My 10 months old PC ran into a problem some days before. My system specs are as follows:

  • Intel Core i7-8700 Processor
  • MSI Z370-A PRO Motherboard
  • 8GB RAM

Here are the details about the issue.

  1. Most of the times, the system restarted instead of shutting down while there was no power from the mains. In other words, while the UPS (Numeric Digital 600EX-V) was on battery.

  2. Sometimes it was unable to turn the system while in the same situation as the above point.

  3. I was able to turn the system on if the second situation didn't occur or if I connect the power cable directly to the source without using the UPS. In that situation, the system randomly restarted while using. This was quite rare and hard to reproduce.

Note: It is not a software issue. I checked it in multiple OSs.

Because the 3rd situation happened regardless of whether it was connected to the UPS or not, I just concluded that either a part other than the UPS or both the part and the UPS is causing the problem.

I wasn't wrong. After a lot of trouble, I could find out that the PSU (Cooler Master MWE 450) was faulty. After replacing it, all three issues are gone.

But I am still confused about one thing: Because 1st and 2nd situations happened, It is clear that the UPS output is different while in normal and backup mode. Is it normal? If not, isn't the UPS faulty too and is it the one which damaged the PSU?

  • When you use the phrase, "Backup mode", you are talking about a situation where you are powering the devices while on battery (i.e. you have lost main power). – Ramhound Dec 6 at 19:15
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    Yes, you are right. – Anees Dec 6 at 19:24
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    That's a 600VA UPS. What does your system draw? It could easily be more. Note that I don't mean "what's its power supply rated to?" but really what does it use in practice, counting all devices attached to the UPS (presuambly monitors, but hopefully no laser printers!). You can guesstimate using this tool if you can't measure it. – Chris H Dec 7 at 10:42
  • It's quite plausible that the UPS can deliver more current from mains than it can when supplying from battery. If you exceed the current that the UPS can supply the voltage will drop one way or another. (Ideally the voltage drops straight to zero in such a case.) – kasperd Dec 7 at 16:25
up vote 10 down vote accepted

I think that only a qualified technician can test the UPS and say whether it is faulty or not. It is always possible, if it is not new, that its battery has weakened over time.

Otherwise, the only way you will ever know is if your new PSU also starts misbehaving.

  • Thank you very much. Do you have any idea about "Should the UPS output be exactly same in normal and backup modes ? Or it is normal to have a slight variation in voltage or something?" ? – Anees Dec 6 at 18:39
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    Ideally they should be the same, but it all depends on the UPS in question. No UPS operates exactly within its parameters. For example, only the best ones really deliver the promised number of watts, others can lack as much as 50 watts. In short, I distrust the specifications of any UPS, as this is not precision equipment. – harrymc Dec 6 at 18:46
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    The UPS you mentioned will have a plus-or-minus 10% variance in output voltage level when compared to mains current and provides a square wave instead of a sine wave. Either can be problematic for some computers. – K7AAY Dec 7 at 1:00

Should the UPS output be exactly same in normal and battery backup modes ?

Of course the AC line power from the electrical utility is true sinusoidal with an accurate frequency of 50 or 60 Hertz.
The typical (standby and line-interactive) UPS will pass through the line power when in normal mode.

Typically only a high-quality UPS will output true sinusoidal AC power when in battery mode.
A UPS of lesser quality will convert the DC battery power to a simulated sinusoidal AC wave.
A UPS of low quality will convert the DC battery power to just a square AC wave.

The typical power supply unit of a PC should be able to cope with low-quality AC waveform such as simulated sinusoidal or square wave.
Or maybe not; YMMV.
See When do I need a pure sine wave UPS? and Sinewave vs Simulated Sinwave - Which is Best? .

Interestingly the web page for your UPS does not bother to specify the type of output that it produces in battery mode. That could be an indicator that it does not produce a true sine wave, and therefore the answer to your question would be "no".

  • Pretty clear. So i guess my UPS functions normal. Thanks – Anees Dec 6 at 20:28
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    This is basically the answer I was about to post. I would add that there are several plausible reasons why a marginal, or defective, PC power supply might have more trouble with a low-quality AC waveform than a clean sine wave. For instance, a noisy waveform puts more strain on all the components on the high-voltage side of the main transformer. – zwol Dec 6 at 21:32

As mentioned by @sawdust in his answer, it could be that your computer power supply was a PFC (Power Factor Correction) one which requires a truer sine-wave UPS to function. These PFC power supplies are higher efficiency, and are getting more common, especially in higher power supplies in gaming rigs, etc. These power supplies are known to shut down if they don't get sufficiently sine-wave voltage. The computer power supply that you listed is a PFC model, and if your UPS is a lesser-cost one that isn't producing a true-enough sine wave, that could likely be your problem, and possibly the cause of death of your old power supply. To be safe, if you have a PFC power supply, you need to make sure your UPS is one that's listed as PFC compatible.

When I hear that a computer reboots or restarts when on backup power, I always suspect that the power supply is low quality, overloaded, or failing.

When the power fails, the UPS has to start supplying power to the computer and disconnect the computer from the power line as quickly as possible. The power line doesn't typically fail open, it typically fails like a dead short as every device in your house is trying to draw power that isn't there. It's the job of the power supply in your PC to continue to supply power to the computer while the UPS is accomplishing these two feats.

This is a specific power supply requirement and good quality power supplies easily meet it. However, cheap power supplies, overloaded power supplies, or old power supplies whose capacitors have lost capacity are not able to maintain the proper hold up times. This results in a drop in their DC output voltage when the power goes out before the UPS can accomplish its magic.

Since replacing the power supply solved the problem, my bet is the UPS is fine. Though, of course, it's possible that the UPS is taking longer than it should to restore the AC supply to the power supply and the new supply is better able to handle the out-of-spec UPS.

My bet is it was the power supply and the UPS is fine. Interestingly, the MWE-450 claims a hold up time of only 14ms while a UPS may rely on the hold up time being 16.6s so technically, the configuration was not really guaranteed to work.

  • I am pretty sure that the hold up time isn't the problem because while the UPS is on the battery, the system is not just restarting at moment of main power loss but continues to restart again and again. If it was managed to operate, it will restart when I try to shut down. – Anees Dec 8 at 8:10

UPS models such as this one are called stand-by or "line interactive" models. The brochure for this one says that while it's on battery, it will output 230 V +/- 10%. While it's online, it will pass whatever the actual line voltage is; this may vary some amount more or less than 230 V as well. So conceivably, if your line voltage is 225 V at a given moment, and the power dies or you unplug it, the UPS could switch onto battery and immediately start putting out, say, 240V. Or vice versa. And this really shouldn't matter to your PC.

But UPS's don't last forever. How old is this one? The switches which engage when it goes on and off battery mode may be rated for a surprisingly low number of cycles; if you have frequent power outages, or if you're in an office which does weekly switchover tests to generator, you can hit this limit in a year. And batteries can fail after a year or so, even if they've been used infrequently. We use a large number of small (1000 VA) UPS units where I work, and they frequently fail rather like this: they simply fail to power the load one day, with no advance warning about battery state or any other error.

Test the UPS by rigging up a purely resistive load (like incandescent lamps, or a heater on a low setting) amounting to some power in the middle of the UPS's range, say 100 to 400 Watts (since this is a 600 VA unit), and see if the UPS can power it for more than a few seconds. It could just be that the UPS is end-of-life, or needs a battery replacement.

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