POWER SUPPLIESCompiled and presented by: Doren Nedrick

The voltages required for circuits containing semiconductor devices seldom exceed 30V and maybe as low as 1.5V. Current demands vary from a few microamperes to many amperes in large systems. Usually the supply must be DC.Batteries are suitable for portable equipment but in general power supply units operated from the a.c. mains are employed. In these a.c. has to be converted to d.c., the process being called rectification.

In most power supply units a transformer steps down the a.c. mains from 220V to a much lower voltage. This is then converted to d.c. using one or more junction diodes in a rectifier circuit.There are two types of rectifiers: half wave rectifier and full wave rectifier.

Half wave rectifierWhen a diode is connected to a source of alternating voltage, it is alternately forward biased and then reverse biased during each cycle. Therefore, when a single diode is used in a rectifier circuit, current passes through the circuit load during only one-half of each cycle of the input voltage (Fig. 17-10). For this reason, the circuit is called a half-wave rectifier.

OutputsThe outputs of half-wave rectifier circuits are pulsating direct currents. Such currents can be used in some circuits. However, they produce a loud humming in radios, television sets, and amplifiers.The ac voltage is given in rms value while the rectified dc voltage (varying dc waveform from types of current) is given in average value (V ave) and is found using the formula: VAVE = VP x 0.318

Full-Wave Rectifier CircuitsA full-wave-rectifier circuit is one that rectifies the entire-cycle of an applied voltage. A basic full-wave-rectifier uses two diodes. The action of these diodes during each half cycle of the applied voltage is shown in the fig below. The diodes may be individual units, or they may both be in a single package.

Transformers used in these circuits must have center-tapped secondary windings. The dc output voltage of the rectifier circuit depends mainly on the voltage across the secondary winding of the transformer, although other circuit components such as output resistors and filter capacitors will affect the output as well.

Full-wave rectifiers have steadier output voltages than half-wave rectifiers do. This is because full-wave rectifiers produce a pulse of voltage in the output circuit during each half cycle of the applied voltage.

A bridge is a full-wave rectifier that uses four diodes in specific configuration. The diode action during each half cycle of the applied alternating input voltage is shown in Fig. 17-15. Note that the rectifier transformer used in a bridge circuit does not have a center-tapped secondary winding. The diodes may be individual units or packaged into a single unit (Fig. 17-16). A filter is also needed to remove the pulses from this rectified dcThe DC value of voltage VAVE for a full-wave rectifier is found using the formula: VAVE = 0.636 x VP

FilteringThe varying d.c. output voltage from a rectifier circuit can be used to charge a battery but must be smoothed to obtain the steady d.c. required by electronic equipment.The basic filtering component of rectifier circuits is an electrolytic capacitor (Fig. 17-11). The action of such capacitors, in half-wave circuits, is shown in Fig. 17-12.

During the time that the diode is forward biased, voltage builds up across the output circuit. The capacitor becomes charged. As the voltage decreases, the capacitor begins to discharge through the resistor R. When the diode is reversed biased, it ceases to conduct. Meanwhile the capacitor keeps discharging through R. When the diode is forward biased again, the capacitor, which was not fully discharged, is brought back to full charge. It then begins the discharge cycle again. Thus, the output voltage and current are smoothed

The ripple voltage in a full wave rectifier circuit is smaller, giving better smoothing. The waveform is shown below; in this case the ripple frequency is twice that of the a.c. supply.

CAPACITOR-INPUT FILTER The smoothing produced by a reservoir capacitor Cl can be increased and ripple reduced further by adding a filter circuit. This consists of an inductor or choke L and another large capacitor C2, arranged as in the fig above. This arrangement is also called a pie filter.

We can regard the varying d.c. voltage produced across C1 (VO in Figs. 36.2 and 36.3) as a steady d.c. voltage (the d.c. component) plus a small ripple voltage (the a.c. component). L offers a much greater impedance than C2 to the a.c. component and so most of the unwanted ripple voltage is developed across L. For the d.c. component the situation is reversed and most of it appears across C2. The filter thus acts as a voltage divider, separating d.c. from a.c. (i.e. acting as a filter) and producing a d.c. output voltage across C2 with less ripple. A resistor may replace the inductor when the current to be supplied small. When resistor is used in applications of high current there is high power (I2R) loss in the resistor

RegulationBecause of its internal (source) resistance (recall internal resistance in cells) the output voltage from an ordinary power supply (and a battery) decreases as the current it supplies to the load increases i.e. the 'lost' volts increase and the terminal p.d. decreases. The greater the decrease the worse is the regulation of the supply. Good and bad regulation curves are shown below.

There are many occasions when a d.c. voltage is required which remains constant and is not affected by load current changes. In these cases a stabilizing or regulating circuit is added to the power supply, as shown by the block in the fig. below.

A zener diode called a regulator can be used to steady the output voltage and is looked at below.

ZENER DIODEA Zener diode is a silicon junction diode designed for stabilizing, i.e. keeping steady, the output voltage of a power supply (p. 85). One is shown in Fig. 27.la with its symbol in b; the cathode end is often marked by a band. It is made so that it can be used in the breakdown region.

Characteristic: The characteristic in Fig. 27.2 shows that, when forward biased, a Zener conducts at about 0.6 V, like an ordinary silicon diode. However it is normally used in reverse bias. Then, the reverse current IR is

negligible until the reverse p.d. VR reaches a certain value Vz, called the Zener or reference voltage, when it increases suddenly and rapidly as the graph indicates. Damage may occur to the diode by overheating unless IR is limited by a series resistor. If this is connected, the p.d. across the diode remains constant at Vz over a wide range of IR, i.e. part AB of the characteristic is at right angles to the VR axis. It is this property of a Zener diode which makes it useful for stabilizing power supplies.

The value of the current-limiting resistor should ensure that the power rating of the diode is not exceeded. For example, a 400 mW (0.4 W) Zener diode for which Vz = 10 V can pass a maximum current Imax given by: