Testing UPS Output Waveforms
lttlabs.com
lttlabs.com
So, I replaced all my UPSes with LiFePO4 batteries supplied by Victron AC->12V chargers. Routed the battery contacts directly to all devices that consume 12V (WiFi AP, network hubs, SLA 3d printers). Used 12V -> 5V adapters to supply 5V / USB2 devices (R-Pi servers). For 19V, Drok DC-DC boost converters work great.
Result: threw away 3 UPSes (different APC models). Overall power consumption with AC present dropped by about 40%. Time on batteries (same Wh battery capacity) increased by a factor of about 20 (yes, 20 times: that's not a typo). Evidently, AC waveform generation is extremely power-hungry
Evidence is the heat from that conversion
I've tested a dozen models from APC. The inverter used in those devices uses roughly 15-20W with no load. Then for any load they have about 85% efficiency. Then you have further losses into any PSU connected there because they tolerate square waves but aren't optimized for it. So yes, in the end, less than 40% of the battery capacity in cheaper UPSes is actually usable.
The reason you're seeing 20x is because obviously you've also greatly increased your battery capacity (typical under-the-desk APC units have 70-150Wh capacity, less than half of which is usable as explained above).
> Overall power consumption with AC present dropped by about 40%.
I'm finding that part harder to understand. The UPS consumes almost nothing when AC is on, so that can't be that. You've replaced multiple PSUs by more efficient, bigger ones, sure that can explain part of your improvement. But 40% drop is wild!
Back in the 1990s, one could buy a "double conversion" UPS that converted AC to DC then back to AC, at all times. This was, supposedly, the best type of UPS (in my experience they were also the least reliable)
It's not a good option for home use because it's always sending power through an inefficient path. The devices we use have power supplies that can handle transients and fluctuations.
If they can do that efficiently, that's great.
If they have to choose between efficiency and outage risk, they always drop efficiency. That's why they use exclusively online UPS.
Disagree. I'm most familiar with Google. For a long time at least their UPSs were built in to each machine and not super reliable. They built entire datacenters without cooling with the understanding that for a few days a year it may be too hot for human access. The reliability is from failing over to other equipment—machine, cluster, even region.
Small/on-prem/co-location datacenters may make totally different choices, but I think the major cloud/AI providers would be similar to Google in this respect.
The sibling comment about other industries was enlightening to me though.
These are also the only variants which will protect you against things like a phase ending up on neutral in a 3 phase power system. I've seen this happen twice. Fried a lot of equipment.
They are "best" in the sense that your output is completely decoupled from your input so you got the most protection from any electrical noise. The trade-off is lower efficiency (AC-DC-AC roundtrip) and more battery wear (it's constantly 'in use').
Any >10kVA UPS is probably double-conversion/online.
You can put them on street power, you can throw them over to the diesel genny, back onto a secondary genny, back onto street power, and the output won't even ripple.
Then theres trivialities like LFP's voltage being higher so there's less resistive losses, etc
The Drok DC-DC did not work for my minipc that needed 19V/130W supply (would cut off with heavy draw), but the JacobsParts LTC3780 130W has been running my minipc's for almost a year now, gaming minipc, server minipc and networking
before that the solar panels barely charged the solix unit, but now my batteries fully charge and I still sometimes have left over solar I feed into the solix
I even made an Arduino-based module that provides an SNMP UPS interface for my Synology NAS. It works surprisingly well and has almost 12 hours of autonomy compared to barely 2 hours for the much heavier lead-acid battery.
One trick that I'm kinda proud of: I powered my server directly from the 96V DC. And I periodically switch the current direction using a DPDT relay to avoid wearing out one side of the rectifier inside the PSU.
If you are seriously worried about this then the whole thing is trash. Either the design is marginal or it is not. You cannot possibly switch a relay fast enough to make a difference here (and have the relay survive).
If you're specifically worried about wear, then you could switch once a month and it would be enough.
(At least not on timescales relevant to individual humans.)
So hearing that makes me get suspicious that something else is going on.
Your other comment says: If you are truly close to the design failure point of the rectifier, it's not safe to run at all. (You are almost certainly not.)
Well there's no reason to assume it's close to the failure point.
Think of it this way: Draw the line in the sand for where you'll approve the design, but just barely. If someone is running a diode close to that line, then it's not trash but trying to improve longevity isn't crazy either.
