In small power supply applications, the main source of power comes from the national power grid. This power is most often created by water or steam driven generators. The term hydroelectric power is generally reserved to describe power generated by falling water. However, since coal, oil, gas, and nuclear power plants utilize water to drive steam turbines, even they may properly be called hydro electric. In any case, the power source is referred to as the ‘mains’.

Most simple power supplies receive their input power from a standard 120-volt circuit consisting of three wires. It is interesting to note that in the electrical trade, it is called a two-wire circuit since the ground wire is not counted.

Black wire this carries the power and is fitted with a circuit breaker or switch.

White wire this is the neutral or ‘return’ wire in the circuit. Its voltage is generally very close to ground potential.

Green wire this is the ground wire and is not used to carry power. It is a safety feature that prevents electrocution in the event of a circuit failure. In home wiring, this is a bare copper wire, and is connected to the copper cold water pipe where it enters the building.

Electric power is generally distributed as AC (alternating current). This is because it is easy to convert to other voltages by means of transformers. However, most electronic devices require a source of DC (direct current) power. Consequently, many types of power supplies isolate the mains from the regulated circuit power by means of transformers.

Power from a generating station is distributed over a 3-phase circuit. This simply means that there are three power conveying wires, each carrying a sinusoidal voltage displaced by 120^o from each other. This offers many advantages.

If the load on each phase is equal (balanced) then the return or neutral wires can be combined, and amazingly enough, this neutral wire will carry no current. In some cases, the neutral wire can be dropped however, it is needed if the phase currents become unbalanced.

Since power follows the sinusoidal nature of the signal, heavy machinery will use all 3-phases. This provides a reasonably constant torque since there will be six power maximums for every motor shaft revolution. Three-phase circuits also power large buildings.

In domestic applications, only 1-phase is distributed. This phase is inverted so that 2 single phase lines separated by 180^o enter the home. This provides a double size voltage for ovens and dryers.

AC Voltage

Several different schemes are used to describe the magnitude of sinusoidal voltages:

• Peak
• Average
• RMS (transformers are specified in rms)

A d’Arsonval movement responds to the average value and is the number obtained if the signal were smoothed out. The average value of any sinusoid is naturally zero however; the average value of a fully rectified sinewave is 63.6% of the peak value.

The RMS (root mean square) value is the DC equivalent which would create the same amount of heat in a load, as the AC signal. The RMS value of a sinusoid is 70.7% of the peak value.

For a full-wave rectified sinusoid:

When using an AC voltmeter, it is important to understand which unit is being measured. Unless a meter specifically says true RMS, the reading should not be taken as a reliable RMS value. Many multi meters measure the rectified average sine waves only or are calibrated to read RMS for sine waves only.

Derivations

Average Value of a Half-Wave Sinusoid

Average Value of a Full-Wave Sinusoid:

RMS Value of a Half-Wave Sinusoid:

RMS Value of a Full-Wave Sinusoid: