Thermal shutdown on DRV8825???

I am running some DRV8825s on my home made CNC machine. I have two Nema 17s on each axis rated at 1.5 Amps. I am driving them close to that rating, probably 1.4 amps. I have heat sinks on the drivers but no fan. I have probably 30 jobs through the machine with no failures until this morning. I had two failures, in both cases it lost steps in the negative Y direction. The first time it seemed like a fairly violent "lock and shudder" as perhaps one motor was fighting to traverse to negative Y but the other one wouldn't, though that's just conjecture. The second time I was engraving some words and it just started writing over itself on my finish pass. In both instances of failure there were a lot of small directional changes (engraving scripts, cutting out wavy features, etc) that were required on the work piece. The cuts I was making at time of failure were light by my machines standards. I was about 2mm deep in hardwood with a V bit at 700mm/min vs. many cuts I've made with a 4 flute fishtail bit at 7mm+ depth at similar speed. In both instances I reran the job was completed successfully. Note that failure occurs a few minutes (10 or so) into the job that might last 15 minutes total.


When I touch the heatsinks on jobs like this they are warm. You would probably would not want to hold your finger on it for more than 2 seconds, but up to that point can be endured.

Each motor is mounted into an aluminum plate that wicks the heat away from it well, so I don't think they are overheating.

I do not believe that the sawdust is obstructing the path. I run a vacuum to suck up most of it and it is a very fine power. My gantry (x axis) is much more exposed and has had no such problem.

Am I correct to suspect a driver issue? How hot would a heat sunk DRV8825 be to the touch before the driver went into thermal shutdown? Can't touch at all or merely just warm to the touch?

I've ruined about $10 worth of wood so far an each failure is another $5 down the drain so I'd love to figure this out!

Heatsinks stuck to the top of chips like the DRV8825 are not very effective, because the plastic case conducts heat badly. These chips need to be cooled from underneath, where they have a thermal pad that is soldered to the PCB. Unfortunately, plug in stepstick type drivers don't have enough PCB area to remove the heat very well. Nevertheless, a fan would probably be more effective than a heatsink stuck to the top of the chip.

Also I suggest you reduce the motor current a little. 80 to 85% of rated current is a reasonable limit.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

It turns out that it was all mechanical. I had two more failures tonight. All because the machine lost steps going in the negative Y direction. The last failure was a complete lock of one side of my Y axis in the negative direction. I could physically push the machine in the Y+ but couldn't even push in the Y- direction.

The set screw on the motor on that side had come out and was getting wedged in the pulley's path as it tried to rotate in the direction required for negative Y travel.

I reset the screw and all is well.

You are probably right about backing the current down though. Probably wise.

Hi,
Just my $0.02, but dc42 is correct all the way: besides lowering the current, you should also add a fan to cool down those DRV8825's. As he explained, the small heatsinks over the chips are practically useless, it's really the airflow over the driver's PCB that is cooling the chips. Since you are driving them at or near their max. current you should really try to keep them as cool as possible, and that means forced air flow over the PCB's.

The jumpers underneath the drivers and the fully closed ports of the drivers don´t help with cooling the underside of the drv8825.
It could help getting fresh air underneath, when you solder the jumper-pins and cut them off to get rid of the jumpers.
-Olaf