So if it is unacceptable, it is unacceptable, and needs to be fixed. I said "trash" because it's going to become trash, and with luck just the power supply. Hope there's a fuse inline! Input rectifier failures tend to take down other stuff without one.
If you are truly close to the design failure point of the rectifier, it's not safe to run at all. (You are almost certainly not.)
If you are worried about the fact that you're only using one element of a multi-element package, again, it's a nonissue. We do this all the time. It's often cheaper to add a second bridge rectifier to get a single diode than it is to add another BOM line item for the "proper" part. As long as that diode isn't operating near absolute maximum ratings (it probably isn't), it doesn't matter that there are or aren't three more in the box.
aka- you add a diode, now you have to add procurement, warehousing, extra time on the pick and place, possibly a more expensive/larger+slower one as well(so even more time), then you have wastage, labor for keeping the machine fed...
The reason I added it in the first place is because I've seen failures from uneven PCB heating. It's not the components themselves fail, but radiators get unglued and PCBs can get damaged.
Yes, it's quite ugly. Open up one of these things and you'll find a big block of four transistors (if not more due to doubling up) on a big heatsink. That's the inverter drive bridge, and it's probably the single largest source of heat in the whole thing. It's not hard to find.
The only thing to be careful with is connecting different voltages to different connectors, but it's at least possible with the APP connectors to "Build your own" with different color housings and different ways of combining the housings.
So maybe 13.8v is red/black and something that's 5V is black/white, etc.
A huge decision on that was that they publish a fat PDF with the full spec of the serial data they emit and can be programmed with, and a service sheet to allow a suitably skilled engineer to fully bench test them if there are any perceived problems.
They're not cheap, but my late father always used to say he didn't have enough money to buy cheap tools.
Edit: handy hint too - many cheap crappy power bricks will work down to 90V and will work just as well off DC as well as AC because the very first thing they do is rectify and smooth the incoming supply.
I do not recommend having 90V DC kicking around unless everyone involved knows what they're getting into.
Entry level differential probes are $300. Less if you shop around or buy used. Micsig makes a good starter probe that would be more than enough for 60Hz AC mains testing and it comes in a generic form that would have worked with this scope.
A lot of things can go wrong, some dangerously so, if you incorrectly probe high voltage lines.
I don't know why they got such an expensive oscilloscope and then proceed to cheap out on the most basic tools needed to use it properly.
For about $300 you can buy a Tiepie differential usb scope: https://www.tiepie.com/en/usb-oscilloscope/handyprobe-hp3
The ground lead on your probes is connected straight to the ground on the power cable. This gets new users in trouble when they're probing power circuits and they don't realize that connecting the ground part of the probe to something will cause a short to ground. If that ground clip pops off and brushes against the high voltage you're trying to probe, you get sparks and maybe a destroyed scope.
The differential probe provides isolation and rejects the common-mode (shared) voltage between the two probe points before it gets to the oscilloscope.
I don't know about that USB probe, but I prefer not to have single-purpose instruments that require their own desktop software to use.
The Tiepie software actually works very well even though it's Windows only, they do have Linux library, just no GUI on Linux. Its not single purpose its a full oscilloscope that happens to use differential inputs.
They actually do sell one more purpose built for power quality analysis which is new. I would love to have a couple on my home power split phases to view and log power quality in detail: https://www.tiepie.com/en/usb-oscilloscope/handyscope-tp450
The CMRR of the Micsig I linked is pretty average, but it's a lot better than the TiePie at low frequencies. Micsig also specifies it at multiple points across the spectrum, while TiePie doesn't even say where they measured it.
It's all the differences like this that make good test gear expensive. The Micsig is not expensive on the scale of these devices. The professional gear will have even better specs, calibration, long-term stability, temperature stability, and many more features.
For playing around, the TiePie thing will do fine.
It worse at high frequencies and the same in the middle. Again this is a whole scope compared to a probe.
Not sure why you think Tiepies are toys, they are use professionally in Europe, popular for automotive. They have a lot more expensive scopes, and the price difference for the differential versions are not that much.
Now, let's say you want to probe two things at the same time (triggered by a common signal source). You can't. And the reason you can't is because the producer took the expedient of floating the entire scope, and there's no trigger input.
In other words, they took the cheap way out by not actually building a differential probe. Related to that, this thing doesn't appear to have a step attenuator, which is why the effective resolution depends on the volts/div setting of the input.
