PC SMPS Fix for UPS Switchover


One of the side effects of this harsh summer is frequent and inexplicable power cuts. It is almost as if the power company is running a disco-lights algorithm with the entire city, where the power goes off in one area after another, circling the city in a matter of few minutes…

The biggest casualty in all this is one of the PC (Pentium-Dual core), which reboots with each power cut, as the UPS switchover takes place. I couldn’t figure this. I have a 2KVA UPS, powering the entire house (yes, it can actually run the Microwave or the Food-processor). My other PC doesn’t reboot. So evidently it had something to do with the power supply of this PC, which tripped up the system every time there is a UPS switchover. I knew changing the UPS with another UPS or even changing the relay in the UPS would solve my problem. But those were complex and expensive solutions to the problem.

So, Did a lot of searching on the web, and found very little useful information. However, this is what I found out. The culprit in this case is low ‘hold up time’, which means, in case the mains power is lost for a few cycles (milliseconds), then the power supply ceases.

The ideal ‘hold up time’ is 20ms or more. Apparently, this SMPS needed an increase in hold up time. My thinking was, there must be a way for me to plug in a capacitor some place to tide things over. So, i pulled out th SMPS out of the system, and opened it up (DON’T TRY THIS IF YOU DON’T KNOW WHAT YOU ARE DOING).

SMPS had two large capacitors of 330uf/200V. I did more search, and found these are called the ‘input capacitors’. Basically a high voltage capacitor which is a store house of some power, in case the power flow isn’t of good quality. I could immediately figure my problems will be over if i changed these capacitors to higher value.

However, I couldn’t figure what would be the right value. Did a lot of searching, but it would be very difficult to calculate the correct value for the capcitor, unless I can accurately calculate the power consumed by the system.

But, i decided to take a chance. I went to market and got two 1000uf/200V capacitors and changed the two in the SMPS circuit. It was a bit of a squeeze to fit them in as they are large in size. Plugged this PS back in to the system. The system was working fine.

Now, went and tripped up the power a few times intentionally to see if UPS switchover will cause the PC to reboot. It isn’t rebooting now!

The solution cost me just Rs.80 and my PC is rock steady now.

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