[originator]

Hello all. The 3986 driver was a great solution to steppers and I have quite a few running daily. There is one non-cnc project I am developing that requires smoother motion at very low speeds than the 3986 can deliver at 1/16, so the next step is to build the Allegro 3938 three phase brushless DC controller. Unless I have missed something, which is possible, the 3938 will serve my purposes of closed loop processor controlled motion. I built the 3986 drivers very closely to the 3986 demo board, and only added a shunt for sensing current into the board, plus I used 540's instead of the surface fet version they used. I am still waiting to see if Allegro has the 3938 demo board schematic available, if so, I will model it very closely(unless there are mods we suggested here), making changes more for the fet selection RC values. I will still use through hole fets at first, as I find that in prototyping, they are the first things to need replacing if there are errors, shorts, etc.

The concept of the driver is similar to the 3986. You send it PWM and DIRECTION. There are a few other inputs like BRAKE and REF for example that will receive info from the processor. The 3938 drives the 3 fet stages in sequence based on Hall effect sensors location. Current is managed by sense resistsors, Speed is set by PWM input, direction by DIR input. Quadrature encoders will be read by the procesoor in my case the Parallax Propeller 32 i/o 80mghz 8*32 bit multiprocessor). I will ultimately use a step and dir input. The processor will move the motor to the desired position based on PID. In stepping this will be done at the highest speed pissible, in my first application, accel, speed, decel rates will be programmed for travel between several specific predetermined positions. A simple app will allow user config of parameters.

I have the bulk of the 3938 driver and processor board done(2 boards). I will post more details and ideas if there is any interest. The driver board shown below. A few values undecided. I am using Eagle btw. No large caps have been added yet, if required.


http://www.allegromicro.com/datafile/3938.pdf

[epineh]

Nice work (as usual ), while I will watch this to see what happens, I will sit on the fence with this one, my head is still spinning after soldering the 3986...lol.

Are you using the standard encoder object for the prop ? I intend to play with this much more when I get the time later on this year, would definately be interested in how you do your code.

Cheers.

Russell.

[luismsm]

Hello!
How do you set the speed by PWM input? Changing PWM frequency or PWM duty cycle?
What value you goes to use in the Rsense?
What is the typical value of voltage that hall sensor inputs support? I don´t find information about maximum inputs rate in datasheet... (hall sensors input, mode, dir, etc)

[krunal_299]

Hi !
I am intrested in more details for this project. :banana:
Originally I am looking for any reference project with Allegro A3930.
But still not found anything helpful.
can any expertise guide me with a3930 refeence design ?


hope to have favorable response earliest.
With best regards!
Krunal.

[miki_a]

Hi originator,
I'm looking for an efficient(easy :-) ) way to control a 1000W BLDC motor to use it in a Electric bicycle application,
and the A3938 IC was looking very promising, but I'm not exactly how to design the circuit.
Your design looks very close to what I'm looking for.
I know there's been 4 years since your last post about this project but I would definitly appreciate any help you can offer to me,
Thank
-Miki

[JAVIER0]

I am intrested in this project, anyone can offer me will be appreciated.

[jbarcelo]

I'm interested in this proyect progress.