The PC reset problem with Active PFC SMPS and UPS

I have a desktop machine with a Corsair SMPS which has active power factor correction. I had a Luminous 675 VA UPS before buying my new desktop machine with this SMPS. The cheapo power supplies available in India (which cost a fourth of the cost of branded ones like Corsair, etc) do not employ power factor correction.

When I was deciding on the configuration of my machine, I decided to buy the latest Intel i5 (i5-4670). The processor had been launched just around a month or two ago. I also bought Gigabyte motherboard and Corsair cabinet after recommendations from many geeky friends I know online who have built their own desktops. They also suggested me that since I was already spending a lot on the machine, I should definitely go for a branded SMPS in order to protect the components instead of going for a cheap one and risk the components.

I did not know that there were compatibility issues with UPSes and Active PFC power supplies. This came to light when my computer started restarting whenever there was a power changeover by the UPS because of high voltage in the input or a general power failure. Because this wasn’t happening for almost 1 year after I bought my computer, I thought something was wrong with my UPS. So I called up Luminous support and they sent an engineer for inspection. He found that the battery terminals had corroded. The Luminous UPS I have supports those big batteries (12V / 100 Ah is the battery I was using) and it claims to be sine wave UPS. He then cleaned them up and things seemed to be back to normal (they just seemed, also known as placebo effect :P). The problem returned back after a few days.

I even tried giving my SMPS for a warranty repair to be sure that the issue is with my UPS and not with the SMPS. Then I started researching about this. I found that this was a known problem (involving big brands like APC!) and the reason was certain PSUs expected pure sine wave at the input but the UPS available in market were outputting modified sine wave, and that’s apparently the oscillator circuit for which is far cheaper and easier to design compared to pure sine wave (Pure sine wave is what you get from the power supply company at homes). I still don’t know if the real cause of the PC getting reset during a changeover is because of the sine wave / square wave thing or it’s because the switch time of the UPS is higher. But if it was a delay, it should happen every time there is a changeover which wasn’t the case.

This clearly indicated that whenever the power waveform at the UPS had an unexpected form, the SMPS was cutting supply to my PC. During the research, I came to know about the kinds of UPS. There are basically two kinds of UPS, one is line interactive UPS and another is online UPS. The difference between the two is that a line interactive UPS will supply AC power directly from the power socket it is connected to as long as there is power and a relay like mechanism is used to switch to battery when there’s an input power failure whereas an online UPS supplies power from the battery all the time. Whenever there is input power available, it will charge the battery. There’s zero switch over time in case of online UPS, while for line-interactive UPS it’s 10-15 ms.

I asked my inverter vendor if he had online UPS and I got shocked when he told me the price for a 600 VA online UPS: ₹25000. That’s way too much for me. Line interactive UPS of the same size costs less than half of that price. So I started manually using the UPS in battery mode whenever I was working on my computer. But this problem needed a solution. So yesterday, I bought a second-hand UPS for ₹250 (yes, that cheap. New ones cost around ₹2000) which supported 600 VA load at output, but the charger inside it cannot charge big batteries. I decided to use my bigger line-interactive UPS as a charger for the battery while this thing will power my computer.

I just bought two wires for connecting the battery and this new UPS. I also added a fan to inside the new UPS’s enclosure to keep the transformer cool as I had the experience of cooking up the transformer during this experiment in another 15-year-old UPS I had. Now things are smooth. Apparently 12V battery chargers are available for ₹4000 on eBay. So why is an online UPS so expensive?! That remains a mystery to me.



14 thoughts on “The PC reset problem with Active PFC SMPS and UPS

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  1. This problem occurs quite often. Even if an APC Smart level UPS is used. All you need is to get a good PSU – Corsair RM1000i, Corsair AX850 (750), Corsair AX1200i, Enermax, Seasonic etc. Note on which platform these power supplies are made and what is the total capacitance of the capacitors they have.

  2. My PC, built an year back, has the similar issue I guess.

    1. PC restarts (like as if you’ve hit ‘reset’ button) randomly, with no predictable pattern. But apparently looks like voltage fluctuation is the reason.

    2. When power supply from the wall (utility power) stops, PC just works fine by taking power from the UPS battery. And when utility power reverts back, PC gets restarted. You’ll feel like while changing over from on-battery to utility power, PC is restarting.

    My PC build:
    Processor: Intel i5 4570
    Mobo: Intel DH87MC
    RAM: Corsair Vengeance DDR3 8 GB 1600(CMZ8GX3M1A1600C10)
    Graphics: Intel motherboard in-built (Intel HD Graphics 4600)
    HDD: WD Blue 1 TB 7200 rpm 64 MB cache (WD10EZEX)
    PSU: Corsair VS550
    Cabinet: Antec X1 T (ATX)
    Monitor: Philips 107C6 (17″ CRT)
    UPS: APC Back-UPS 600 VA (360 W)(BX600C-IN).
    OS: Win-7 64-bit

    I have my old UPS with me which is APC Back-UPS ES500, and I tried making a Daisy-chain connection, and PC worked just fine during power outages and at power returning. But the random restarts still there, which occurs randomly, as I earlier said.

