Power TransferAnd Efficiency.

Basic Electronics
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internal resistance of the source. This theory is illustrated in the table

and the graph of figure 41 (on the previous page). When the load resistance

is 5 Ohms, matching the source resistance, the maximum power of 500 watts is

developed in the load.

The efficiency of power transfer (ratio of output power to input power) from

the source to the load increases as the load resistance is increased. The

efficiency approaches 100 percent as the load resistance approaches a

relatively large value compared with that of the source, since less power is

lost in the source. The efficiency of power transfer is only 50 percent at

the maximum power transfer point (when the load resistance equals the

internal resistance of the source).

The efficiency of power transfer

approaches zero efficiency when the load resistance is relatively small

compared with the internal resistance of the source. This is also shown on

the chart of figure 41 on the previous page.

The problem of a desire for both high efficiency and maximum power transfer

is resolved by a compromise between maximum power transfer and high

efficiency.

Where the amounts of power involved are large and the

efficiency is important, the load resistance is made large relative to the

source resistance so that the losses are kept small.

In this case, the

efficiency is high.

Where the problem of matching a source to a load is

important, as in communications circuits, a strong signal may be more

important than a high percentage of efficiency.

In such cases, the

efficiency of power transfer should be only about 50 percent; however, the

power transfer would be the maximum which the source is capable of

supplying.

You should now understand the basic concepts of series circuits.

The

principles which have been presented are of lasting importance.

Once

equipped with a firm understanding of series circuits, you hold the key to

an understanding of the parallel circuits to be presented next.