alt.hn

5/12/2026 at 4:50:59 PM

Testing UPS Output Waveforms

 https://www.lttlabs.com/articles/2026/05/12/ups-exploration

by LabsLucas

5/12/2026 at 7:07:07 PM

I've noticed that ALL the devices I plug into my UPSes have external power bricks. Most of them are either 5V, 12V, or 19V

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

by chimpontherun

5/12/2026 at 8:19:17 PM

> Evidently, AC waveform generation is extremely power-hungry

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!

by tredre3

5/12/2026 at 8:52:50 PM

> The UPS consumes almost nothing when AC is on, so that can't be that.

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)

by michaelt

5/12/2026 at 9:42:24 PM

> 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)

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.

by throw0101c

5/12/2026 at 10:02:42 PM

If built correctly this design also suffers no transition transients. You can switch the external power off/on all day and downstream equipment will never see a glitch.

by dreamcompiler

5/12/2026 at 9:07:10 PM

As far as I know the more expensive UPS models are all still "online" (ie. double conversion) UPS'es.

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.

by jsiepkes

5/12/2026 at 9:04:09 PM

This is called an online UPS and it's still a thing.

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.

by Aurornis

5/12/2026 at 10:23:28 PM

Where is it a good option? I'd expect datacenters to be more focused on efficiency, not less.

by scottlamb

5/12/2026 at 10:54:54 PM

> Evidently, AC waveform generation is extremely power-hungry

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.

by exmadscientist

5/12/2026 at 11:25:55 PM

I've been in the process of cutting the DC side of power bricks and crimping anderson power pole (APP) connectors onto both sides of the wire. For camping and ham radio, it's really nice to hook up to a battery without the AC inverter taking DC -> AC and the power brick sending that to DC again.

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.

by bb88

5/13/2026 at 10:59:15 AM

theres also anderson's SB series- APC seems fond of knockoff versions.

by butvacuum

5/12/2026 at 7:15:42 PM

I toyed with this too, but I guess I have a slightly more diverse set of devices than you do. A few more weird voltages, and some things that expect mains. I looked into finding a DC version of their power supplies (e.g. the pico-box X9-ATX-500 to replace a conventional ATX PSU, tracking down DC versions of network switch hot-swappable PSUs from eBay) but decided it wasn't worth it. I just bought a stock LifePO4 power station. I found that I got most of the benefit [edit: measured in terms of runtime after power outage, not power draw while input power was available] just from switching to LifePO4 rather than from avoiding DC->AC->DC, and it was cheap and easy.

by scottlamb

5/12/2026 at 7:19:09 PM

If you get your battery pack up to 48VDC, it opens up a whole world of low voltage power converters, since this is standard in telecom/PoE.

by nomel

5/12/2026 at 8:35:17 PM

I did something similar but made the batteries and solar priority, solar charges battery and wall power only used to top off as needed, otherwise always running on batteries

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

by aspbee555

5/12/2026 at 7:50:00 PM

> Evidently, AC waveform generation is extremely power-hungry

Evidence is the heat from that conversion

by dylan604

5/12/2026 at 10:06:05 PM

This is the way to go IMHO: Keep the outputs DC. Put a bunch of USB-C PD ports on the thing and you're good with a lot of modern equipment.

by dreamcompiler

5/12/2026 at 9:36:47 PM

I have thrown out my Tripplite UPS because its battery has degraded to the unusable levels. I replaced it with a 5kWh LiFePo4 rack-mounted battery and an AIMS rack-mounted inverter. I'm really surprised that there are no off-the-shelf solutions for this. The traditional UPS makers are just neglecting the recent 10 years of battery advances.

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.

by cyberax

5/12/2026 at 10:53:34 PM

> 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).

by exmadscientist

5/13/2026 at 2:34:58 AM

I don't understand what you're saying.

If you're specifically worried about wear, then you could switch once a month and it would be enough.

by Dylan16807

5/13/2026 at 4:22:55 AM

Silicon rectifiers don't wear out.

(At least not on timescales relevant to individual humans.)

So hearing that makes me get suspicious that something else is going on.

by exmadscientist

5/13/2026 at 5:01:21 AM

I'm also suspicious of the idea of that part wearing out, but if it doesn't matter at all then there's no reason to call things trash.

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.

by Dylan16807

5/13/2026 at 5:40:05 AM

The point is that the dance with the relay doesn't move the needle on design acceptance. If it is acceptable with the relay, it will be acceptable without the relay, because the component stresses will be the same.

