Testing UPS Output Waveforms

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

by LabsLucas • 2 months ago

72 100 2 months ago

LabsLucas• 2 months ago

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

exmadscientist10• 2 months ago

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.

Aurornis7• 2 months ago

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.

zorgmonkey• 2 months ago

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.

elevation• 2 months ago

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?

zokier• 2 months ago

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.

lazide2• 2 months ago

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.

Dylan168079• 2 months ago

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

bombela2• 2 months ago

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

cyberax4• 2 months ago

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.

teraflop• 2 months ago

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.

hex4def6• 2 months ago

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!)

Aurornis7• 2 months ago

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.

exmadscientist10• 2 months ago

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.)

mbesto3• 2 months ago

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.

lazide2• 2 months ago

‘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.

markus92• 2 months ago

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

dylan6047• 2 months ago

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.

exmadscientist10• 2 months ago

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.

Dylan168079• 2 months ago

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.

toast0• 2 months ago

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.

bob1029• 2 months ago

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.

mikeyouse• 2 months ago

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...

alnwlsn• 2 months ago

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.

ak217• 2 months ago

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.

garaetjjte• 2 months ago

Could it be that it directly uses AC for flame rectification and gets confused with ugly waveform?

krunck• 2 months ago

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

quickthrowman• 2 months ago

I had a weird situation with a 480V three-phase 250kW generator, it passed an 80% load bank test (the load is resistive heat) with flying colors, almost rock solid 60 hz frequency with smooth operation.

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.

chimpontherun• 2 months ago

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

scottlamb8• 2 months ago

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.

nomel• 2 months ago

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.

dylan6047• 2 months ago

Evidently, AC waveform generation is extremely power-hungry

Evidence is the heat from that conversion

tredre3• 2 months ago

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!

michaelt• 2 months ago

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)

butvacuum5• 2 months ago

it's even worse: SLA's can only discharge to ~half their nameplate capacity or so- and if you're pulling tons of amps, it's even less. LiFePo4 otoh you'll get the nameplate capacity.

Then theres trivialities like LFP's voltage being higher so there's less resistive losses, etc

aspbee555• 2 months ago

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

cyberax4• 2 months ago

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.

exmadscientist10• 2 months ago

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).

dreamcompiler4• 2 months ago

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.

exmadscientist10• 2 months ago

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.

bb88• 2 months ago

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.

butvacuum5• 2 months ago

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

ErroneousBosh2• 2 months ago

I've been looking at Victron gear for an off-grid power system up north (initially for a caravan, will get migrated across to a house once the upgrading and repairing is done).

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.

cwillu3• 2 months ago

Yeah, 90vdc is an immediate nope from me: way too easy to suffer a painful death.

scottlamb8• 2 months ago

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.

LeifCarrotson• 2 months ago

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.

scottlamb8• 2 months ago

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.

Aurornis7• 2 months ago

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.

Dylan168079• 2 months ago

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.

dylan6047• 2 months ago

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

scottlamb8• 2 months ago

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.

mbesto3• 2 months ago

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.

Aurornis7• 2 months ago

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.

SigmundA4• 2 months ago

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

Aurornis7• 2 months ago

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.

dreamcompiler4• 2 months ago

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.

exmadscientist10• 2 months ago

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!

exmadscientist10• 2 months ago

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.

dreamcompiler4• 2 months ago

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.

xenadu02• 2 months ago

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!

butvacuum5• 2 months ago

LiFePO4 generally has a 0.5 or 1C discharge. You'd end up with a 500W UPS that could run for 10 hours, or 48v+ battery packs.

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.

Saris• 2 months ago

Depends on the type of LiFePo4 cells, ones designed for higher power can easily do 5-10C or more. But that's not needed for a UPS. Most common high energy cells can handle 3C.

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).

butvacuum5• 2 months ago

I notice nobody- OP/tfa included, is mentioning that that 30ms transfer is 10-20ms longer miliseconds longer than an ATX3.0/3.1 PSU's hold-up time.

Testing UPS Output Waveforms

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