In the previous tutorial, Online UPS was assembled from its basic building blocks like Lead Acid battery Charger, Square Wave Inverter and Modified Sine Wave Inverter. Now in this tutorial, offline UPS will be designed. The Offline UPS is one of the topologies of UPS (Uninterruptible Power Supply). In UPS system, the term topology refers to the switching mechanism of the UPS system.  Contrary to online UPS, in offline UPS, the load circuit is initially directly powered by the AC mains. On the failure of power supply, the power failure is detected by a switching circuit and the power is provided to the load through the battery and inverter after the power failure has already occurred. So, in the offline UPS, the load experiences the power cutoff once before it receives the power backup from the UPS.

The Uninterruptible Power Supply (UPS) is an electronics device which supplies power to a load when main supplies or input power source fails. It not only acts as an emergency power source for the appliances, it serves to resolve common power problems too. Any UPS has a power storage element which stores energy in the form of chemical energy like the energy is stored in batteries.It is like energy is stored in the form of motion in a flywheel. That is why these devices are also called battery backup or flywheel backup. The UPS not only provides emergency power, they also help to sort out common power related issues like providing protection from input power interruptions, protection from overvoltage, output voltage regulation and stabilization.