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High power, low capacity UPS circuit

jimduchek

Mar 29, 2022
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I'm looking to design a UPS/transfer switch/inverter circuit that can supply ~1kW 115 VAC for around 5-10 seconds. Such a thing does not exist commercially, unfortunately (and not surprisingly, really, pretty niche here) The use case is this: I live in an RV and my whole place is battery-backed. If external power is shutoff via a breaker, my inverter transfers to battery power quickly enough that nothing has any problems. However, if I'm running my generator (Generally, I'm solar-powered, but trees and rain do happen), and I shut it off... the power goes 'bad' (as the generator spins down, frequency drops) and while eventually (and 'eventually' is like 100ms) the inverter takes over, it doesn't happen quick enough. Almost everything I have either A> is fine to lose power for a quarter second (laptops, screen, etc), or B> is running directly off battery anyway. However, my main workstation is AC-powered, and it inevitably reboots when I turn the genny off if I don't go through a whole sequence of tripping the breaker first, which is annoying. An off the shelf (lead acid) UPS fixed this problem, but it takes up too much space and weight in my rack (as you can imagine, every U is precious in an RV rack :D) and I'd like to get it gone.

So I've got a basic idea in my head on how to design a UPS/transfer switch, and there's some example schematics and such out there. I'm hoping to fit this little project into my existing PDC (A 1U Pyle PDBC70), but we'll see I guess. I'm debating a few issues and I'd be happy to have anybody weigh in with any thoughts/ideas before I really start digging in. I've not seen anybody doing what I'm trying to do, but if you know of anyone who has, please let me know.

A> I don't even really need 5-10 seconds. A 1 second "capacity" would probably do me just fine. But I may be running a lot of power at the time.
B> I have access to power from my house batteries (albeit, 10ga cable and about 15 feet of run away) in and near the PDC I want to fit this into. I was originally thinking capacitors, but now I'm thinking skipping having a 'charging' circuit or energy storage at all, and simply drawing on the house battery. I would not want to run 1000W through this cable for very long, but a couple seconds should be fine.... If I go this route, I'll want to accept 12-26VDC or so -- I'm on a 12V nominal bank now but I'm debating moving to 24).
C> I could probably get by with 8 or 900W, but it's always best to build in some headroom. Likely in most cases my graphics cards won't be fired up and really, 150W is fine, but I'd rather do this right and make sure it works even if I'm sucking power full-tilt.
D> I suspect somebody's gonna say something like "you need enough capacity to shut down cleanly -- a minute or two" -- like most UPSes -- I say no, I don't. I'm absolutely fine with my workstation just losing power if it takes more than a couple seconds for things to stabilize -- that's gonna be really rare.

tldr: I need a UPS that can provide 1000W of 115VAC for a _very_ short time (<1s) while a generator spins down.
 

Bluejets

Oct 5, 2014
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Use a ups that is always supplying ac and run charger on the battery.
 

kellys_eye

Jun 25, 2010
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You know the difference between an online and an offline UPS system?
 

ramussons

Jun 10, 2014
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Make an Online UPs.

Unreliable AC ----> Battery Charger -----> Battery --------> Invertor (1 KVA) -----> Load

1 KVA invertors are commercially available. Google Solar off grid 48 volt to 110 volts 1KVA Invertor
Batteries - 4 x 12 Volts = 48 volts
Charger 48 volts / 30 A
 

kellys_eye

Jun 25, 2010
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Can you not take a signal wire from the genset key (ignition) line to use as a UPS 'on' signal?
 

jimduchek

Mar 29, 2022
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I feel like there's a lot of not reading what I wrote for my use case. Am not interested in ANY solution with batteries. More space/weight than I want to give and it's not necessary anyway. I already HAVE an off the shelf UPS that works JUST FINE. It also weighs 30 lbs and takes up 3U of rackspace and I don't need most of it's capability, so I want to design something that meets MY specific need. I need a UPS to provide 1 kW of power for like, 2 seconds. I am perfectly happy for it to shutdown if power is not restored within 2 seconds. And want it to be appropriately small/lightweight for that.

I am NOT going to use an off the shelf 1kW inverter. They don't really make them that small with transfer switches anyway, and I have no reason to believe that any of them (unless I have full access to their schematics/firmware) will do any better than my whole-house inverter at recognizing the generator fall-off quick enough to respond. Plus they all come with giant heat-sinks and whatnot because they're designed to supply 1kW for hours and hours. I need 2-3 seconds.

Anyway. Figured I'd post here first to see if there might be some constructive advice, but I guess there's not so I'm gonna dig in and I'll post build details as I go.
 

Harald Kapp

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kellys_eye

Jun 25, 2010
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ISTR you used to be able to get an internal UPS-like card for desktop PC's (if I recall it just buffered the main supply rails) that allowed sufficient time for a proper shut-down process in the event of power failure. A few Li-ON's and some charging/diode-switching could achieve this quite easily. Rather like larger smoothing capacitors in the PSU..... maybe even exploring just 'that' would be sufficient? i.e. super capacitors?
 

jimduchek

Mar 29, 2022
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?
Any UPS will require a battery to provide the power, for whatever time you envision. You may get away with capacitors but these will take up even more room for any useable power range.

Still feel like you are not (like everybody else) not actually reading the whole long use case. Yes, it's long, sorry it's long, but if you're not willing to read the whole thing, don't comment, because you're not being helpful. I'm not some kind of dumbshit who thinks a UPS can have no power supply and magically make power from nothing.

Yes, a UPS requires a power source to maintain AC voltage "in the meantime" while source AC voltage is not available. Yes, I know some kind of power source is necessary during a "power outage".

But read my use case, please, thoroughly, before you comment. I am wanting ONE piece of equipment which MUST be 115VAC powered to have SPECIAL protection during one VERY SPECIFIC KIND (frequency failure) of power outage for a VERY VERY SHORT amount of time (2-3 seconds).

I am maybe not explaining my need very well. Apologies for that.
 

Bluejets

Oct 5, 2014
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Yes well, debatable just who is not listening here...might have to bring out my favourite " I know boats, Norm video" once again but I fear that would go unabsorbed as well. Good luck once again.....
 
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