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Testing UPS Output Waveforms

lttlabs.com

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Re: Testing UPS Output Waveforms

#61

Post by dylan604 »

scottlamb wrote: post #52
dylan604 wrote: post #49

Earlier quoted context omitted.

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.

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 t…

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:

Re: Testing UPS Output Waveforms

#62

Post by gruez »

dylan604 wrote: post #49
mbesto wrote: post #41

Earlier quoted context omitted.

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.

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.

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

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

Re: Testing UPS Output Waveforms

#63

Post by scottlamb »

dylan604 wrote: post #61
scottlamb wrote: post #52

Earlier quoted context omitted.

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 t…

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 sim…

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!

Re: Testing UPS Output Waveforms

#64

Post by SigmundA »

Aurornis wrote: post #29
SigmundA wrote: post #24

Earlier quoted context omitted.

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

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 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

Re: Testing UPS Output Waveforms

#65

Post by Aurornis »

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…

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.

Re: Testing UPS Output Waveforms

#66

Post by Dylan16807 »

exmadscientist wrote: post #56
cyberax wrote: post #43

Earlier quoted context omitted.

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 m…

> 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).

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.

Re: Testing UPS Output Waveforms

#67

Post by Dylan16807 »

dylan604 wrote: post #25
mbesto wrote: post #6

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.

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 overload any components even if it keeps running at a really low voltage.

Re: Testing UPS Output Waveforms

#68

Post by dreamcompiler »

exmadscientist wrote: post #58
dreamcompiler wrote: post #48

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 sho…

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 sim…

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.

Re: Testing UPS Output Waveforms

#69

Post by Dylan16807 »

LeifCarrotson wrote: post #12
scottlamb wrote: post #10

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…

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.

> 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.

Re: Testing UPS Output Waveforms

#70

Post by Dylan16807 »

scottlamb wrote: post #16
LeifCarrotson wrote: post #12

Earlier quoted context omitted.

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.

> 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 lim…

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.

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