Power supply

The DC power supplies used for electronics and computers are of two main types, switching and linear, both supplied from the 120V AC mains. Switching supplies include a regulator, and are a special study, so they will not be mentioned further here. The linear supply is relatively simple and inexpensive, and in many cases is completely adequate. For some applications, a regulator is not required, either because the load is constant or because small variations in the output voltage do not matter. The basic ideas of linear power supply design will be presented here, and you should make a small supply just for the experience, if you have never done so before.
A linear supply consists of transformer, rectifier, surge-limiting resistor, filter capacitor, and bleeder. The transformer should have separate primary and secondary windings so that the output is isolated from the power-line ground. It is very dangerous otherwise, so isolation is important. A transformer cannot usually take a DC current through a winding, but if a winding is center-tapped, equal DC currents can flow from the center tap to the ends of the windings without inconvenience. Small currents can, of course, be tolerated. The rectifier is usually a full-wave bridge of silicon diodes, though two diodes at the ends of a center-tapped winding can also be used. The capacitor is a large aluminum electrolytic, up to 10 000 μF, and a voltage rating of 450 V. For higher voltages, capacitors may be used in series (halving the equivalent capacitance but doubling the voltage rating); the leakage in the capacitors will equalize the voltages. A resistor is used in series with the capacitor to limit the surge of current in the first half-cycle of operation, when the capacitor is uncharged. To handle the sharp power pulse, a wire-wound power resistor is required, though after the turn-on surge, it hardly dissipates any power at all. The bleeder resistance is connected across the capacitor to discharge it when the supply is turned off. It should carry up to 10% of the rated output current, and helps to stabilize the operation of the supply, eliminating any rises in voltage at very low currents that may occur. For high-voltage (> 50V) supplies, a bleeder resistor is essential to remove the hazard of an unexpected voltage in the filter capacitor when the supply is turned off. The bleeder resistor should be rated to dissipate the necessary power in steady operation.
The AC input should be fused, using a slow-blow fuse of about two or three times the normal input current. The purpose of this fuse is to save the power transformer if a filter capacitor fails, usually by becoming a short circuit, or if the rectifier fails, also usually by presenting a short circuit. This can happen even if the output is protected against a short by a voltage regulator. Usually, insufficient current flows in this case to open the protective device of the line, which may be 15 A or more, and is designed to prevent fire. The current may be more than sufficient to burn out the transformer primary, if it is rated for an ampere or less. The transformer is usually the most expensive part of the power supply, and is worth saving. The fuse may be in a fuse block inside the chassis; it is not worth much to have front-panel access to the fuse. It is also nice to have a pilot light to show when power is applied to the transformer primary, across which the pilot light should be connected. A neon lamp with dropping resistor is very satisfactory for this purpose.
It is not necessary to have a polarized plug unless the chassis is grounded to one side of the line–in that case, a polarized plug is mandatory. A metal chassis or box should be grounded to the green wire of a 3-conductor power cord for protection against shock. Any fault to the hot wire should trip the protective device of the AC supply circuit in this case. It is general practice to put a switch only in the hot wire, but with an unpolarized plug, both wires should be switched using a DPST switch to make sure the hot wire is switched. The white wire should never be directly connected to the chassis, even if it is ground. If the chassis is grounded, then it is safest to connect one side of a high-voltage DC output to the chassis, so that no fault placing the chassis at high voltage can occur. For voltages over 500 V, special care must be taken with protection, since such voltages can be lethal. For voltages less than 50 V, practically anything goes, since the shock hazard is minimal.

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