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.

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