Testing UPS Output Waveforms • Hacker News Board

Re: Testing UPS Output Waveforms

#81

Post by butvacuum » Wed, May 13, 2026, 11:15 AM UTC

exmadscientist wrote: post #73

cyberax wrote: post #71

Earlier quoted context omitted.

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.

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…

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...

Re: Testing UPS Output Waveforms

#82

Post by felixgallo » Wed, May 13, 2026, 11:43 AM UTC

scottlamb wrote: post #50

Aurornis wrote: post #38

Earlier quoted context omitted.

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.

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

Music, medical, engineering. Wherever the waveform consistency and noise floor are the optimization point.

Re: Testing UPS Output Waveforms

#83

Post by ErroneousBosh » Wed, May 13, 2026, 1:22 PM UTC

chimpontherun wrote: post #9

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 con…

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.

Re: Testing UPS Output Waveforms

#84

Post by ErroneousBosh » Wed, May 13, 2026, 1:26 PM UTC

michaelt wrote: post #34

tredre3 wrote: post #28

Earlier quoted context omitted.

> 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 ac…

> 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)

Our UPSes at work are like that, but the smallest is the size of a sofa and the largest is the size of a minivan.

You can put them on street power, you can throw them over to the diesel genny, back onto a secondary genny, back onto street power, and the output won't even ripple.

Re: Testing UPS Output Waveforms

#85

Post by cwillu » Wed, May 13, 2026, 2:19 PM UTC

ErroneousBosh wrote: post #83

chimpontherun wrote: post #9

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 con…

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 pr…

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

Re: Testing UPS Output Waveforms

#86

Post by cwillu » Wed, May 13, 2026, 2:26 PM UTC

ak217 wrote: post #47

mikeyouse wrote: post #35

Earlier quoted context omitted.

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...

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…

Breakers are there to protect the in-wall wiring, not the connected equipment.

Re: Testing UPS Output Waveforms

#87

Post by cwillu » Wed, May 13, 2026, 2:28 PM UTC

Dylan16807 wrote: post #67

dylan604 wrote: post #25

Earlier quoted context omitted.

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.

> 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 overl…

Low voltage requires higher current to maintain a given output power.

Re: Testing UPS Output Waveforms

#88

Post by dylan604 » Wed, May 13, 2026, 3:29 PM UTC

Dylan16807 wrote: post #77

dylan604 wrote: post #75

Earlier quoted context omitted.

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 rea…

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…

Unfortunately, there are a lot more things in the world that need a power supply than a PC. Sorry if my use of PSU unintentionally narrowed the focus, just faster to type. The power supply for these high end video machines were not small, nor come with a cheap price tag. I would not have expected them to be made poorly as they are specialty units designed to run precision analog electronics. That would be comparable to expecting nylon seats in a Ferrari.

From all of that, I have learned the lessons of how dips can ruin electronic equipment (even if not the exact why back then), so for me and my house all electronics are hooked up to a UPS or power conditioner. Appliances are on their own though as that's the landlords problem! Multiple times a week, I get noticeable dips where the lights visibly dim and I can hear all of the UPS units kick in and back to mains a few seconds later.

Re: Testing UPS Output Waveforms

#89

Post by labcomputer » Wed, May 13, 2026, 3:44 PM UTC

SigmundA wrote: post #64

Aurornis wrote: post #29

Earlier quoted context omitted.

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…

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…

The thing you posted isn't the same thing as a differential probe. It's a single input low bandwidth scope that floats at the common mode voltage, with an optically-isolated data output.

Now, let's say you want to probe two things at the same time (triggered by a common signal source). You can't. And the reason you can't is because the producer took the expedient of floating the entire scope, and there's no trigger input.

In other words, they took the cheap way out by not actually building a differential probe. Related to that, this thing doesn't appear to have a step attenuator, which is why the effective resolution depends on the volts/div setting of the input.

Also, they don't specify the CMRR, which is one the main figures of merit people look for on differential probes. Any capacitive coupling between the scope and ground is going to degrade CMRR. So who knows if it can actually measure anything useful.

You can buy a scope with multiple optically-isolated channels as well as a trigger input, but those end up costing as much as a differential probe. Because it turns out that achieving good CMRR when you have multiple inputs is as hard a problem as making a good differential probe.

This is not to say the product you linked shouldn't or can't be used for anything, but it is a niche product. That's probably why it is advertised as a "power quality monitor" and not an "oscilloscope".

Re: Testing UPS Output Waveforms

#90

Post by butvacuum » Wed, May 13, 2026, 3:50 PM UTC

xenadu02 wrote: post #54

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 w…

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.