Uninterruptible Power Supply, UPS

- overview of the basics and technology behind Uninterruptible Power Supply, UPS systems and how uninterruptible power supplies operate and may be used.

An uninterruptible power supply, UPS, sometimes also called an uninterruptible power source is a form of power supply that utilises the main power source such as the line supply, but is also able to maintain power to the equipment being powered when the main source fails or is interrupted.

The key feature of an uninterruptible power supply, UPS is that it provides instantaneous, or near instantaneous protection from power interruptions.

Uninterruptible power supplies are used in applications where maintaining power is of paramount importance. Typically UPS systems are used for data centres, computer systems and for some medical applications where continuity of power is crucial.

Uninterruptible power supply basics

The aim of an uninterruptible power supply is to provide AC power from the normal line or mains connection when it is available, but in the case of power failures, the uninterruptible power supply will use back-up alternatives, often in the form of batteries. Using techniques such as inverters, they will provide a simulated AC supply to maintain the power to the equipment.

There are several different types of uninterruptible power supply, and it is necessary to choose the correct type for the particular application in mind.

An uninterruptible power supply differs from an auxiliary power supply in that the UPS provides instantaneous or virtually instantaneous protection from power interruptions. An auxiliary power supply may need some time to replace the power.

Often a UPS will provide the replacement power from batteries. These will only have a limited run time - often between 5 and 20 minutes, but this should be sufficient to allow for an orderly shutdown of the system to prevent data loss, or to allow an auxiliary power supply to be started up. It is possible to extend the power capacity in many instances to provide protection for longer periods of time. This is normally achieved by using larger batteries, or adopting other strategies that will be able to provide power over longer periods.

Uninterruptible power supplies, UPS range in their power ratings from small systems that may protect a single computer to much larger ones used to protect complete data centres, etc.

Uninterruptible power supply technologies

There are several different uninterruptible power supply technologies that are available and can be used:

  • Stand-by or Off-line UPS technology:   This form of uninterruptible power supply technology often referred to as an SPS - standby power supply - is used to provide a low cost solution to overcome the risk of data loss, etc from power failures. It is one of the most basics types. It provides surge protection along with battery back-up.

    The equipment being protected is normally connected directly to incoming line or mains power. To provide the transient or surge protection, voltage transient clamping devices like those used in a common surge protected plug strip are employed - these are connected across the power line.

    When the incoming utility voltage falls below a predetermined level the uninterruptible power supply detects the low voltage condition and activates its internal DC-AC inverter circuitry, which is powered from an internal storage battery. The UPS will normally use mechanical switches or relays to change the equipment to be powered to the new supply. The switchover time can be as long as 25 milliseconds depending on the amount of time it takes the Standby UPS to detect the lost utility voltage and for the switches to change over.

    It is also worth remembering that the power provided from this form of uninterruptible power supply technology is a square wave form factor rather than a sine wave as supplied by the normal line supply.
  • Line-interactive UPS technology:   This form of UPS technology is aimed at the mod-range cost bracket. It builds on the technology of the Standby UPS, adding a multi-tap variable voltage transformer to provide protection against sustained low or overvoltage situations.

    This form of uninterruptible power supply technology uses the autotransformer with selectable taps to compensate for any long term fluctuations in voltage. By altering the tap on the transformer the correct output voltage can be maintained. By doing this battery power in conserved as the UPS is able to operate from the mains or line power supply even when the input power does not conform fully to the normal specified voltages.

    This form of UPS technology is particularly valuable in areas where the line or mains supply is not particularly reliable and may exhibit considerable fluctuations and occasional drop-outs. It provides the protection against all situations, although for extended failures, an auxiliary power supply may be needed to be incorporated as battery power is unlikely to be able to support long periods without mains power..
  • On-line or double conversion UPS technology:   This form of uninterruptible power supply technology provide a greater level of protection. Initially this type of uninterruptible power supply technology was normally used for large installations, however costs and technology improvements have enabled them to be viable for smaller installations.

    The On-Line or Double conversion UPS technology uses the same building blocks as the other forms of uninterruptible power supply technology. However the big difference with an online UPS is that the batteries are in circuit all the time, and this means that no power switches are required, and the problems with drop-outs during switch-over are removed.

    In essence the incoming line or mains power is transformed and rectified and applied to the batteries. The power for the load equipment is then taken from the batteries / rectified and through an inverter which brings the voltage back up to the required line input voltage. This form of UPS gains its name of a double conversion, because the power is converted from AC to DC to float charge the batteries and also power a DC to AC inverter.

    In this way it can be seen that the batteries are always in circuit and this means that no switches are required. When power loss occurs, the input rectifier receives no supply, and as a result power is drawn from the batteries. Once line power is resumed, the rectifier will supply the equipment and also re-charge the batteries.

These different uninterruptible power supply technologies represent the major ones that are used. Other UPS technologies exist and are used, but as widely as those described.


Uninterruptible power supplies are used in many areas where a continuous supply is essential. Uninterruptible power supply technology has advanced in recent years as the need for reliable power sources has grown. A wide range of applications require UPS technology, but some of the chief users are data and computer centres as well as cellular telecommunications or land line telecommunications centres where continuity of service is essential.

By Ian Poole

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