed does not mean a great deal. Have you checked with a CRO, exactly what is on the pot?
Frank

chuckey said:

BLDC motors contain all sorts of stuff, oscillators, power stages flux detectors. . . Just because there is an externally available pot to control the speed does not mean a great deal. Have you checked with a CRO, exactly what is on the pot?
Frank

Click to expand...

what is a CRO?

potentio varied a reference voltage, 0 meant full speed, 5 v meant stop, and from what i can understand it is connected directly to the microcontroller, along with some pullups, and pull downs (or is might be the internal pullups and pull downs of MC, i cant be sure), and caps....
however it was WORKING fine for a week or two, and now it is not,

Cathode Ray Oscilloscope. Perhaps its time to go back to basics. Disconnect your circuit, put in the pot, and make sure that motor is working OK. this will give you two choices, either the motor is broken, or your circuit is broken. Test your circuit to make sure it delivers 0-> 5V smoothly without any jumps. You have got enough decoupling on your rails? BLDC motors take big current surges.
Frank

chuckey said:

Cathode Ray Oscilloscope. Perhaps its time to go back to basics. Disconnect your circuit, put in the pot, and make sure that motor is working OK. this will give you two choices, either the motor is broken, or your circuit is broken. Test your circuit to make sure it delivers 0-> 5V smoothly without any jumps. You have got enough decoupling on your rails? BLDC motors take big current surges.
Frank

Click to expand...

dont have a CRO,
with pot it works fine,
i am using a driver module, which was made in china, so there are no schematics etc,
i just wanted to replace the pot with dac,
dac independently works dine.

chuckey said:

Cathode Ray Oscilloscope. ....................

Click to expand...

Of course an oscilloscope with an LCD display would also work. :wink: