Friday 15 July 2016

DuetWifi Thermal Testing

The DuetWifi is our new advanced 3d printing electronics board based on the Duet 0.8.5 but completely redesigned with David Crocker (DC42 on the reprap forums). We are currently running a pre-order which is due to end 17 Jul 2016 with the first boards delivered in the first week of August. We have also had a batch of pre-production beta boards made which are with beta testers right now. We put one of the boards through thermal testing by Andy Hingston (who was part of the Duet 0.6 design team). This blog post will deal with the thermal testing, I have future posts planned to detail the hardware design of the DuetWifi.

The main concern for heat generation on the board are the stepper driver chips. The TMC2660s are rated to 2.8A RMS, however we have limited them in firmware to 2A for now. What our testing has shown is that the TMCs drivers, coupled with the board design mean these drivers will run cool in most "normal" desktop size 3d printers (~1A motor current) and have the capacity to scale for significantly larger printers comfortably (~2A motor current).

In order to do a comprehensive test we decided to test the TMCs with 1A, 1.5A and 2A (RMS) of stepper motor current in three conditions:

  1. With the motors held in a "half step": in this condition there is 100% of the current flowing through half of the stepper driver with 0% through the other half.
  2. With the motors held in a "full step": in this condition there is 1/Sqrt(2) of the current ~70.7% of the maximum current flowing through both halves of the driver.
  3. In a normal microstepping mode with a step frequency of 8000 steps/min at 16 microsteps, interpolated by the drivers to 256 microsteps.
In all cases we allowed the temperature to stabilise for 20 mins from an ambient temperature of ~25C

The results: in summary the drivers and heatsinking design on the board have performed really well. At 2A RMS in the "half step" condition (the worst case) we saw the temperature rise to 87.7C (a 62.7C rise from ambient). In the actual use case of the normal stepping at 2A it was a rise to 71C (a 46C rise from ambient). All these tests were performed powering X,Y and Z drivers on the board at the same time so that the Y driver had hot drivers on either side of it. We expected it to get the hottest but actually Z was generally the hottest by about a degree indicating the importance of the PCB to dissipate the heat.

The thermal camera output is below; click on the pictures for larger images.

1A Half Step hold
DuetWifi Thermal Test 1A Half Step Hold - 48.7C

1.5A Half Step hold
DuetWifi Thermal Test 1.5A Half Step Hold - 62.1C

2A Half Step hold
DuetWifi Thermal Test 2A Half Step Hold - 87.7C - Front View
This view of the back of the board shows just how effective the heatsinking on the back layer is.
DuetWifi Thermal Test 2A Half Step Hold - 71.0C - Back View

1A Full Step hold
DuetWifi Thermal Test 1A Full Step Hold - 51.0C

1.5A Full Step hold
DuetWifi Thermal Test 1.5A Full Step Hold - 62.1C

2A Full Step hold
DuetWifi Thermal Test 2A Full Step Hold - 79.8C

1A Normal Stepping
DuetWifi Thermal Test 1A Normal Stepping - 41.8C

1.5A Normal Stepping

DuetWifi Thermal Test 1.5A Normal Stepping - 55.5C

2A Normal Stepping

DuetWifi Thermal Test 2A Normal Stepping - 71.0C

The other area of interest as far as power dissipation on the bard was the heated bed MOSFET and associated power traces. We wanted to carry at least 15A so I increased the width of the trace and doubled it up (both front and back). With 15A on constantly we saw a stable temperature of 88.5C at the hottest point.

DuetWifi Thermal Test 15A Bed MOSFET - 85.3C on Front power (-) trace, 81.9C on MOSFET

DuetWifi Thermal Test 15A Bed MOSFET - 88.5C on Back power (+) trace