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2.4 Trick in PC power supply selection

We would like to show you several tricks in selecting PC power supply.

<< Category of power supply >>
PC power supply can be categorized into mainly three sections due to dimension and output connector variations. For more details, refer to section 2-2 "Category and Differences of PC power supplies."

<< Input voltage >>
Attention must be paid to input voltage as PC is used worldwidely as well.

<Selection switch system>
Many power supplies adopt this system. Selection switch (Photo 2.8) is equipped on the main body. You can select input voltage, 115V or 230V, by the switch. Generally 115V covers 90V to 130V, and 230V covers 180V to 260V approximately. However, it does not cover the input between the two, for example, 130V to 180V.

NOTE: Nominal voltage in Japan is 100V, and when vacuum cleaner and dryer are used at a time, the voltage may drop to 90V. When used at input voltage close to or less than the lower limit of 115V PC power supply, proper output cannot be gained causing malfunction of system. If power supply does not work properly, verify the input voltage as well.

<Automatic selection system>
This system has no switch like selection switch system. However, this does not cover the middle range of input voltage likewise.

<Worldwide range system>
This system covers 85V to 265V of input voltage with no selection switch equipped. With this type of power supply, any input voltage is acceptable for proper output so that you do not need pay attention. However, it makes circuit design complexed and brings higher cost in comparison with other systems in general.
We have adopted worldwide system to our products except some products dedicated to domestic use only. Refer to "Nominal voltage of the world" on page D-13.

<Output current>
PC power supply provides various voltage such as +3.3V, +5V, -5V, and so on. And each output voltage has rated current.

<Rated output current>
Rated current can be derived continuously, but cannot exceed this value. However, in the case that multiple outputs influences each other in terms of circuit operation, even if more current than rated current (max. current) is described, all max. current cannot be derived at a time. In general, it is described in this way, that total output of +5V and +12V shall be XX max. or total current shall be XX A max.,for example.
In addition, instantaneous value (instantaneous output current) is shown depending on power supply. In this case, how long period or how much current must be confirmed.

<Min. load>
Min. load is min. current to be derived to stabilize the power supply. If the output current is less than that, feedback loop to stabilize output voltage does not work properly and may cause unstable or abnormal output voltage. That is, to gain stable operation, the load current must be between rated and min. current. Also, as it is not a good idea to use the power supply on the borderline, giving ample margin is recommended
in selection

<< MTBF (Mean Time Between Failures) >>
PC power supply has its own lifetime as well as other electronic equipments. It is too hard to know the exact lifetime of individual product. MTBF is a rough guide to know lifetime of the same type of product or series product, although it is too hard to know the exact lifetime individually. MTBF is just a rough guide as it comes from reciprocal of failure probability under specific conditions. Depending on operation condition, 70% of MTBT, in general, is recommended period of maintenance, replacement, etc. However, it may be out of order within the period. Periodic check points to be recommended at least are "Is fan working?," "Is temperature low enough?"
"Any abnormal sound?"

<< Output harness >>
This is used to provide power to motherboard from PC power supply. If the harness does not fit, the power supply may not be used even though output specification is good enough. Or, the power supply may not give enough margin for stable operation.

<< Other trick >>
Power supply is a indispensable component to drive computers. When failed, system fails naturally. On a daily basis, maintenance of power supply is important, and safety measures to shut down system safely must be implemented in case of power failure.