Chapter Experiment Half and Full Wave Rectification

This experiment in half and full wave rectification begins your study of AC or alternating current energy principles. Alternating current, as opposed to direct current, which you studied in the first three experiments, is used primarily for the transmission of high voltage electrical power over long distances. In addition, low voltage alternating current is used to directly power many common household devices such as light bulbs and motors. While your television and computer are also powered by conventional 110 volt, 60-cycle AC house current, they both need a device to convert the AC voltage to DC. This device is called a power supply, and all power supplies use a principal called "rectification" to convert AC to DC, which is what this experiment is all about.

Rectification = Modifying a signal so that either the positive or the negative portions of the

signal are eliminated. There are two types of rectification, half-wave and full-wave.

Half-wave rectification = Only the positive (or negative) parts of a signal remain, the other

portions having been truncated:

Full-wave rectification = Half of the signal is inverted, so the entire signal is either positive


(or negative).

Unrectified Signal Half-wave rectification Full-wave rectification

In this context the word rectification means, "to change"; that is, to change a signal from alternating current (AC) to direct current (DC). The two basic forms of rectification, half wave and full wave, both use a diode, like the one we used in our Battery Charger circuit, to begin the conversion from AC to DC. You can think of a diode as a kind of one-way valve that allows current to flow in only one direction, from positive to negative. Engineers call this "forward-biasing" the diode. There are a wide variety of diodes ranging from small signal diodes used in transistor radios to power diodes that can pass huge amounts of current as is needed for medium to large power supplies. Regardless of the type, every diode has a positive anode and negative cathode, like the diode pictured in Figure 5-1; current flows from the positive anode "A" to the negative cathode "B"

Silicon Diode Conducting (Breakdown) Voltage Point

Reversed Biased

Forward Biased

Figure 5-1: Diode Diagram and Signal Characteristics

And because most small signal diodes, like the ones we use in this text, are made from silicon, a certain minimum voltage must be applied before the diode "conducts" or begins to pass current from anode to cathode. In the case of a silicon diode this minimum "breakdown" voltage is between 0.6 to 0.7 volts. Diodes made from germanium, another semiconductor material, have a lower breakdown or conducting voltage of around 0.4 volts. Both types are used in electronic circuits, and the choice of one over the other is usually a decision based on either lower-cost (silicon) or lower-conducting voltage (germanium); a lower-conducting voltage may be necessary for the other parts of the electronic circuit to function correctly thus justifying the extra cost. Generally, germanium diodes are more expensive as compared with silicon diodes; however both have their place in modern electronics. You are using silicon diodes for the experiments in this text.

If you're familiar with power supplies as a product, you would think by now that every possible power supply has already been developed and is on the market for sale. That may be true up to a point; however industry is constantly demanding more efficiency (power) from power supplies for less money spent. Remember, you can find a power supply in every computer, computer monitor, television and stereo that uses regular house current for its primary source of input power. Because of the huge volumes that are sold, this "economy of scale" serves to drive the cost of power supplies down lower and lower each year. At the same time, engineers are hard at work to make power supplies even more efficient and inexpensive. So in spite of current-day technologies being so advanced as viewed in the context of the present, much more is left to be done in terms of electronic design and cost reductions in the very near future. Therefore, the power electronic industry has an ongoing demand for clever power supply designers. Perhaps this experiment in half and full wave rectification will inspire a career in the power electronics industry for you. We hope so, because such a career is both exciting and rewarding.

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DIY Battery Repair

DIY Battery Repair

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