How does a UPS (Uninterruptible Power Supply) work?

Almost everyone who uses a computer knows what a UPS is and its purpose for those who don’t; in simple words, a UPS (uninterrupted Power Supply) serves as a backup power system in case of a power failure. They are used for protecting sensitive devices which need stable, continuous power, such as PCs, networking systems, workstations, and extensive company servers.

Now a UPS power back is not the same as a generator; it is only meant to provide support for 20-30 mins until the auxiliary power kicks in.

There are many kinds of UPS currently in the market, but they can all be classified into three main types based on how they supply power to the devices from the mains and their batteries.

Offline/Standby UPS
This is the entry-level setup, with a more linear design, best used for less sensitive devices like fans, light, TVs, etc. These kinds of UPS use the power from the mains as it is and only tap into the battery when there is a power loss, surges, or slight voltage fluctuations. This type is perfect for users whose incoming utility power is stable and has the same voltage as the devices connected because offline UPSs cannot actively manage voltage; instead, they resort to their batteries as soon as a voltage variation is detected.

These UPS have two paths; one is from the AC main to the output, the other from the battery to the output. Even a slight voltage disturbance will trigger the UPS to switch to the battery DC then convert it to AC for the output. Offline/Standby UPS handles voltage fluctuations by switching to the battery, meaning there will be more frequent erratic switches. The transfer time can be as high as 6-25 milliseconds, depending on how new the model is. Recent models have a power-saving backup ups mode which decreases the switching time by a few milliseconds but shortens the voltage range.

Line-interactive UPS
Line-interactive UPS have an automatic voltage regulation(AVR) system which can handle a wide range of voltage fluctuations without switching to battery backup. The system can detect when the voltage hits a preset burnout(low voltage) or swell(high voltage) value and uses a transformer to either boost or decrease the voltage depending on the output configuration. They only need to switch to battery when there is a power loss and provide stable output with 8-15% variation in input or 145-290V depending on the model.

The autotransformers can be programmed to handle any range depending on the use; the wider the range, the more time it takes to switch to battery backup in case of a power failure, but it typically takes between 2- 6 milliseconds. Chances are the one backing your PC is a line-interactive UPS.

Online/Double-Conversion UPS
This is the most expensive of all three, primarily used to back up critical commercial-grade equipment that runs 24/7 where even a millisecond of delay can cause problems, like in data centers, telecommunications and company servers, etc. These are the Smart-UPS On-line offered by many companies as flagship products and they are usually the industry best.

Online UPS always provides stable filtered power no matter the input. In these UPS, there are no switches used in regular operation because the battery is always connected to the inverter and the rectifier. The input is converted to DC and stored in the batteries, and then the inverter again converts it to AC pure sine wave for the output; that’s why it’s called double conversion; it continuously converts AC to DC and DC to AC in each cycle. Due to this system, the devices only draw power directly from the battery, not from the wall even if the main power is on; the battery backup is always online and running 24/7, unlike an offline UPS.

All the voltage fluctuations, surges, and noise are filtered as the AC turns to DC, and then it returns back to AC for the output. They provide multi-layer protection and clean energy to the connected devices with zero transfer delays regardless of what goes on in the main AC line as the batteries are constantly in use. But as a fail-safe, there are manual/automatic bypasses included in the system to power the load without the batteries, actively filtering the fluctuations if any problems arise from within.

Pure Sine Wave Output vs Simulated/Modified Sine Wave Output
Now, these do not come into play if the devices draw power directly from the wall, but when there is a power failure, the UPS reverts to the battery for power. So when the stored DC is converted to AC, it is either converted into a pure sine wave AC or a simulated sine wave depending on the type of UPS.

However, there is a slight but important difference between the two; pure sine wave always ensures a clean top-of-the-line AC output that can be used by any device from PCs to server-grade components. But a simulated sine wave is choppy, unrefined, and a bit rough around the edges; due to this, certain devices work with good efficiency while others produce more heat and suffer from performance loss. Only the most sensitive equipments need a pure wave setup like those with AC motors, servers, and telecommunications devices. Besides, most UPSs use a modified sine wave output as it comes at half the cost of a pure sine wave system.

At last, we have reached the end, and we hope you have learned quite a bit about UPSs and how they work from this read.