Articles: Cases/PSU

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The ratings of the capacitors suffer, too. A reduction of the capacities of the input capacitors worsens the reaction of the power supply to minor sags of the input voltage; a reduction of the capacities of the output capacitors leads to a stronger pulsation at the PSU’s output.

Sometimes this cost cutting can be observed with your own eyes: there are fewer output connectors, and the section of the wires in the power cables is 20AWG rather than the required 18AWG (in the AWG system, a bigger number means a smaller section of the wire). This results in a higher voltage drop on the wires, which leads to stronger pulsations of the voltage right on the power connectors of the consumers. If the load is high, thinner wires become perceptible hot, too.

The filtering chokes are the last to be abandoned – reducing their size doesn’t bring any serious cost advantages, so these chokes are present until the manufacturer decides they are unnecessary at all. A replacement of the chokes with straps leads to a stronger pulsation at the output of the PSU (if these were the output chokes) or to more noise the PSU is issuing into the power grid (if these were the chokes of the input filter).

One of the most memorable ways of making cheaper low-end PSUs was the realization of the standby +5v power source as a blocking generator with an electrolytic capacitor in the feedback circuit. In this scheme, which is a switching power supply based on a blocking generator, the output voltage is determined by the frequency of the impulses, and this frequency in its turn is inversely proportional to the capacitance of the capacitor in the feedback circuit. Cheap capacitors, intended to work at a temperature of 85°C at most, couldn’t stand the hard thermal conditions of the standby unit, typical for cheap PSUs (the standby is always on, while the PSU fan is only working when the computer is running), and after a year and a half of such work, the capacitor began to dry up, and its capacity – to decline. With the reduction of the capacity, the output voltage of the standby source grew up, and this voltage powers up the main regulator of the PSU. So in the end the main regulator just breaks down on your turning the computer on, and this breakdown is accompanied with an output of two- or threefold voltage along all of the power rails. Of course, the computer was just burned out after that – you could even see the burnings on the mainboard’s chips, in the hard disk drive and so on. The CPU and the memory were the only devices to have any chance at all – if their own regulators located on the mainboard endured the trial.

Of course, the manufacturers came to their senses at last and began to install near-infinite film capacitors into the standby circuit instead of electrolytic ones, since small capacity was quite sufficient there. Unfortunately, they had already produced quite a number of such “delayed-action” bombs which were a serious argument in favor of purchase of more expensive and high-quality PSUs which were free from such dubious engineering solutions.

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