@dc42 I wanted to wrap up some informations about this topic but found that I am not allowed to post links - I need two "reputations".

That's a pity. Would anyone be so kind to enable posting links for me? At least for this time. (Assuming that my findings are of general interest for the furure).

@jay_s_uk You did a great job identifying the right place to customize the software (at least for me, it would have been a lot of work).

Anyways, considering that in this case I would need to host the whole programming environment for this minor change, which would not be persistent at the next release, I decided to rather try to create a macro and use the BtnCmd plugin to make it available in DWC.

Thanks for your help.

@dc42 Of course! Why didn't I myself come to the idea of combining several capacitors, so having even more design flexibility.

You gave me the decisive impulse to start this task. I would even use the remaining power to run the hotend fan a bit longer to avoid nozzle clogging after a sudden heater shutoff.

By the way: What I most appreciate using Duet hardware is... this help forum, definitely (beside their indisputable quality). Sincere thanks!

@stuartofmt I just read that discussion, looks a bit chaotic but the hint about the BTT Supercap UPS is valuable.
@T3P3Tony Thanks!

I just wanted to clarify that it is really not necessary to build a pack of supercaps yourself. I was looking around and found some solutions. Often, supercap modules are offered to be used in automotive applications to protect the vehicle battery from to sudden current surge or to stabilize board voltage to satisfy hifi equipment (music listening people tend to be very demanding when it comes to sound quality).

For example, this one:
5PCS 1Set Super Capacitor 13.5V 12F Single Row Farad Capacitor2.7V 60F Automotive Super Farad Capacitor Module Supply Rectifier
Of course this would only be suitable for a 12V power supply, but you can connect two of them in series. This boards have voltage protection and balancing built-in. They are even oversized for this case (remember: the purpose is to allow a controlled shutdown, not to run the printer on UPS).

For more ambitious people, a 24V module is also available, for about $100 on Amazon, or around $21 on AliExpress:
GDCPH 24V5.5F Supercapacitor Electronic Rectifier Module 2.7V50F Super Farad Capacitor Backup Power Supply 24.3V Electrolysis
Note that it is composed of 9x50F supercaps in series (resulting in 5.5F), which is plenty of energy.
Still, these are not full fledged UPS - you still need the combo of resistor and Shottky (in parallel to each other) between the module and the PSU.

I even found a supercap module exactly as @dc42 proposed, if using two of them in series:
[16V 1F/2F Farad Capacitor Module 2.7V 10F Super Capacitors With Protection Board]
(https://www.aliexpress.com/item/1005002715223142.html)
You get one module for $6 or two for about $12, fairly affordable. That is currently my preferred option. The BigTreeTech 24V UPS mentioned above does not add any value because the Duet already has a power failure detection built-in.

My impression is that supercap based UPS are becoming popular. There are some professional solutions, for example customized for a Raspberry Pi (if you use one):
Andino-UPS - Supercapacitor UPS for the Raspberry Pi
but I also found some DIY projects. They should be considered as "emergency power supply" rather than UPS due to the limited length of time they can keep a printer running.

Engaging in this matter I have some new questions and ideas, namely: I would like to switch my printer on remotely (it is in an attic and I always have to go upstairs to switch it on) - a reversed question in comparison.

But also: What happens when I just switch the printer off in a regular manner when resurrection has been configured? The voltage will drop to the defined threshold in this situation too- will then the power failure procedure defined by M911 be executed? I have never checked that. OK, as a rule, no print job is running when I deliberately switch my printer off, so I suppose the procedure is only triggered in this case.

Why I come up with this question?
Assume that I have a small UPS that is able to detect failure of the mains power. It does not have to wait until DC voltage drops below say 22.5V to kick in. I can then save current coordinates and take some other precautions to prepare the resurrection, even before the voltage threshold is reached. Will then the power failure procedure defined with M911 still be executed a 2nd time when the DC voltage falls?

A real UPS has one key advantage compared to an emergency power supply, even when it is not meant to run the printer accross the powerless time, since it allows to keep the hotend fan running for a couple of minutes until the hotend gets cold enough, so to avoid nozzle clogging. In this case, the question whether M911 kicks in and saves the current state a 2nd time would be relevant.

I am done.
Thanks again.

@stuartofmt I am looking at different options and learned about a switch with a ESP8266 NodeMCU microprocessor, which is more elegant and flexible. It runs a Webserver as well.
If you are interested:
ESP8266 Button

@stuartofmt I just read that discussion, looks a bit chaotic but the hint about the BTT Supercap UPS is valuable.
@T3P3Tony Thanks!

I just wanted to clarify that it is really not necessary to build a pack of supercaps yourself. I was looking around and found some solutions. Often, supercap modules are offered to be used in automotive applications to protect the vehicle battery from to sudden current surge or to stabilize board voltage to satisfy hifi equipment (music listening people tend to be very demanding when it comes to sound quality).

For example, this one:
5PCS 1Set Super Capacitor 13.5V 12F Single Row Farad Capacitor2.7V 60F Automotive Super Farad Capacitor Module Supply Rectifier
Of course this would only be suitable for a 12V power supply, but you can connect two of them in series. This boards have voltage protection and balancing built-in. They are even oversized for this case (remember: the purpose is to allow a controlled shutdown, not to run the printer on UPS).

For more ambitious people, a 24V module is also available, for about $100 on Amazon, or around $21 on AliExpress:
GDCPH 24V5.5F Supercapacitor Electronic Rectifier Module 2.7V50F Super Farad Capacitor Backup Power Supply 24.3V Electrolysis
Note that it is composed of 9x50F supercaps in series (resulting in 5.5F), which is plenty of energy.
Still, these are not full fledged UPS - you still need the combo of resistor and Shottky (in parallel to each other) between the module and the PSU.

