Hi there. Is there a way to slow down Z axis steppers when deenergized? I’ve just burned my SKR Pro yesterday. My 300x300 bed is pretty heavy and drops down like a rock when power turn off or, as in my case, hit reset button to prevent overtravel. This is my first coreXY printer and bed leveling + no endstops on Z axis, so a lot of new and different stuff from my previous printers and CNC machines. I was having hard time with BL Touch as it was not recognized at the startup and wouldn’t engage when bed leveling button was pressed. It was working because at the turn on it moved a couple times up and down but wasn’t talking to Marlin, I guess.
I need a way to restrict speed of the steppers when deenergized or I’ll most likely modify MP3DP v3 Z mounts for my 2020 frame.
I thought the SKR had protection against that. I know my bed has dropped several times and my board is just fine.
The breaker on board is fine, but when the bed dropped (wasn’t first time also) this time all the way - approx 300mm - I saw some sparks flying from under the cover. When powered on after, only 12V led is on.
You should always end your gcodes with a move to Z max. then there is nowhere for it to fall.
If you are going to kill power I would put my hand in there to catch it. Once you are done testing killing power is extremly rare.
That should not happen, there is reverse current protection to some degree. It would have to fall really far really fast to do that. I have never heard of that happening before and some people mistakenly slam around the CNC gantries for what I would assume is faster and longer.
They do actually sell some back current stepper brakes these days though. I am not sure where to buy one. but it feeds back power to slow steppers in this exact scenario.
I always put my hand under the bed when powering off, except the last one 
. My first free bed fall taught me that. I just want to prevent that situation because blackouts are not uncommon these day in SoCal and it could happen again while printing. My bed is really heavy - top plate Alu 310x310mm, 3/16" thick + Alu support plate the same thickness (with lightening pockets but still heavy) + magnetic base + steel printing surface + silicone heating pad.
My bed is the same setup as yours. Except 1/4” and no lightning holes. When it falls it damn sure falls. Like Ryan said I do have it set in my gcode to lower to z max at the end of the print. But any time you have to cancel or a power flash its crashing down. I’m still extremely surprised you smoked a board with it.
Believe me, I was surprised too.
I tightened the belts and it slowed down a little, but it can fall almost 400 mm. I seriously might print you stoppers for the bottom. But that won’t slow down the speed of the drop.
If you look up Z axis brake, it looks like there area few boards out there.
You could counter balance each one a bit to slow it down, but you still want gravity on the bed. Or you can get constant tenion springs (they look like inside a tape measure).
Those three are probably the easiest.
The easiest and cheapest is a constant tension spring, so after I’ll try those. McMaster have quite a few different strength and width types so I can experiment with them. I have a time now before I get the board replacement.
Sorry to hear about your SKR Pro board. Curious to see what you end up with, would like to prevent hard bed drops for all situations as well.
I must say, tension springs are amazing, thanks Ryan for the tip. Installed today these - McMaster-Carr - one for each motor. My table weights about 7.5 lbs (with the modified motor mounts and brackets). It is exactly what I wanted - table still dropping when off power but much slower - videos here: https://www.youtube.com/shorts/1SWS--e2MTE and https://youtube.com/shorts/zRqqBFRCHQc. How long they’ll hold the tension will time tell. Happy with the table, now onto hooking up and configure my new BT Octopus board.
You got a good tension there! I was worried it would be overkill. Nice work, seems like a good amount of gravity is still involved and is needed.