Resume print after power fail question
This peeked my interest. Thinking about the new feature in experimeltal fw 1.19beta9.
On what rail does it measure the voltage drop? I have a separate 5V PSU that I'm not sure how long it will keep its voltage. For all I know the drop could be instant.
But on the 24V-rail I could throw in some capacitors and a diode before and a few behind to simulate a voltage drop if PSU goes down. This would give the printer a bit more time to do its pause.
Hmm… I would need a very big capacitor to power the bed. It would be paramount to kill it as soon as possible when a voltage drop is detected. In theory all the heaters could be killed as there would be enough thermal capacity to keep them hot enough to pause.
But the more I think of this the less it would be of any help with a PEI bed surface. The parts would pop off if bed is cooled sufficiently
So my thought this would only be of interest when printing without a heated bed.
The Duet measures the incoming VIN voltage. The firmware relies on 5V and 3.3V power continuing to be available for a little while after VIN power is lost, which is the case when using the on-board 5V regulator.
Ok, for me I would need a UPS and keep the bed hot and we rarely have a power outage so in my case this is more hassle than the gain.
But for anyone else printing on a cold bed this should be a possibility. Adding separate capacitors for any external 5V. And add a rather large capacitor array on the VIN having a single diode before the cap and a few diodes in series after the cap. Having this in parallel to the PSU VIN cables you get a voltage drop equal to the voltage across all the diodes.
But I'm not sure how quick we can disable the heaters. With heaters enabled the voltage would drop to quick to re-position the nozzle reliably.
Even with the heaters disabled there will not be many seconds of power. At least half a second from 22V to 12V should be easy.
I would rather just spend my money on a UPS.
David, is there a way to interface a UPS to the Duet?
The VIN voltage is measured every 1ms. The firmware turns off the heaters, saves the resume information and starts the auto pause process as soon as VIN is measured to be below the threshold configured using M671.
On my delta, when I turn off the power there is enough left in the PSU capacitors to raise the head a couple of mm, but not to move it to the usual park position. But this printer uses an AC mains bed heater, so the 24V PSU is only rated at 100W and won't store much energy.