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.

by exmadscientist

5/13/2026 at 3:56:15 AM

I'm worried about long-term thermal wear from uneven heating, so I switch the current direction once a day. Simply because that's the maximum time I can set on a time delay relay that controls it.

by cyberax

5/13/2026 at 4:25:49 AM

Don't bother. That's not how these things fail, and one day is so so far beyond the thermal time constants involved that it's not doing anything useful. You would have to switch on timescales of seconds to minutes to do anything meaningful, and that would kill your relay in short order.

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.

by exmadscientist

5/13/2026 at 11:15:21 AM

note: its the additional component and its knock on effects that are the cost- not the half cent diode.

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...

by butvacuum

5/12/2026 at 7:56:51 PM

> This can be done safely with high voltage differential probes like the R&SRT-ZHD, but we don't have any.

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.

by Aurornis

5/12/2026 at 8:03:51 PM

Why are differential probes so expensive and why don't more scopes just have differential port built in?

For about $300 you can buy a Tiepie differential usb scope: https://www.tiepie.com/en/usb-oscilloscope/handyprobe-hp3

by SigmundA

5/12/2026 at 8:19:47 PM

You can create a pseudo-differential input by combining two input channels on almost every scope. That's not the problem the differential probe is solving, though. The differential probe exists to provide a differential measurement between two voltages that may be isolated or significantly different than the ground voltage of your oscilloscope.

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.

by Aurornis

5/13/2026 at 12:29:35 AM

I understand what a differential probe does, what I don't understand is why they are so expensive. Again one can get a whole USB differential scope for the same price or less. Seems like just a differential probe should be relatively inexpensive well under a $100.

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

by SigmundA

5/13/2026 at 1:02:35 AM

A good differential probe has to use precision components combined with careful factory trimming and calibration to get good common mode rejection ratio.

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.

by Aurornis

5/12/2026 at 6:24:33 PM

Cool graphs.

> 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.

by teraflop

5/12/2026 at 7:10:29 PM

Agreed.

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!)

by hex4def6

5/12/2026 at 7:58:22 PM

> 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.

They should have spent $300 on a differential probe.

The higher end scopes can have some nice power analysis packages.

by Aurornis

5/12/2026 at 11:03:17 PM

> the same job could be done with an isolation transformer

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.)

by exmadscientist

5/12/2026 at 10:49:13 PM

For some time I've been curious why none of the major UPS manufacturers offer LiFePO4 UPS units. They'd be smaller, lighter, and have a longer run time all else equal (dramatically lower shipping charges). Batteries wouldn't need replacing nearly as often.

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!

by xenadu02

5/12/2026 at 6:50:11 PM

Curious - what actual real life issues do real world people encounter with dirty AC waves? Like I always hear the proverbial "this could cause harm to electronics" but are there real world tests of electronics failing? Does it fail over time or because of a one time instance? Same thing with under/over voltage.

by mbesto

5/12/2026 at 8:55:11 PM

Modern furnaces are weirdly sensitive to ‘clean’ AC power. Mine won’t work on bad non-inverter backup generators and interestingly to me, doesn’t work on non-bonded (ground-neutral) power from an inverter generator. Had to chop the cord off a drill and build a bonding plug last winter when I finally figured out why it wouldn’t run.

https://rvelectricity.substack.com/p/diy-generator-bonding-p...

by mikeyouse

5/12/2026 at 10:16:15 PM

My furnace is protected by an old school fast-acting fuse. One day it blew and at first I thought it was an anachronism from the house's original wiring but then realized it's intentional - the standard breaker upstream of it is not fast enough. Not clear if it mainly protects the blower fan motor or the circuit board - I suspect it's the motor. At least one other fan motor in the house got fried previously.

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.

by ak217

5/12/2026 at 9:03:15 PM

I discovered the same thing a few months ago with my home's furnace (which was a bit of a shock, because one reason for picking natural gas is so you can still have heat when the power is out). It runs just fine on a pure sine wave inverter though.

by alnwlsn

5/12/2026 at 8:05:09 PM

If you get dips in voltage below the range that the PSU can handle, it will kill the PSU. If you get spikes higher than the range that the PSU can handle, it can kill not only the PSU but things attached to the PSU as well. Most people are familiar with spikes with things like surge protectors, but most are unaware of how damaging voltage dips can be as well.

by dylan604

5/13/2026 at 2:38:45 AM

> If you get dips in voltage below the range that the PSU can handle, it will kill the PSU.

