Sine Wave and Square Wave Uninterruptable Power Supply (UPS)

By Lawrence

iStock_000013014656XSmallLast time when I wrote about UPS, I left out one of the more technical but important aspect of UPS equipment. I guess I should fill in the blanks here in case some of you find it confusing why some UPS are significantly more expensive than the others even though the supposed functionalities are about the same. There is such a big difference in price because there are 2 classes of UPS and they are built with completely different technologies.

Some Background on AC Electricity

The power we get from the power outlet is called AC (alternating current). They are provided by the local electricity company. As we already know, the electricity coming from the power outlet can be problematic at times hence the need of surge protector and UPS. The electricity we get from the wall outlet is in a waveform called sine wave.

In case you wonder what the 50Hz and 60Hz rating on the power outlet means, that describe the sine wave frequency of the current available in your country.

Sine wave is a characteristics of AC power that many electronic equipment are built to depend on. Electricity flowing into these devices are best to be in sine wave. One of these devices is the transformer in your computer. These transformers change the AC power into DC power to support a computer. Higher end transformers needed to power computers with high-end processors and graphic cards must be supported by AC in sine wave.

Power From Battery Are Not In Sine Wave

The battery used in all UPS can only provide direct output of electricity. That means it is a constant flow of electricity in a straight line. This is fine if the battery is hook up to equipment expecting DC (direct current). But this does not work at all if we are talking about powering equipment that are supposedly to be powered by AC.

Solution to this problem leads to two distinct types of UPS.

Square Wave UPS

The cheapest way to emulate AC sine wave from a battery is to produce a square wave of same frequency. Very simple circuitry will do the job. As long as the equipment does not need true sine wave AC power, square wave will do the job. It is a quick and dirty solution.

UPS built to provide square wave output from the backup battery are cheaper because of this simpler design and hardware cost.

The problem is that sensitive electronic equipment overtime will be damaged by the square wave electricity even if they can function under the square wave current. It is like feeding your car with diesel but that your car engine is not built for that.

Long term damage includes shortened life span of your computer equipment.

Sine Wave UPS

The more expensive way to build UPS is that they can reverse the DC power into true AC first. Then feeding the AC to the equipment connected to the UPS. This way, the sensitive equipment would not be impacted at all due to the change in power source. In fact, the artificially generated AC power by a good UPS has cleaner sine wave AC for sensitive electronic equipment which can potentially prolong the life of the equipment themselves.

Since there is not much breakthrough in technologies related to AC power generation for many years, the components necessary for AC generation from DC power is still very costly. As a result, sine wave UPS are more expensive.

General Guideline

Backup power supply UPS are likely square wave. Online power supply are likely sine wave. To find out for sure, you can check the specification of the UPS to make sure. It becomes important if you are protecting high-end computer equipment.

As of 2013, most mid to low range computers do not require the high-end UPS as backup power. As long as a decent transformer is used within your computer, they can likely take the impact of square wave power during the occasional power outage. Those equipment depending on a small DC adaptor are also pretty safe with basic UPS.

I know computers with i7 (and higher end models) using the latest power hungry graphic cards do not function properly with square wave UPS. If you have these high-end computers, it is best you shop for a matching high-end UPS.

Put it nicely, these computers are "high-maintenance".


  • MidKnight December 22, 2013 at 7:19 pm

    I bought an APC UPS just a few months ago. I didn’t buy the “smart” due to the cost. Ended up getting the 900V backup UPS PRO. In the specs they say the waveform is “Stepped approximation to a sinewave”. So does that mean it is part square?

    • Lawrence Chan December 22, 2013 at 8:00 pm

      It is an in-between of a true sine wave and square wave.

      Square wave jumps from one extreme to the other, no in-between steps.

      The stepped version adds multiple steps there to make it look more like the sine wave. Just 10 steps or so going from one extreme to the other side already make a big difference and likely to work with the more sensitive equipment.

  • smfb6 March 20, 2014 at 5:39 pm

    Lawrence is correct in the different types of UPS’ available on the market. Most computers I’ve had to put on UPS didn’t mind the square wave ones except for a certain series of Dell desktops I have come across in my previous positions.

    Another, and more technical way to back up the AC power from the wall is to use only batteries for your computer. After all, most electronic equipment only ever uses (regulated and smoothed) DC in differing voltages; meaning electronics only likes DC to operate. They will fail to run or go up in smoke if AC is applied to the power rails. Line voltage AC is for hair dryers and toasters.

    Most consumer electronics steps down the line (wall outlet) AC voltage to common voltages of 6,14, 26… Volts-AC rms (Root Means Squared: effective voltage). After that, it gets rectified, smoothed (optional) and regulated to meet specific operating voltages within the device. All this takes place in the ‘power supply’ of the device. And since the power consumed in any electronic device is proportional to the size of the power supply, smaller devices will usually a power supply that may be larger than the actual device itself (because there’s a minimum size any transformer & associated rectifiers can be to be effective). And this leads me to my beef with why a regular power bar cannot accept some power ‘plugs’ due to the odd sizing. Hence a new class of power bars on the market with additional receptacle spacing and fancy schmacy surge and line protections on them to help lighten the load in your wallet.

    In essence, it is up to the power supply to regulate the different DC voltages in your computer so there’s no need to re-invent the wheel. All one needs to do is to isolate and by-pass the secondary stage of the main transformer, rectifiers, and attach the your 12V DC to the main power rails. Of course, your 12V-DC supply should be clean power directly from the DC batteries of any inexpensive UPS. While there are battery cell capacity / computer load types to consider, it is relatively easy for any electronic technician to develop and implement.

    Keep in mind that a ‘battery’ is a collection of multiple cells. I.e. a lead acid (PbsO4) car battery consists of 8 cells of 1.5V each for combined voltage of 12V nominal whereas some UPS batteries are 6V nominal, so they would need to be wired in series (2x6V=12V) to obtain the proper voltage compatible for a computer at 12V.

    Also keep in mind that most UPS batteries (because they’re of a lead acid variant; gel type) lasts on average of about four (4) years from the date of manufacture (the batteries, not the UPS), in which time, they tend to degrade to the point of being non-usable and may cause your computer to behave erractically. So the amount of time the batteries sit on the shelf is also a factor of longevity.

    I’ve done this to a prior computer using an APC UPS but the batteries died (as expected) and I was too cheap to buy replacement batteries, weighing the expense with the amount of times I actually needed the UPS (which was none) at the time. All in all, it served me well for a few years.

    I’m just saying. :=)

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