    1. Find a PSU without active PFC correction. only that works. I’m using xpro after my corsair went kaput after many such incidents.

    2. I can tell you for sure – APC SUA1000I and SUA2200I and PSU like Enermax Platimax 1500, Corsair AX850, Corsair 1200i, Seasonic 750-850 W models got no problems, even if I’ll will start to on/off the circuit breaker without stopping. The uninterruptible power supply will go many times to the batteries and back, and the computer power supply will not notice this.

    3. That’s the catch here. My PC never used to reboot with the Luminous UPS I have when I manually triggered a power failure using the main circuit breaker.

    4. If you take such a bundle, as described above, then there will be no problems at all. As long as the batteries in the UPS are alive 🙂

    5. Hi, i think there are 2 possible issues …

      1) your UPS isn’t working properly .. (. ( more likely ) .. if the PC restarts “everytime” when the AC power returns .. then the problem is with UPS relays taking too much time to switch when it switches to AC mode from battery.. Try it with a different UPS.


      2) Your PSU primary capacitor is failing ( VS series uses cheap components + fluctuations puts a lot of strain on primary capacitor) so it is not holding enough charge to give you a long “hold up time” .. ( was it working fine when the PSU was new ? )

      IF the problem was with APFC / non-sinewave output .. then your PSU would have shut down/restarted the PC when it switched to battery mode .. but it works fine when the AC goes off and keeps on working alright on the Stepped/ Square sinewave output of UPS

      RMA your PSU ..

  3. Hi ,

    Thanks for this article , very very helpful 🙂

    Which PSU was that ? was it corsair VS450 ?

    So you connected your new ups ( 250rs ) battery input directly to Bigger inverter battery and running that ups( 250rs) on “battery mode” ? While the inverter ( luminous ) simultaneously charges those battery from mains ..?

    am i interpreting it correctly ? if yes then aren’t you putting more strain on your PSU this way ?
    because that ups is outputting square/stepped sine wave .. and Active PFC don’t like square sinewave , some PSU don’t even work at all with square sinewave , Some work while giving buzzing sound .. but regardless square/stepped sinewave does put a lot of strain on Active PFC .. Using it for a short while doesn’t do much harm but running it continuously ( for hours ) on that is risking it too much ?

    I hope you’ll reply 🙂


    1. Eventually the corsair vs450 died and I switched to a xPro 500W SMPS. Going by the weight of the box this doesn’t seem to be of the same quality as Corsair but at least now I don’t need the weirdo setup I’d made. It works directly using my Luminous UPS.

    2. Thanks for replying ..
      Your setup was amazing .. good thing is when VS450 died it didn’t kill your other components – a decent PSU 🙂

      “I still don’t know if the real cause of the PC getting reset during a changeover is because of the sine wave / square wave thing or it’s because the switch time of the UPS is higher. But if it was a delay, it should happen every time there is a changeover which wasn’t the case.”

      The switch time of your UPS was fine .. but the primary capacitor used in VS 450 is of cheap quality .. it looses it’s capacitance very fast over time .. So it was not able to provide you enough “hold up time” long enough for UPS to switch .. this is probably why you didn’t notice it for the first year because it had enough charge and was working fine

      RMA would have solved your problem

      The switch time of UPS is not constant , APC mentions it is 4ms -8ms , so if your PSU hold up time is 6ms .. Sometimes it will work when UPS switches in 4-5ms and sometimes it will not when it switches in 7-8ms … which i think was the case with you .. when the PSU was new it might have had a hold up time of 16-17ms during average load .. so it wasn’t giving problem then

      If the problem was with square wave incompatibility .. Then the PSU should not have worked at all and restarted/Shut down PSU “everytime” and not sometimes …

      Also, your Luminous UPS was a Pure sinewave , although these are not “truly pure sinewave” , but they are very close to pure sinewave … so there should’t have been a problem at all if it was waveform incompatibility as you were already feeding the PSU with close enough sinewave output , rather than Square/stepped wave .. Square/stepped wave on the other hand is very different and damaging , even tubelights will emit humming sound when they are connected to square wave..

      because i have read many people replies, that using VS450 with UPS and inverters working fine for them .. and in a 1-2 cases on amazon reviews , people said who had this prob was resolved by RMA

      Also generally most APFC power supply works fine with stepped square wave UPS .. The only thing required is higher VA UPS ..

      Hopefully it will be helpful to anybody reading this ..thanks for your amazing Hack/article .. 🙂

  4. Hi,
    I had the same problem till I changed my battery from 130AH to 175AH. Try 175AH battery your problem will be gone 🙂 I’ve simulated sinewave UPS and corsair VS550 psu, I’ve never seen PC restarting itself with main breakdown ever ! and yes I’m in Pakistan, the power goes out after every 1 hour.

    When I was using 130AH battery the PC restarted itself on random AC cut offs. Like it will not restart 10 times, but randomly after 10th or whatever 17th time it will restart itself.


    1. I have tried that, the giant battery you see there is a 150 AH tubular battery that can power a refrigerator. I don’t think a computer consumes more power than a refrigerator.

    1. Akshat, you don’t know this side of me since our chat interaction doesn’t involve this. I do all sorts of crazy things 😛

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