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.

by Dylan16807

5/13/2026 at 5:32:51 AM

My experience is from older analog equipment. Specifically, I worked in a video/film post house that also had a VHS duplication department. Analog tape machines run at a certain rate. For those old enough, VHS had SP, LP, and SLP modes. Those were defined speeds that the tape was fed through the system. If the voltage dipped, the speeds would slow down. When played back at a constant rate, the signal could not be read as the signal wasn't recorded at that speed. So our meter would sound an alarm when the voltage dipped too low that would introduce that problem which would tell the operators to stop the recordings and rewind/restart. That's more of an annoyance than damaging, however, some of the records of a certain model would fail which was usually a power supply issue.

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.

by dylan604

5/13/2026 at 5:44:34 AM

Well, I can think of a lot more ways for losing a phase to break things than a voltage drop.

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.

by Dylan16807

5/12/2026 at 10:50:19 PM

This really should not happen, at least for units that are qualified to the relevant IEC standard.

But, certainly, garbage devices are all over the place.

by exmadscientist

5/12/2026 at 8:11:29 PM

over voltage (beyond reasonable tolerances) has a tendency to let the smoke out of components directly.

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.

by toast0

5/12/2026 at 8:35:18 PM

Audio amplifiers can be strongly affected by noisy waveforms.

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.

by bob1029

5/12/2026 at 7:42:03 PM

Capacitive touch screens when plugged in with cheap chargers to crappy waveforms, behave weird.

by markus92

5/12/2026 at 9:07:18 PM

I've had servers that would not power on with non sinusoidal power.

by krunck

5/12/2026 at 6:58:44 PM

‘It lets the smoke out’ is a classic, and happens periodically. Bad waveforms cause weird heating issues, (literal) audio noise, and sometimes sporadic stability issues with computers.

It typically shows up ‘randomly’ unless you know how to attribute it.

by lazide

5/12/2026 at 7:07:30 PM

I would be curious to see how LifePO4 power stations compare.

* 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.

by scottlamb

5/12/2026 at 7:51:19 PM

Can these LifePO4 batteries be safely drained to 0% and then charged numerous times?

by dylan604

5/12/2026 at 9:24:18 PM

Yes and no. 0% you should be fine, but generally speaking you'll get longer life out of LifePO4 if you stay between 10% and 80%. Most battery based PV systems are installed to shut off (or switch to grid) at the 10% SOC mark for this reason.

by mbesto

5/12/2026 at 10:23:25 PM

This is my issue with LiPo whatever flavor where they tell you it has a "runtime" of X minutes, yet you are strongly advised to only use 70%-80% of that value. It's worse than hard drives using 1000 vs 1024.

by dylan604

5/12/2026 at 10:42:50 PM

I've found the difference in runtime between similarly-priced low-end units with similar power rating is hour+ (LifePO4 power station) vs not advertised but actually just minutes (lead acid UPS). And you can spend a bit more on the LifePO4 power station and get a proportional increase in runtime and power, vs. the lead acid UPS where the cost would quickly become prohibitive. And the LifePO4 power station gives you the choice to cut off above 0% or not, where the lead acid unit doesn't give you any control. So you can trade off 30% of your capacity for increased longevity if you choose and still come out way way ahead on runtime. Or you can not and still have much better battery longevity than lead acid. You can choose a spot on a Pareto frontier that lead acid can't even approach.

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.

by scottlamb

5/12/2026 at 11:33:10 PM

Both you and @mbesto here are persuading me to let go of my long held boat anchors based on LiPo tech is a bad fit for deep cycle battery use. I have several expensive SLAs that I have used with an inverter to get power remotely. Replacing that with a lighter/better battery chemistry is something I'd be willing to trade. I guess I need to quit being so curmudgeonly about the new batteries. I bet there's some with similar thinking I can unload these SLAs and recoup some to spend on the new batteries. :thinking-face:

by dylan604

5/13/2026 at 12:06:28 AM

You're reminding me that I have a couple conventional UPSs sitting in the garage. I should unload them before convincing everyone they're junk!

by scottlamb

5/12/2026 at 10:34:56 PM

I have most certainly used 100% of the runtime. So you're more than welcome to do so as well, you might just have to replace it in 8 years instead of 10. YMMV.

