"Backup" for 12V systems if powerloss?

dc42

The problem is that you would need a lot of capacitance. The stepper drivers shut off at 9.5V. The energy stored in a capacitor is 0.5CV^2. So let's consider a couple of cases:

24V PSU, 10000uF capacitor (0.01F), power fail threshold set to 22V.

Initial energy stored = 0.5 * 0.01 * 22^2 = 2.42 Joules
Final energy stored = 0.5 * 0.01 * 9.5^2 = 0.45 Joules
Energy available = 2.42 - 0.45 = 1.97 joules

12v PSU, 40000uD capacitor (0.04F), power fail threshold 11V (a 40000uF 16V capacitor should be about the same size as a 10000uF 25V capacitor).

Initial energy stored = 0.5 * 0.04 * 24^2 = 2.42 Joules
Final energy stored = 0.5 * 0.04 * 9.5^2 = 1.80 Joules
Energy available = 2.42 - 1.80 = 0.6 joules

Both capacitors store the same amount of energy, but in the case of 24V power we can use 81% of it, and with 12V power we can only use 25% of it.

Iamturbo1978

I was wondering about this question. I came up with an idea, but I don't know if the Duet can work with it.

Making an interface that connects to the high voltage side of the power supply using an opto isolator, then connecting the output of the isolator to a free input of the Duet. The Duet would just look for the input to change, and then start the loss power script. Since the Duet would not be comparing the input voltage to detect a loss power event, there would be no wasted time in the event of loss power.

dc42

Yes that could be done, and the input smoothing capacitor of the PSU would then provide additional energy storage.

Iamturbo1978

@dc42 that's great. Looks like I got some work in my future.

PuterPro

Or you can just cheat, and use a 1500VA UPS, like I do, for the whole thing. You also get surge protection, and on the better AVR ones, it smoothes during slight dips to preserve the battery on brownouts.

If you really want to let out your "Inner Engineer" and want a little project, almost all of the better ones have a USB port that can communicate status & power loss. I imagine an Arduino or Raspi could grab that and send a signal to save and shutdown the printer when the battery gets low.

If you do it, post it on Thingiverse and here, OK?

Iamturbo1978

@puterpro that one will take a little more time, but does sound like it would be fun, and a nice idea.

PuterPro

Always happy to help you burn up your time on earth ... LOL!

Seriously, I've had that thought floating in my head for a couple years. Never got into the Arduino thing despite having roots as an Electronics tech, too many other things in my life...

But it strikes me as a great project for someone who know it.

DocTrucker

Car battery? Generally around 13 volts but that should be fine. Ensure the supply (/charge) is fused.

PlasticMetal

While these ideas are certainly worthy, I think they're being over-thought. As @PuterPro mentioned, surplus UPS units are pretty inexpensive if you keep your eyes open. I purchased two 1000VA/800W rack-mount TrippLite units off eBay for less than $50 each, with shipping. Right now, I'm only using one, so I replaced its internal batteries and added 2 parallel 18Ah UPS-rated deep cycles externally for about $100 (commercial units such as these frequently are designed with external battery pack connections; batteries were on sale).

So, I have at least a few hours worth of power for the printer, PC and monitors, all with pure sine-wave, double-conversion smooth electrical goodness, and don't worry about power loss routines. The one concession I did make is the 120V bed heater is not on the UPS - that would just be wasteful for a home printer. It's a good chunk of aluminum, though, so it takes a while to cool down.

PuterPro

@PlasticMetal - Those are sweet units! Wish I has room for one.

Back in the day I was trained on UPS repair; 200KVa units with racks of deep cycle batteries.

The company had an "Instant Dismissal" policy if you tried to work on them alone. The first thing they taught us in the 2 week school was CPR! You had to have a CPR trained tech sitting there watching you, no talking, reading or horsing around when you opened it up to work on, he was there to start your heart if you F-up'd. Fun stuff!

PlasticMetal

@puterpro said in "Backup" for 12V systems if powerloss?:

@PlasticMetal - Those are sweet units! Wish I has room for one.

Well, I certainly don't have them in a rack. One is on a metal shelf (restaurant-style wire shelves, plenty of air circulation), & the unused one is over in a corner.