Two ways of running a low-voltage lamp from the mains
The following example illustrates the advantage of a.c. over d.c. when it is necessary to reduce the available voltage to a lower value.
Suppose we wish to run a 24 V 12 W lamp from 240 V a.c. mains. The required voltage may be obtained by using a 10: 1 step-down transformer. The current taken by the lamp may be calculated from the formula, Let us now consider how this lamp could be run from 240 V d.c. mains. In this case it would be necessary to connect a limiting resistor in series with the lamp, the
value of which is calculated as follows: The p.d. across the lamp must be 24 V. Hence the p.d. across the limiting resistor
must be (240 – 24) = 216 V. Since lamp and resistor are in series. the same current, 0.5 A, passes through both.