Machine Design

Common power-supply forms

There are no standards for constructing power supplies, but common forms are produced by many manufacturers. For example, regulated power supplies in encapsulated packages having solder pins for PCB mountings are widely available. These typically have ratings of 15 W or less. Similar encapsulated units having screw terminals (instead of solder terminals) are also listed by some manufacturers. In addition, power supplies are commonly constructed on printed-circuit cards for mounting, along with other circuit cards, in standard card cages. Printed-circuit-card units are typically rated less than 30 W.

Many power supplies are housed in perforated modular steel or aluminum cases. These typically are shoebox-shaped and have dimensions that vary with power ratings. The case acts as a heat sink. Most of these modular power supplies have screw terminals on one end for I/O connections. Threaded mounting holes are generally provided on bottom, side, and rear, permitting various mounting arrangements.

Many manufacturers make open-frame power supplies with ratings similar to those of enclosed units but having their components mounted exposed on "U" or "L" brackets. These are less costly than enclosed units, partly because enclosure cards are eliminated. Sometimes, these open-type power supplies may be less costly as a result of using lower-cost components. This difference is reflected in reliability. Warranty periods for open-type power supplies usually run from 60 days to 1 year, whereas enclosed unit warranties generally go for 5 years. Many open-type power supplies require relatively more derating for high-temperature ambients than do enclosed power supplies.

Plug-in power supplies are also widely available. These typically have an octal or 11-pin plug similar to that used on plug-in relays. Regulated plug-in units generally are rated up to 60 W, while unregulated plug-in supplies have ratings as high as 140 W.

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