Need a 0.5 sec UPS (prolonging agony for resurrection)

Triet

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.

droftarts

@Triet See https://docs.duet3d.com/en/User_manual/Tuning/Resume#power

Adding a 10000uF capacitor across the VIN rail (between positive and negative terminals) can help increase the amount of stored power giving enough time to write to the SD card. Some PSUs have several sets of output terminals all connected in parallel, so you can connect the capacitor to a spare pair of terminals. However, the PSU may false-detect a short circuit and turn off when the machine is powered on. In this case, we suggest using a high-current Schottky diode and parallel resistor, connected between the VIN line and the capacitor. The resistor is for charging the cap. When the power goes off, the capacitor powers VIN via the diode. Make sure the diode is sufficient to handle the bed heater current (if powered from VIN) for at least a few seconds; in the fraction of a second between the mains failing and the VIN voltage dropping low enough for RRF to detect the loss of power, the capacitor will be supplying the bed through the diode.

Ian

samlogan87

You could try using a DC UPS. @deckingman who unfortunately passed away but you can search his posts, where I pointed him to one we used on remote sites to keep PLC’s and RTU’s up for compliance in pump stations. We used Phoenix contact UPS’s but he had good luck with a mean well one. The beauty of the DC UPS, is everything is 24VDC, they are cheaper than a traditional one, and you can change the size of the battery you need

Triet

@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).

Triet

@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.

dc42

@Triet you can connect supercaps of the same type and value in series, with parallel resistors to help keep the voltages balanced. For example, ten 10F 2.7V caps in series with a 100 ohm resistor in parallel with each one will give you a 1F 27V capacitor. Charge/discharge them through a 100 ohm 6W resistor and parallel Schottky diode.

Don't forget that after turning power off the supercaps will still power the board for some time, so wait until the LEDs have all turned off before doing any work on the system.

Triet

@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!