Also, they don't specify the CMRR, which is one the main figures of merit people look for on differential probes. Any capacitive coupling between the scope and ground is going to degrade CMRR. So who knows if it can actually measure anything useful.
You can buy a scope with multiple optically-isolated channels as well as a trigger input, but those end up costing as much as a differential probe. Because it turns out that achieving good CMRR when you have multiple inputs is as hard a problem as making a good differential probe.
This is not to say the product you linked shouldn't or can't be used for anything, but it is a niche product. That's probably why it is advertised as a "power quality monitor" and not an "oscilloscope".
Yes this is very inexpensive single input scope, they make more expensive multi input scopes you can stack as many as you want and have them synced:
https://www.tiepie.com/en/usb-oscilloscope/handyscope-hs6-di...
The difference in price between their single ended and differential scope is not the much so it seems the actual differential part is not the expensive part, back to my original point.
>Related to that, this thing doesn't appear to have a step attenuator, which is why the effective resolution depends on the volts/div setting of the input.
They sell differential step attenuators: https://www.tiepie.com/en/usb-oscilloscope/accessories/diffe...
>Also, they don't specify the CMRR, which is one the main figures of merit people look for on differential probes.
CMMR 60db from thier spec sheet: https://download.tiepie.com/Documents/Datasheets/Datasheet-H...
>This is not to say the product you linked shouldn't or can't be used for anything, but it is a niche product. That's probably why it is advertised as a "power quality monitor" and not an "oscilloscope".
From their spec page: "The tables below show detailed specifications of the Handyscope TP450 high voltage oscilloscope."
and
"The Handyscope TP450 is delivered with the versatile Multi Channel oscilloscope software, which transforms the Handyscope TP450 into an oscilloscope, spectrum analyzer, data logger, multimeter and protocol analyzer."
They make quite a few differential scopes, that just happens to be a inexpensive one physically deigned for power quality analysis.
> Our previous reticence to measure UPSs was centered around the connection of our very nice $50,000 Rohde & Schwarz MXO58 oscilloscope directly to mains power. [...] What we do have is a Chroma 61507, a programmable AC power source, capable of generating its own isolated Alternating Current(AC) signal. The AC signal created by the Chroma 61507 is galvanically isolated from the "earth"/ground, providing a floating source.
This too seems to be a pretty expensive piece of gear (the price I found with a quick Google was >$28,000) so I think it's worth mentioning that the same job could be done with an isolation transformer, which costs maybe a couple hundred bucks.
For such low frequency stuff, it feels way safer to just buy a cheap <$500 scope for this kind of work. Using a $50k scope when it's not needed just seems needlessly risky.
Also, float the DUT, not the scope... Sometimes that's not possible, and the temptation is there, but it's really not worth it. Just buy the right gear like a diff probe. You can get one for a few hundred bucks if you don't mind going downmarket.
You can also use two probes and do CH2 - CH1. (Disconnect the GND clips!)
They should have spent $300 on a differential probe.
The higher end scopes can have some nice power analysis packages.
It really cannot -- the isolation transformer doesn't have control of its output, so it can't start or stop cleanly, and it can't ramp voltage cleanly. (An autotransformer kind of can, but it's still not really good enough.) The AC source can stop on a dime, with no inductance of its own, so it is the correct way to do this test.
Source: I have had to do this and refused to use the autotransformer anymore because it was just too much of a pain in the butt. (We rented the AC source.)
Yet as far as I can tell none of them offer anything in this area except at the extremely high end. Even Ubiquiti's UPS offerings are garbage simulated sine wave with lead acid batteries.
Are UPSes such a niche product there's no money in it? Are they really content to just give up the whole "power station" market to upstart competitors?
Even aviation jump packs (that connect to aircraft ground power ports) offer lithium versions and that's an industry that moves like sloths toward new technology!
and yes, they're also happy to heave to: plenty of DC ups' use NMC. Apc can't even be bothered to make a consumer class UPS that works with PFC- at all.
3C or 20 minutes of runtime would be in the realm of most UPS ratings (remember the LiFePo4 cells have a lot more energy/runtime in the same space).
It typically shows up ‘randomly’ unless you know how to attribute it.
But, certainly, garbage devices are all over the place.
How does that happen? Let's say it's running and I drop the input to 80 volts. Why do I get any behavior that isn't "runs at reduced capacity" or "shuts off"? And what part of the circuit is failing?
