DRV8825 near max V and A

Support Motor controllers/drivers and motors
1

I’m building a desktop CNC machine and am interested in Pololu’s DRV8825 drives. I want to run them around 40 V and 2.0 A – I’d like to check that my plan for this is a good one.

My overall plan is: Grbl > Uno > 3x DRV8825 > 3x Nema 23 motors.

I searched for low inductance and economical steppers and the best I found is a 2.0 mH, 3.3 A, 252 oz-in, 4 wire (57BYGH318). 2.0 mH corresponds to a max motor voltage of 45. I want to run the motors up to their max speed (thus near 45 V), and around 2.0 A.

Other system parameters:

  • 48V 7A switching power supply, turned down to 40V. Large capacitor on DC output to absorb generated braking/Estop energy. Motor leads less than 6".
  • Enough microstepping to minimize noise and resonance.
  • 470 uF, 63 V caps on the power inputs to the DRV825s.
  • Heatsinks and thermal paste on the DRV8825s, fan cooling.

Some questions:

  1. I see the PCB bottom has a silver-colored plate area below the DRV8825’s PowerPad. I’ll have airflow both above and below the PCB. Should my additional heatsink be mounted to that plate or on top of the DRV8825? I was thinking on top, since that conduction path is more direct, but perhaps the heat is mostly created near the bottom surface of the DRV8825.
  2. 40 V should get the motors to around 1000 rpm (13 kHz at 4x u_step) – any issues with that?
  3. For a soft start switch, can you recommend an economical one rated for 40+ V? A relay would be ideal, since I’ll have a main switch for the power supply.
  4. Any other suggestions or concerns on this system?

Thanks for any insights.
David

2

Hi, David.

I have numbered my responses according to your questions:

  1. You can add a heat sink to either side of the board and probably expect to receive reasonable dissipation for each one. We have not characterized which way is better.
  2. I did not review your math, but it does not seem unreasonable to expect that kind of speed.
  3. Unfortunately, we do not have any recommendations for a switch that can help you with a soft start.
  4. Since you plan on operating this board near its limits, I recommend monitoring your parameters to prevent damaging the driver. I also recommend testing your system with a single driver and then integrating each additional driver into your system.
3

Hi Jonathan,
Thanks for the reply and info – helpful and makes sense.
David