UPS Dissected

The electric power that comes from the utility provider is not the smoothest. In other words, the power that comes to the wall unit fluctuates continuously. This is also called alternating current (AC) because it keeps alternating between the highest voltage to the lowest voltage and back (approximately 60 times in a second). 

A sensitive electronic unit like PC requires non-fluctuating current. That is direct current (DC), which flows uni-directionally with a constant voltage. A 'power supply' unit inside the PC converts the fluctuating AC to DC before it is supplied to the remaining parts of the PC.

However, the power supply unit inside the computer cannot handle sudden increase or decrease in voltage caused by lightning strikes or heavy power consuming units like air-conditioners or heaters. When an air-conditioner or a heater is switched off, the voltage suddenly goes up. 

This sudden increase in voltage is called a spike or a surge, which can cause sensitive circuits inside the PC to burn out or cause data loss or corruption. A voltage spike can be handled by using a surge protector. But if the power fails, neither the surge protector or the supply unit can be of any help. Here comes the role of uninterruptible power supply (UPS) units. 

As the name itself suggests, the UPS's primary role is to maintain an uninterrupted supply of current in the event of a power failure. UPS are nothing but a bunch of batteries which takes over as soon as the main power fails. UPS come in various sizes ranging from the size of a brick to that of a room.

Parts of a UPS

Converter/inverter circuit: This circuit basically converts AC power to DC, to be stored in the battery and back again to AC for use by the equipment.

Battery: Batteries are the main component that hold energy, which is eventually used by the equipment. 

Transfer switch: This part of the UPS plays a crucial role when the power fails. When the power fails, the switch almost immediately turns to the battery unit.

Control panel: The control-panel switches primarily control the functions of the UPS. It also has LED indicators showing various status -- online, battery, overload and errors.

Output point: These are plug points where you connect your sensitive electronic units like PCs.

Types of UPS

Standby/Offline UPS: The battery and inverter don't supply power to the equipment till the main power fails. Instead, the battery charger uses the main power to charge the batteries and waits. The moment power fails, the transfer switch makes the battery take over as the power source. When the line power is restored, it switches back. These are the least expensive models.

Ferroresonant Standby UPS: In this UPS the transfer switch is replaced by a Ferroresonant transformer. This avoids the small delay during transfer from the main source to the battery source at the time of a power failure. The transformer stores energy and acts a buffer in the event of a power failure and give enough time to switch over to the battery.

Line-Interactive UPS: Line-interactive UPS use a totally different design than any other type of standby UPS. The separate battery charger, inverter and source selection switch are replaced by a combination inverter/converter, which charges the battery and converts its energy to AC for the output as required. When the line power is operating, the inverter/converter charges the battery; when the power fails, it operates in reverse.

Online UPS: In Online UPS, the primary power source is the battery. In the event of a power failure, the transfer time is almost zero. The main power continuously charges the battery and the battery powers the equipment.



However, in this design a lot of power is dissipated as heat. This disadvantage is overcome by a new design in which the charger is replaced with a delta converter. Instead of providing all of the output from battery under normal circumstances, some of it is provided directly by the delta converter from the main power. In the event of a power failure, the delta converter stops operating and the unit acts like a regular Online UPS.





The role of software: Most modern UPS are microprocessor-controlled and this enables linking of software that can monitor and control their functions. This includes checking the battery, the temperature, conducting self-tests, setting alarms and controlling auto shutdowns. 

To get the software work with the UPS, a cable is connected between the UPS and the PC using a special interface. In modern UPS, this can be done through USB port.

Nelson Johny in Mumbai