11 ms·

Testing UPS Output Waveforms

72 points by LabsLucas 5mo ago

LabsLucas 5mo ago
Testing the output of some UPSs from around the office. Checking out the results and finding avenues for further exploration.
- exmadscientist 5mo 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.
- Aurornis 5mo ago
  Please spend $300 on a differential probe https://www.micsig.com/DPA/ 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 5mo 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 5mo 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 5mo 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.
- lazide 5mo 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.
- Dylan16807 5mo 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://www.powerstream.com/DC_PC.htm https://synoceantech.com/index.php?page=lotinfo&lot=36 https://synoceantech.com/index.php?page=lotinfo&lot=36
  - bombela 5mo ago
    How much does it cost and where to buy? If I have to call, I cannot afford it.
    - Dylan16807 5mo ago
      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".
      - bombela 5mo ago
teraflop 5mo 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 5mo 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!)
  - Aurornis 5mo 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.
- exmadscientist 5mo 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.)
mbesto 5mo 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.
- lazide 5mo 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 5mo ago
  Capacitive touch screens when plugged in with cheap chargers to crappy waveforms, behave weird.
- dylan604 5mo 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.
  - exmadscientist 5mo 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.
  - Dylan16807 5mo 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.
chimpontherun 5mo 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
- scottlamb 5mo 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 5mo 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.
- dylan604 5mo ago
  > Evidently, AC waveform generation is extremely power-hungry Evidence is the heat from that conversion
- tredre3 5mo 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!
scottlamb 5mo 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 5mo 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.
  - scottlamb 5mo 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-supply-units/atx-30-vs-atx-31-whats-the-difference/ https://www.corsair.com/us/en/explorer/diy-builder/power-sup... I haven't dug through the spec itself.
    - Dylan16807 5mo ago
      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.
Aurornis 5mo 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.
- SigmundA 5mo 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 https://www.tiepie.com/en/usb-oscilloscope/handyprobe-hp3
  - Aurornis 5mo 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.
    - SigmundA 5mo ago
      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 https://www.tiepie.com/en/usb-oscilloscope/handyscope-tp450
dreamcompiler 5mo 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.
- exmadscientist 5mo 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!
- exmadscientist 5mo 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.
xenadu02 5mo 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!
- butvacuum 5mo 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 5mo 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).
butvacuum 5mo 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.

72 points by LabsLucas 5mo ago

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

exmadscientist 5mo 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.
Aurornis 5mo ago
Please spend $300 on a differential probe https://www.micsig.com/DPA/ 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 5mo 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 5mo 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 5mo 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.

lazide 5mo 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.
Dylan16807 5mo 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://www.powerstream.com/DC_PC.htm https://synoceantech.com/index.php?page=lotinfo&lot=36 https://synoceantech.com/index.php?page=lotinfo&lot=36
- bombela 5mo ago
  How much does it cost and where to buy? If I have to call, I cannot afford it.
  - Dylan16807 5mo ago
    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".
    - bombela 5mo ago

teraflop 5mo 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 5mo 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!)
- Aurornis 5mo 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.
exmadscientist 5mo 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.)

mbesto 5mo 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.

lazide 5mo 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 5mo ago
Capacitive touch screens when plugged in with cheap chargers to crappy waveforms, behave weird.
dylan604 5mo 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.
- exmadscientist 5mo 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.
- Dylan16807 5mo 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.

chimpontherun 5mo 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

scottlamb 5mo 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 5mo 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.
dylan604 5mo ago
> Evidently, AC waveform generation is extremely power-hungry Evidence is the heat from that conversion
tredre3 5mo 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!

scottlamb 5mo 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 5mo 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.
- scottlamb 5mo 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-supply-units/atx-30-vs-atx-31-whats-the-difference/ https://www.corsair.com/us/en/explorer/diy-builder/power-sup... I haven't dug through the spec itself.
  - Dylan16807 5mo ago
    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.

Aurornis 5mo 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.

SigmundA 5mo 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 https://www.tiepie.com/en/usb-oscilloscope/handyprobe-hp3
- Aurornis 5mo 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.
  - SigmundA 5mo ago
    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 https://www.tiepie.com/en/usb-oscilloscope/handyscope-tp450

dreamcompiler 5mo 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.

exmadscientist 5mo 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!
exmadscientist 5mo 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.

xenadu02 5mo 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!

butvacuum 5mo 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 5mo 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).

butvacuum 5mo 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.