One of the most difficult problems to find in PC is power shortage. The symptoms may be that the motherboard is "not working" and the software causes frequent system crashes. These symptoms may be manifested by the abnormalities of the motherboard, CPU or memory, and sometimes seem to be problems of hard disk, CDROM, floppy disk, etc. Imagine that every component of a PC system has the same source of electrical energy - that is, the power supply. The power supply must provide a steady, continuous current for all equipment uninterruptedly. If the power supply is excessive or insufficient, the connected equipment may not work properly, and it looks as if it is broken. For example, memory can not be refreshed, resulting in data loss (resulting in software errors); while the CPU may be deadlocked, or restarted randomly; the hard disk may not rotate, or even more strange - rotate, can not normally handle control signals. Since so many devices are closely related to power supply, it is not excessive to regard power supply as the most important component in PC hardware system. Unfortunately, most people don't realize that they sometimes prefer old chassis (chassis usually have power) when they buy power, expecting "low price and good quality". According to experience, this is a common phenomenon. The old power supply cannot be as effective as it was when it was first used, and the energy provided cannot be as high as the nominal value. Many power supplies are UL-free and may only "extrude" 50-75% of the nominal value. Even if there is a power supply in the famous chassis, there may be problems, which we have encountered in our daily life.