I even found a supercap module exactly as @dc42 proposed, if using two of them in series:
[16V 1F/2F Farad Capacitor Module 2.7V 10F Super Capacitors With Protection Board]
(https://www.aliexpress.com/item/1005002715223142.html)
You get one module for $6 or two for about $12, fairly affordable. That is currently my preferred option. The BigTreeTech 24V UPS mentioned above does not add any value because the Duet already has a power failure detection built-in.

My impression is that supercap based UPS are becoming popular. There are some professional solutions, for example customized for a Raspberry Pi (if you use one):
Andino-UPS - Supercapacitor UPS for the Raspberry Pi
but I also found some DIY projects. They should be considered as "emergency power supply" rather than UPS due to the limited length of time they can keep a printer running.

Engaging in this matter I have some new questions and ideas, namely: I would like to switch my printer on remotely (it is in an attic and I always have to go upstairs to switch it on) - a reversed question in comparison.

But also: What happens when I just switch the printer off in a regular manner when resurrection has been configured? The voltage will drop to the defined threshold in this situation too- will then the power failure procedure defined by M911 be executed? I have never checked that. OK, as a rule, no print job is running when I deliberately switch my printer off, so I suppose the procedure is only triggered in this case.

Why I come up with this question?
Assume that I have a small UPS that is able to detect failure of the mains power. It does not have to wait until DC voltage drops below say 22.5V to kick in. I can then save current coordinates and take some other precautions to prepare the resurrection, even before the voltage threshold is reached. Will then the power failure procedure defined with M911 still be executed a 2nd time when the DC voltage falls?

A real UPS has one key advantage compared to an emergency power supply, even when it is not meant to run the printer accross the powerless time, since it allows to keep the hotend fan running for a couple of minutes until the hotend gets cold enough, so to avoid nozzle clogging. In this case, the question whether M911 kicks in and saves the current state a 2nd time would be relevant.

I am done.
Thanks again.

@dc42 I wanted to wrap up some informations about this topic but found that I am not allowed to post links - I need two "reputations".

That's a pity. Would anyone be so kind to enable posting links for me? At least for this time. (Assuming that my findings are of general interest for the furure).

@dc42 Of course! Why didn't I myself come to the idea of combining several capacitors, so having even more design flexibility.

You gave me the decisive impulse to start this task. I would even use the remaining power to run the hotend fan a bit longer to avoid nozzle clogging after a sudden heater shutoff.

By the way: What I most appreciate using Duet hardware is... this help forum, definitely (beside their indisputable quality). Sincere thanks!

@samlogan87 Yes, I have already read that article a couple of hours ago. I found the solution very attractive but my concern is not to run the printer any long time at an outage. There are virtually no power outages in the country where I live, except very short ones most people won't even notice (probably caused by a more challenging management of the public power grid due to a large and increasing share of renewable, but discontinuous power sources). Right now I am running a 23 h print and I have no ambition to use a UPS for that. Frankly, I even suspect something is wrong in the electrical installation of my house.

After looking at the DC UPS mentioned in the article I found them to be oversized solutions for a 0.5 sec problem, and I thought I would find something similar, but self-powered, essentially like a supercap. I could only find supercaps for 12v only, for whatever reason.

Deckingman passed by? Well that's a loss indeed. He was an enthusiastic and respected comrade in the 3D printer community.

@droftarts Thanks a lot! OK, a 10mF capacitor, connected to the two 24v leads via a Schottky diode and a resistor (both in parallel to each other) would do the trick. I understand how this works. I would figure out the value of the resistor to limit the current, but I have no idea how to choose the Schottky diode.

Would anyone indicate a particular Schottky diode as a possible example for this purpose? Otherwise this will be trial and error for me.

And yes, I know that I have to care about the polarity . This simple circuit is nothing more than a kind of downsized DC UPS controler, and I would even have expected to find a board with a similar function but I couldn't. I don't think I am the first one coping with such a question, as many different devices need to be shut down in a controlled fashion (think of a NAS storage for example).

Hello
I have experienced short voltage outages leaving the 3D printer in an undefined state or knocking it off. While implementing the resurrection procedure I found that the printer does not live long enough to prepare for the resurrection, so I need a way to extend the working time of the printer when voltage gets below 22.5v for, let's say, 0.5 sec (I actually don't know how long I need).

My 3D printer uses a Duet 2 Wifi and there is nothing special to consider regarding power consumption. Of course power consumers are switched off or cut down as the first step when the outage procedure is triggered at 22.5 v.

In short, I don't really need a UPS, but a way to dampen the voltage decay. What would be the best way to achieve this? Should I simply connect a capacitor in parallel to the 24v, something like a supercap? How many farads would be appropriate? Or perhaps a small 24v pack of rechargeable Li-Ion batteries instead? I am afraid it won't be that simple, which is why I am asking help here. If there is a DIY solution I would willingly try to build it.

Thanks in advance.

I know the M221 command, which allows me to multiply the current extrusion rate defined in the slicer by a factor to increase or decrease it.

But how to I set the extrusion rate directly? I mean how to set it so that 100% would mean all e-steps per rotation (200 for example or as calibrated). I would then rather redefine the extrusion rate replacing (not correcting) what is set in the slicer, for the current gcode session.

Sorry if this is a stupid question, I did not find the answer looking at the RRF gcode dictionary.

@DIY-O-Sphere OK, I knew I would make a fool of myself with this question, now I feel a bit embarrassed but well informed. Thanks!

If the printer is in the process of shutting down via "M81 S1", or perhaps due to a set dwell time, how do I cancel that? How do I stop the waiting without proceeding with what would have happened if the time had passed as originally designated?

I cannot find the command in the documentation because I am missing the right keywords when searching.