Understanding Offline and Online UPS Diagram

In our separate blog, we discussed differences between online and offline UPS and their applications. Today, we will dig deeper into these UPS diagrams. We will try to explain these two diagrams and functionalities for better understanding for the beginners.

Online UPS vs Offline UPS in a nutshell:

Before going further, here we will explain the basic differences between Online and Offline UPS. So that you can easily understand the diagrams in the next section. We will break down these differences into some basic points. Let’s hop on them.

Functional Difference:

The batteries in an offline UPS are constantly charged. The load is powered by the inverter if the electricity goes off. When on inverter/battery backup, most low-cost offline UPS produce a square/pseudo sine wave output waveform.

The online UPS converts the incoming AC mains supply to DC, which is then fed to the battery and load via the inverter. If the mains power goes out, the batteries feed the load through the inverter, ensuring that the output supply remains uninterrupted. It means that instead of getting power from the AC mains, our electronics get power from the online UPS. As a result, even if the main AC power goes out, our electronic equipment can continue to operate.

Furthermore, an Online system’s dual conversion design (AC to DC / DC to AC) assures a far high level of load isolation from supply source anomalies. As a result, the Online system consistently produces a fixed and reliable output.

Difference in voltage variations:

Online UPS: Voltage distortion has no effect on the performance of an online UPS.

Offline UPS: The more voltage fluctuations there are, the more the offline UPS is needed. A frequent switch may result in switching delays or a reduction in performance.


Offline UPS are less expensive than online UPS’s.

Block Diagram of Online UPS:

Let’s start with the Online UPS diagram. Below there is a block diagram of an Online UPS.

As you can see in the diagram above, there are nine blocks. Let’s take a look at each block individually.

  1. Inductors and capacitors are used to create an EMI filter. This EMI filter circuit’s main purpose is to decrease or filter electromagnetic interferences.
  2. To convert AC to DC, this rectifier circuit is used. As this UPS contains a battery, and the battery can only store DC, we must convert the input AC supply to DC.
  3. The DC filter circuit is used to filter the impure DC that is produced by the rectifier circuit. The rectifier’s DC output contains an AC component. As a result, the filter circuit is utilized to filter out any AC components from the DC supply.
  4. The battery is linked to the DC filter circuit’s output. The battery will charge when the UPS is connected to the power supply.
  5. We have a DC supply now, but we need an AC supply as an output to drive the load. To convert DC to AC, an inverter circuit is required. High-speed solid state switches, such as MOSFETs and SCRs, are used in the inverter circuit. The Inverter Circuit is not necessary if your load requires DC power.
  6. The AC filter circuit is used to filter the impure AC that comes from the inverter circuit.
  7. Between the AC filter circuit and the Critical Load there is a static switch. According to the stated circumstance, this allows or disallows power flow from the UPS to the load.

Just after EMI filter supply, another static switch is connected between the essential load and the primary power source. The power flow from the main supply to the load is accepted or denied by this switch.

The lower static switch on an Online UPS is generally ON, whereas the upper static switch is normally OFF. As a result, in normal circumstances, power flows from the main supply to the load via the UPS circuits. When the main power supply is unavailable, the load uses the battery for power.

The upper static switch will be ON and the lower switch will be OFF if the UPS is unable to transmit power to the load. As a result, power will flow directly from the main supply to the load in this situation.

Block diagram of Offline UPS:

Now let’s come to the block diagram of Offline UPS.

The block diagram of Offline UPS is identical to that of Online UPS. There is a slight distinction between them.

The upper static switch on an Offline UPS is generally ON, whereas the lower static switch is normally OFF. In normal circumstances, power flows directly from the main supply to the load. The battery will charge at the same time. The upper static switch will be turned off and the lower static switch will be turned on when the main power source is unavailable. As a result, the load draws energy from the battery.


When a UPS is required for a longer period of time and the main power source changes significantly, an online UPS is the ideal option. However, if you are concerned about cost and operating temperature, it is logical that you should choose an offline UPS.

This article is only for basic understanding for getting an idea about these two UPS. When you need to buy any of these, please talk to an expert first then decide.

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