Power Supply: Definition, Functions & Components

Definition: A power supply is an electronic circuit designed to provide various ac and dc voltages for equipment operation.

Proper operation of electronic equipment requires a number of source voltages. Low dc voltages are needed to operate ICs and transistors. High voltages are needed to operate CRTs and other devices. Batteries can provide all of these voltages.

However, electricity for electrical and electronic devices are commonly supplied by the local power company. This power comes out of an outlet at 115-volt ac, with a frequency of 60 Hertz. Different voltages are needed to operate some equipment.

Power Supply Functions

The complete power supply circuit can perform these functions:

1. Step voltages up or step voltages down, by transformer action, to the required ac line voltage.
2. Provide some method of voltage division to meet equipment needs.
3. Change ac voltage to pulsating dc voltage by either half-wave or full-wave rectification.
4. Filter pulsating dc voltage to a pure dc steady voltage for equipment use.
5. Regulate power supply output in proportion to the applied load.

Power Supply Components

A block diagram illustrating these functions is shown in Figure 1. Note that certain functions are not found in every power supply. See Figure 2 for a typical commercial power supply components.

Figure 1. Block diagram for power supply components. Input is 117 volts ac. Processes used in a typical power supply are shown below the blocks. The output of the power supply can be dc or ac. The output of this supply is five volts dc.

Figure 2. Regulated dc power supply. (Knight Electronics)

Power Transformers Diodes

The first device in a power supply is the transformer. Its purpose is to step up or step down alternating source voltage to values needed for radio, TV, computer, or other electronic circuit use.

Most transformers do not have any electrical connection between the secondary and primary windings. See Figure 3. This means that the transformer isolates the circuit connected to the primary from the circuit connected in the secondary.

Figure 3. Isolation in a transformer.

Using an isolation transformer is a safety feature because it helps prevent shocks in the secondary. Our body or hands must be joined across both leads of the secondary connections in order to receive a shock.

The safety condition described above does not hold true in the primary with commercial ac provided by the power company. One connection is hot, which means that the connection is electrically energized. The other is grounded, or neutral. Standing on the ground while touching the hot connection will result in a shock. Touching the ground connection alone will not result in a shock.

Many power supplies use a center tap secondary transformer winding. The tapped voltages, Figure 4, are 180 degrees out of phase with respect to the center tap.

A variety of transformers can be found in nearly all electronic devices. You should understand the basic theory and purpose of the transformer. Review Chapter 12 if necessary.

A Lesson in Safety

Transformers produce high voltages that can be very dangerous. Proper respect and extreme caution must be used at all times when working with, or measuring, high voltages.

Figure 4. A center tap transformer.

Half-Wave and Full-Wave Rectification

After a voltage has gone through a power supply’s transformer, the next step is rectification.

When changing an ac signal to dc, there are two types of rectification: half-wave rectification and full-wave rectification.

With the half-wave rectifier, only half of the input signal passes on through the rectifier. With the full-wave rectifier, the entire input wave is passed through.

Half-Wave Rectification

In Figure 5, the output of a transformer is connected to a diode and a load resistor that are in series. The input voltage to the transformer appears as a sine wave.

The polarity of the wave reverses at the frequency of the applied voltage. The output voltage of the transformer secondary also appears as a sine wave. The magnitude of the wave depends on the turns ratio of the transformer. The output is 180 degrees out of phase with the primary.

The top of the transformer (point A) is joined to the diode anode. Note that the B side of the transformer is connected to ground.

During the first half cycle, point A is positive. The diode conducts, producing a voltage drop across resistor R equal to IR. During the second half cycle, point A is negative. The diode anode is also negative. No conduction takes place, and no IR drop appears across R.

Figure 5. Basic diode rectifier schematic.

An oscilloscope connected across R produces the waveform shown to the right in Figure 6. The output of this circuit consists of pulses of current flowing in only one direction and is at the same frequency as the input voltage. The output is a pulsating direct current.

Figure 6. Input and output waveforms of a diode rectifier.

Only one half of the ac input wave is used to produce the output voltage. This type of rectifier is called a half-wave rectifier.

Look at the polarity of the output voltage in Figure 6. One end of the resistor R is connected to ground. The current flows from the ground to the cathode. This connection makes the end of R connected to the cathode positive as shown in Figure 5.

A negative rectifier can be made by reversing the diode in the circuit, Figure 7. The diode conducts when the cathode becomes negative causing the anode to become positive.

The current through R would be from the anode to ground making the anode end of R negative and the ground end of R more positive.

Voltages taken from across R, the output, would be negative with respect to ground. This circuit is called an inverted diode. It is used when a negative supply voltage is required.

Figure 7. An inverted diode produces a negative voltage.

It is possible to have a power supply that provides half-wave rectification without the use of a transformer. This circuit is not isolated. There is no step up or step down of current voltages. This circuit is a simpler, less costly design, and since there is no transformer, it can be used in smaller spaces, Figure 8.

Figure 8. Half-wave rectification without a transformer.

Full-Wave Rectification

The pulsating direct voltage output of a half-wave rectifier can be filtered to a pure dc voltage. However, the half-wave rectifier uses only one half of the input ac wave.

A better filtering action can be obtained by using two diodes. With this setup, both half cycles of the input wave can be used.

Both half cycles at the output have the same polarity in this full-wave rectifier. Figure 9 follows the first half cycle. Figure 10 follows the second half cycle.

Figure 9. Arrows show current in full-wave rectifier during the first half cycle.

Figure 10. The direction of current during the second half cycle.