Are we assuming a combination of missing current limiting and a heavy load? If I'm just watching youtube then it shouldn't overload any components even if it keeps running at a really low voltage.
Same facility was fed 3-phase power, but due to some construction mishap nearby, one leg was cut. That lost us a lot of expensive power supplies that day for some of the more expensive equipment.
Those are examples, but not really an answer to the how question. I'm not an electrical engineer, so :shrug-emoji: I asked the engineers that question at the time, and they told me it was something along the lines of the equipment tried really hard trying to function instead of just shutting off. The dip was low, but not that low. Described like a ceased electrical motor where it keeps pulling more power where normally a breaker/fuse would trip, but something different. It wasn't a satisfactory answer then any more than it is now.
For a voltage drop, the main idea that comes to mind is something trying to keep up a constant wattage and drawing increasing current to make that happen. But you have to do quite a bad job to design that circuit and not have a current limit.
And a PC power supply is inherently flexible on the input voltage, so it would never have the problems you get with a fixed ratio transformer on that old equipment.
From all of that, I have learned the lessons of how dips can ruin electronic equipment (even if not the exact why back then), so for me and my house all electronics are hooked up to a UPS or power conditioner. Appliances are on their own though as that's the landlords problem! Multiple times a week, I get noticeable dips where the lights visibly dim and I can hear all of the UPS units kick in and back to mains a few seconds later.
under voltage can do lots of things. Browning out with partial functionality can cause lots of problems. Some devices will pull about the same watts regardless of input voltage, so lower voltage means more current, and significant under voltage may require much higher than rated current and can damage connectors, leading to thermal runaway (loosened connector has more resistance -> more current -> more heat -> connector loosens). Brown outs during control sequences can lead to controlled loads running for longer than intended and over current situations too.
Class D amplifiers and other topologies that depend upon SMPS for power delivery are usually unaffected. Class A/B is where you will typically hear it.
https://rvelectricity.substack.com/p/diy-generator-bonding-p...
I think the quality of your power is determined mainly by the size of the transformer serving your neighborhood as well as the presence of noisy heavy power equipment like AC with poor/no soft starters or big brush motors among the consumers. It's noticeably worse on our street compared to where we previously lived.
This generator is meant to back up a 400A electrical service that runs a 480V 200 HP three-phase water pump motor, which draws ~225A and is controlled by a VFD. This pump motor and drive run just fine on utility power, but when utility power was cut to let the generator drive the pump, the generator output frequency started jumping between 57-63 hz and a manometer placed on the generator showed the gas pressure fluctuating wildly.
Now, a 480V 250kW generator puts out (250000/480/1.732)=300.712 amps, which should be more than enough to run the pump. I asked the generator manufacturer what size generator they would recommend for this 200 HP pump, and they said for a basic 6-pulse VFD, it would need a 350kW generator with a 600kW alternator, but the existing generator could be used if a harmonic filter was added to the variable frequency drive.
I don’t really understand exactly what the issue was, but something with impedance and harmonics that could be corrected with a harmonic filter, and once that harmonic filter was added, the generator was able to run the water pump just fine.
* These power stations are better than conventional (lead-acid battery) UPSs in the sense that they're cheaper, more flexible, have dramatically longer battery life, and require battery replacement less often.
* ...but I haven't seen any that claim to be "line-interactive" or even say specifically when they fail over (other than a total power cut). They do talk about how long it takes to fail over: older models are >20ms (long enough that your machine will probably reboot); many newer ones are <10ms. I'm not sure how high-quality their sine wave is when on battery.
20 milliseconds is barely distinguishable from a single 60 Hz sine wave period. 10 milliseconds just over half a cycle.
I've read that the newest PSUs are only guaranteed to last 12ms. Of course they may last much longer, especially if running near idle, but I'd prefer something that works well with any compliant device.
Here's one source: "Measured in milliseconds, hold-up time indicates how long a PSU can sustain its output within specified voltage limits after a loss or drop in input power. ATX 3.1 features a shorter hold-up time of 12ms, compared to ATX 3.0's 17ms hold-up time. This results in a small improvement in the PSU's efficiency." https://www.corsair.com/us/en/explorer/diy-builder/power-sup...
I haven't dug through the spec itself.
