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My computer (sometimes) shuts down for a fraction of second and then restarts at time of power outage. The most weird thing happening is that when I switch off the electric supply of UPS (ie. where UPS is connected to socket), it back up the computer normally, I mean without any re-boot happening. But when there is power outage the computer re-start.

I have the best available UPS (I have just replaced the battery), connected to my IBM Think Centre desktop. Recently I got computer SMPS (power supply) replaced. Can that be the reason of this problem ? because this issue has surfaced only after I got the power supply replaced. My previous SMPS was 230 Watts (which got fried due to power surge) the new one is of a company called CIL Odyessy (ATX) 450 watts.

Do you think the new SMPS is not doing what it should have done at time of power outage. I read somewhere that SMPS have capacitors which provide power back up to CPU for milliseconds (like 1-2 ms) till the UPS takes over. Can that be the cause ?

Please note at time of power outage, out of 10 re-starting happens 7 times.

What is the reason behind this strange problem and how can I solve it ?

I have tried changing the UPS, connected UPS to other points in house, plugged CPU to different points behind UPS, isolated other equipments from UPS (like directly plugged TFT monitor, speaker to main lines so that CPU gets dedicated power) and done many other things, but no success !!

Thanking you all in advance.

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What is the model of your UPS. Is your UPS line-interactive? –  Zoredache Jun 20 '12 at 0:01
    
tried UPS with some other system? –  tumchaaditya Jun 20 '12 at 9:00

2 Answers 2

up vote 1 down vote accepted

I would guess that your power "outages" are not causing power to be suddenly cut to the UPS but instead involve an initial "sag" of the power that is severe enough to trigger the computer's power supply sensing of power lost, but is not severe enough to cause the UPS to kick in. This could indeed be due in part to changing computer power supplies, with the new one having a slightly different threshold.

It may be that there is a setting for the UPS to tell it to kick in sooner. Otherwise, you should probably replace the UPS with one that will more reliably trigger.

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Do you think the SMPS is not doing what it should have done at time of power outage. I read somewhere that SMPS have capacitors which provide power back up to CPU for milliseconds (like 1-2 ms) till the UPS takes over. Can that be the cause for further evaluation ? –  arayman Jun 19 '12 at 20:49
    
You could get a wattmeter to hook in between the UPS and your computer, which might show the power dip. –  Bryce Jun 19 '12 at 22:39
    
Yes, it could be that the replacement PS does not have the same capacity to handle dips of a given time duration that the old one did (though normally one with a higher wattage would be expected to do better). (Note that all power supplies must be capable of handling a "dip" of about 1/120 second, in order to sufficiently filter the 60Hz power line.) –  Daniel R Hicks Jun 20 '12 at 11:33
    
Also note that the original power surge may have damaged the UPS. –  Daniel R Hicks Jun 20 '12 at 11:43
    
Thanks a lot DanH for taking out time for answering to my question. Your feedback really helped me to move in right direction. I will surely change the power supply and try to find a quality equivalent or better than the original one. :) –  arayman Jun 20 '12 at 20:05

If the new PSU is not rated at something like 65-250 V then it is sensitive to undervoltage. You should either change the PSU (which you might not want to as you say it is rather new) or reconfigure the UPS to kick in at a lower (RMS) voltage.

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