Could be worse - could be lead acid and weigh 2x as much and you only get half the Ah.

by mbesto

5/12/2026 at 11:48:26 PM

>It's worse than hard drives using 1000 vs 1024.

So like SSDs? If you fill them up near 100%, performance tanks.

by gruez

5/12/2026 at 8:45:01 PM

Yes, thousands of times, an order of magnitude improvement over lead acid. And the increased capacity means that they're much less likely to hit 0% (or whatever defined cut-off you set) during a typical outage anyway.

by scottlamb

5/12/2026 at 7:12:47 PM

The capacitors in your PSU's rectifier have to float through 8.333ms interruptions every. single. cycle.

20 milliseconds is barely distinguishable from a single 60 Hz sine wave period. 10 milliseconds just over half a cycle.

by LeifCarrotson

5/12/2026 at 7:52:26 PM

> The capacitors in your PSU's rectifier have to float through 8.333ms interruptions every. single. cycle.

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.

by Aurornis

5/13/2026 at 2:59:54 AM

> The capacitors in your PSU's rectifier have to float through 8.333ms interruptions every. single. cycle.

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.

by Dylan16807

5/12/2026 at 7:23:14 PM

> 20 milliseconds is barely distinguishable from a single 60 Hz sine wave period.

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.

by scottlamb

5/13/2026 at 3:06:43 AM

How is that supposed to improve efficiency? Also that change sucks, it's cutting the margins way too tight.

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.

by Dylan16807

5/12/2026 at 4:50:59 PM

Testing the output of some UPSs from around the office. Checking out the results and finding avenues for further exploration.

by LabsLucas

5/12/2026 at 6:42:46 PM

Please just buy a pair of mains voltage diff probes. They're not expensive (around $500 each new, much less used) and they will eliminate the crazy connection scheme and give you true input -> output fidelity.

by exmadscientist

5/12/2026 at 8:00:40 PM

Please spend $300 on a differential probe https://www.micsig.com/DPA/

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.

by Aurornis

5/12/2026 at 8:41:00 PM

Great to see LTT in this space, you're well positioned for it (access to a variety of hardware.) Would love to see a more developed experiment design.

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?

by elevation

5/12/2026 at 8:08:45 PM

Please just buy a proper differential probe for stuff like this, you definitely don't need the R&SRT-ZHD mentioned in the article. Otherwise loved the article btw.

by zorgmonkey

5/12/2026 at 6:22:21 PM

its a shame that we don't have mainstream dc ups standards (telcos are their own niche). its kinda silly to generate fancy sinewave, manage transitions, and maintain phase of ac just to get immediately converted to dc.

by zokier

5/12/2026 at 7:17:38 PM

There's not much to standardize, basically just pick a plug shape for your desired voltage and current, it's really about building enough desire for manufacturers to take interest.

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

by Dylan16807

5/13/2026 at 6:24:59 AM

How much does it cost and where to buy? If I have to call, I cannot afford it.

by bombela

5/13/2026 at 8:03:04 AM

Okay so click on the first link I gave you.

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".

by Dylan16807

5/12/2026 at 7:01:31 PM

Issue is mostly lack of standard dc power distribution standards - outside of old telco ones anyway.

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.

by lazide

5/12/2026 at 9:28:59 PM

You actually can run most ATX power adapters directly from 96V DC! You can get that by connecting two 48V batteries in series.

They do rectification as the first step anyway, and then they use high-frequency switching that works just fine with the DC current.

by cyberax

5/12/2026 at 10:21:12 PM

The crossover distortion seen here suggests an analog Class-B output stage and that surprises me, because a digital output stage would be much more efficient. Class-D in other words. I've built digital inverters using IGBTs that produced an output sinusoidal power wave with lower distortion than the mains power. Granted these were one-offs and probably not cheap enough for production, but modern IGBTs and MOSFETS should be cheap enough nowadays that medium-priced UPSes could just use Class-D as the default solution.

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.

by dreamcompiler

5/12/2026 at 10:46:07 PM

Phase-shifted full bridge is the way to go. (It might have another name in this area of power electronics, these things do have lots of names....)

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!

by exmadscientist

5/12/2026 at 10:57:10 PM

Also I probably should have actually addressed your Class B comment:

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.

by exmadscientist

5/13/2026 at 2:58:26 AM

I don't remember the details on mine but I might have had the luxury of letting the entire output voltage be positive. Hence no crossover distortion.

by dreamcompiler