At least I can be comforted that for my current power supply, the most recent version of it made for ATX 3.1 actually increased the hold-up time. So not all manufacturers are cutting that corner.
They do not. You must be thinking of very old power supply technology with a simple bridge rectifier in front of some capacitors.
Switch mode power supplies with power factor correction spread the current draw across the cycle to keep the power factor high. They are drawing power from the line for most of the cycle. There is not a 8.3ms interruption.
> 20 milliseconds is barely distinguishable from a single 60 Hz sine wave period. 10 milliseconds just over half a cycle
The ATX 3.1 power supply standard only requires 12ms of hold up time.
It's not an 8ms interruption, it's 8ms between peaks. The part you could call an interruption is more like 2.5ms and even then it's not zero power draw. You need an order of magnitude more buffering to handle a 20ms dropout.
> 20 milliseconds is barely distinguishable from a single 60 Hz sine wave period.
Right, so think about that harder. A 60Hz sine wave has two wide periods of power and two narrow gaps. And 20 milliseconds is longer than that entire process combined.
Could be worse - could be lead acid and weigh 2x as much and you only get half the Ah.
The rationale I've heard to justify conventional UPSs not even trying to compete on runtime is that they're just for giving you a few minutes to cleanly shut down your crap software that isn't crash-safe and/or for your auto-start generator to start up. But what I actually want is to keep working for an hour+ after the power goes out without owning/installing/maintaining a generator.
So like SSDs? If you fill them up near 100%, performance tanks.
I hope nobody sees this article and tries to replicate the experiments as presented. You can get away with it when everything goes correctly, but a diff probe is good insurance.
Would love to see how the waveform changes over load -- perhaps test at 0, 10, 20, 40, 80% load.
Also, how does waveform vary as the battery depletes?
Another metric is how capacity varies with load. If a UPS gives me 1 hour @ 100w, will it give me 10 hours @ 10w? How long will it power an idling rpi5 (<1w)? How long will it give my workstation PC?
Assuming you really need a sinewave at the output at all. DC output UPSes are the most efficient way to go if you can bypass the switched-mode power supply at the input of your equipment. Which most equipment has these days unless AC motors are involved.
We did a "big" inverter design a while back (500 VA was big for us; perhaps not for you). The guy who did the concept architecture suggested a PSFB design. He then quit to take a a great offer from a startup. Not really being a power electronics team, we hired a specialist consultant. The first consultant did... honestly, I don't know what he did. But it was weird. (This was a problem.) It wasn't a PSFB anymore. It also didn't work. The design then went through five more lead engineers and two more consultants, plus one more if you count me on the side watching and occasionally pitching in (I was the sister subsystem lead). It ended up being a full digitally programmable bridge and we had to figure out how to switch it. Guess how it ended up working?
Phase-shifted full bridge. Just like the first guy (and I!) said it should have been all along!
No, they're not Class B. It's all digital PWM stuff inside. But the duty cycle gets tiny near zero cross, there's very little power in the waveform there, and there's overhead to have a switching device on at all (this is much more noticeable for IGBTs).
So it ends up being a massive simplification to just not care about that section. And it's a simplification that works pretty great, so people do it!
We had to get this truly right in the inverter I mentioned in sibling comment (as it wasn't a grid-feed or backup inverter, it was doing Something Else™ *) and just that piece was actually way harder than the entire rest of the waveform output design.
* hopefully NDA-OK spoiler: let's just say I know way, way more than I'd like to about what's inside that Chroma 61507 mentioned in the article.
It’s cheap and easy (relatively) to transform AC voltages, and hence to manage AC power distribution. DC is trickier, and voltage switching is relatively more expensive and flakier. Hence why DC distribution tends to be within a device/controlled setup.
It's worth noting that there's already ATX power supplies that are built to run directly off battery power. They don't look all that impressive but they exist. https://www.powerstream.com/DC_PC.htm https://synoceantech.com/index.php?page=lotinfo&lot=36
And then scroll down to the model you want.
The second to last column is price. Click on that (Or click on the first column where it has the model number. Either one works.).
Then click "add to cart".
I can click on the picture to get the details and volume pricing. The 300W model has no price and a minimum order of 30 units. So I thought it ended there.
But it recommends to check out https://www.powerstream.com/DC-PC-24V-400.htm where I ultimately found the add to cart for $300.
They do rectification as the first step anyway, and then they use high-frequency switching that works just fine with the DC current.