Offline UPS (Uninterruptible Power Supply) is also a type of uninterruptible power supply device. However, it differs from online UPS in terms of working principles, performance characteristics, and more. The following is a detailed introduction about it:
I. Basic Working Principle
- When Mains Power is Normal:
- When the mains power is supplying electricity normally, the offline UPS will directly transmit the mains power to the load equipment for use. At this time, components such as the inverter inside the UPS are basically in standby mode. The mains power is supplied to the load end without going through the complex conversion and processing procedures inside the UPS. Only the mains power may first pass through the simple filtering circuit equipped with the UPS for a certain degree of filtering to remove some relatively obvious power noise, but the processing of mains power voltage stabilization and the like is relatively limited.
- When Mains Power is Abnormal:
- When abnormal situations such as power outages, high or low voltage occur in the mains power, the offline UPS will quickly detect such changes. Then, it will start the internal inverter to convert the direct current stored in its built-in battery into alternating current, and then supply it to the load equipment for use. Thus, it provides temporary power support to the load equipment during mains power failures, preventing the load equipment from immediately shutting down due to mains power interruption. However, compared with the online UPS, this switching process is usually a bit slower and may cause a brief power fluctuation or even a short power outage in the load equipment during the switching instant (but usually the time is very short and has little impact on most ordinary equipment).
II. Main Characteristics
- Relatively Low Cost:
- Since the offline UPS basically just simply transmits the mains power when the mains power is normal, its internal circuit structure is relatively simple. It doesn’t need to be equipped with complex conversion circuits such as rectification and inversion that work continuously like the online UPS. Therefore, the manufacturing cost of the offline UPS is relatively low, and its selling price in the market is often more affordable. This gives it an advantage in some application scenarios that are sensitive to cost and don’t have extremely high requirements for power uninterruptibility.
- Relatively High Efficiency (When Mains Power is Normal):
- During the period when the mains power is supplying electricity normally, because the offline UPS mainly directly transmits the mains power to the load equipment without going through a complex conversion process, the power loss during the transmission process is relatively small. So its working efficiency is relatively higher than that of the online UPS when the mains power is normal. However, once the mains power becomes abnormal, its efficiency will decrease during the process of switching to battery power supply and outputting alternating current through the inverter.
- Limited Improvement of Mains Power Quality:
- As mentioned before, the offline UPS only performs simple filtering on the mains power when the mains power is normal and cannot perform comprehensive processing such as voltage stabilization, filtering, and harmonic removal on the mains power like the online UPS. So when there are quality problems such as voltage fluctuations and harmonic interference in the mains power, the power quality received by the load equipment will still be significantly affected. It is not very suitable for precision electronic equipment with very high requirements for power quality.
- Risk of Brief Interruption during Switching:
- Although the offline UPS can switch to battery power supply when the mains power is abnormal, due to its switching mechanism and circuit response speed, etc., there may be a brief power interruption or fluctuation phenomenon during the switching instant. Although the time is usually very short, for some key equipment with extremely high requirements for power continuity (such as some high-end servers, medical emergency equipment, etc.), it may bring certain risks, such as data loss, equipment failure, etc.
III. Application Fields
- Home Office and Ordinary Civilian Equipment:
- For ordinary computers, printers and other equipment in the home office environment, as well as some small ordinary civilian electrical appliances, these equipment don’t have particularly high requirements for power uninterruptibility and are sensitive to cost. The offline UPS can provide a certain degree of power guarantee in case of mains power outages, preventing inconvenience or data loss caused by sudden power outage and shutdown of the equipment (although there may be brief fluctuations, but they have little impact on this type of equipment). So it is widely used in the home office and ordinary civilian fields.
- Some Equipment in Small Commercial Premises:
- In some small commercial premises, such as small retail stores, small restaurants, etc., for cash registers, ordinary lighting equipment, etc. in the store, the offline UPS can also provide temporary power support when the mains power has problems, preventing the equipment from shutting down due to power outage and affecting normal business operations. Considering the cost factor, the offline UPS is also a relatively suitable choice.
The offline UPS, with its characteristics such as relatively low cost and relatively high efficiency when the mains power is normal, plays a certain role in some application scenarios where the requirements for power uninterruptibility are not very strict and are sensitive to cost.