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How an Uninterruptible Power Supply Works

written by: Swagatam • edited by: Lamar Stonecypher • updated: 11/30/2011

An uninterruptible power supply is far superior to some other power guards like CVTs since it can provide power backup for long durations of power failures. We will try to understand more about this technology here.

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    Uninterruptible power Supply (UPS) vs Constant Voltage Transformer (CVT)

    Through one of my earlier articles, you should have received a thorough knowledge regarding the salient features of a CVT or constant voltage transformer, but it’s always an Uninterruptible Power Supply, or UPS, that wins the race against a CVT. For the ultimate protection of their computers, people normally prefer a UPS rather than a CVT due to the following reasons:

    • All good quality UPS devices include a built-in voltage stabilizer that is as good as a CVT.
    • A UPS can provide power back up for long durations of voltage failures (more than an hour). CVTs are effective only against very brief power interruptions (5ms).
    • An absence of an output load can in no way harm a UPS. CVTs can easily get burnt in an absence of an output load.
    • An uninterruptible power supply not only provides protection against power failures (blackouts), but also its built in voltage stabilizer provides total safety against low voltages (brownouts). A CVT is totally helpless against these problems.
    • A UPS generally is long lasting and does not require much after sales service. A CVT requires lot of maintenance and is more vulnerable to internal malfunctions.
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    A UPS is Your Computers Best Friend

    A Typical UPS  

    An uninterruptible power supply can be considered the “best friend of a computer" since it safeguards your PC against all possible power related hazards.

    According to a survey conducted by IBM, on an average a computer may be forced to face as many as 120 power related problems per month. We all know how detrimental these can be to our computers, so a UPS is strongly recommended for computers which are operated for long hours.

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    How a UPS Functions

    UPS Block Diagram  

    An uninterruptible power supply basically functions through the following main stages:

    • Rectifier: Here the stepped down AC mains is converted into DC and fed to the inverter stage.
    • Inverter: In this stage the rectified DC is oscillated at the rate of the input frequency and is fed to a high power transformer where it is stepped up to the level of the input AC mains and supplied to the load. During a power lapse, a battery voltage connected to the rectifier stage immediately replaces the “rectified voltage" and keeps the inverter ON, so that an uninterruptible power supply is available at the output, without a break even for a millisecond.
    • Battery: A sealed maintenance free battery is permanently connected to the rectifier stage. Here it is continuously charged and kept topped up during the presence of the AC mains, through the rectifier stage itself.
    • Static Bypass Switch: During an accidental overload this switch transits the load smoothly so that no instantaneous blackouts are created.
    • DC Link Filter: This stage comprises of an inductor and a capacitor filter circuit. It is responsible for a smooth DC to the battery so that it remains long lasting and provides a consistent uninterruptible power supply to the load.

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