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Technical jargon about the different types of UPS can be confusing, but it’s worthwhile to find out which type of UPS will best meet your needs.  What’s the difference between an online and an offline UPS?

Offline UPS, including Line Interactive technologies

In this type of UPS the load is directly connected to the incoming AC power supply. When the mains supply fails or goes below a minimum level, the offline UPS disconnects the incoming AC mains and delivers power to the load via an internally connected battery using DC-AC inverter circuitry. Although all manufacturers try to minimise the switching delay, it could be as long as 25 milliseconds.  The Line Interactive UPS is designed as a low-cost solution  to supply backup  power to computer equipment.

A  filter is incorporated into the machine that offers limited protection against spikes and surges, as well as an automatic voltage regulator to correct high or low voltage fluctuations on the input. The use of an Automatic Voltage Regulator will help to increase the battery life of the UPS. Switching time is normally well within the tolerance of modern computer power supplies.   Generally, for home desktops, the offline UPS is the most used.

Online UPS

This type of UPS always delivers power to the load via the DC-AC inverter. Therefore in these UPSs no switching mechanism is required, and hence transfer time has no role during a power failure. To maintain the charge of the battery, a battery charging unit is incorporated in the system. So when mains supply fails, the UPS continues to deliver power to the load using its battery, however, charging of the battery stops. This is similar to what happens when a plugged-in laptop keeps on running without interruption when mains power fails.

The incoming mains is converted directly into DC. This DC power is used to charge the batteries and to drive the inverter, which in turn runs the load. Should the mains fail the batteries will simply carry on driving the inverter, and start to discharge as opposed to charging.

The units have a built-in static bypass feature which enables the machine to transfer the load to normal mains under certain fault conditions. There is no break on the output on transfer to or from the mains.

In the power range between 600VA and 3000VA, both types of UPS technologies are available, so the decision for which topology to use is primarily based on the specifics of the customer application.