What is an uninterruptible power supply (UPS)?

by Nyal Mellor April 08, 2013

A UPS is a device that provides battery power to connected electronics in the event of a power outage. There are three different types of UPS systems: standby, line interactive and online ‘dual conversion’.

Pros:

• UPS systems are the only solution that can protect your projector  or hard drive equipped electronics in the event of a power outage.
• Line interactive UPSs provide voltage regulation and online dual  conversion UPSs go one step further by providing substantial input to output isolation.

Cons:

• Ongoing costs due to battery wear and tear.
• Most UPSs are designed for computers, not audiovisual systems, and may actually increase powerline noise. Specifications provided  on datasheets are often into loads representing 50% of less of capacity.
• Inability to supply peak current requirements for dynamic loads  such as amplifiers.
  
  

In a standby UPS, a battery charger converts AC to DC to keep the  battery full for when it is required. The battery is not in the AC circuit. When the line voltage goes below a certain predefined level the  inverter switches on and is connected via the transfer switch to the AC  line. The inverter converts DC to AC.

The main advantages of a standby UPS are efficiency and low cost. The only things that are consuming power are the line voltage monitoring circuitry and the battery charger. Battery life is longer than for other designs since it is generally out of the circuit. They cannot protect from under or overvoltage conditions and offer no input to output noise reduction or surge isolation.

Line interactive UPS systems appear similar at first glance but they have a crucial difference – the inverter is always on, generating AC from the battery which is kept full by the battery charger. A sensing circuit monitors the AC waveform and determines how much current to draw from the inverter to keep the voltage within tolerances.

Line interactive UPS systems have the advantage of being able to support brownouts and undervoltages which standby UPSs cannot. Some of the more sophisticated line interactive solutions provide ‘bucking’ circuitry, which pulls down the AC voltage if it is too high.

The final type is an online or dual conversion UPS. In this design there is no direct path for AC. Power is converted from AC to DC by a  rectifier which feeds the battery. An inverter then converts the AC back to DC.

The advantage of this approach is that it offers a very high amount of noise and transient isolation between input and output. Online UPSs are energy inefficient however, and batteries need replacing more frequently than in other designs since they are constantly being charged and discharged.

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