A transformer consists of a coil of 1200 turns and another coil, with a total of 120 turns, which can be tapped at various places. Primary voltage is 240 V. At which pair of terminals would you connect to a 12 V, 24 V lamp for it to be lit normally?

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Solution

Secondary terminal

The given transformer is a step down transformer as the number of turns in the secondary is less than primary. So, the voltage gets reduced.
Given,
Number of turns in the primary, $N_{1}$ = 1200
Number of turns in the secondary, $N_{2}$ = 120
primary voltage, $V_{1}$ = 240V
We have,

$\frac{N_{1}}{N_{2}} = \frac{V_{1}}{V_{2}}$
Where,
$N_{1}$ is the number of turns in the primary winding
$N_{2}$ is the number of turns in the secondary winding
$V_{1}$ is the voltage across the primary winding
$V_{2}$ is the voltage across the secondary winding
Substituting the given values in the equation we get,

$\frac{1200}{120} = \frac{240}{V_{2}}$

$1200 V_{2} = 28800$

$V_{2} = \frac{28800}{1200} = 24 V$

So the lamps will connect as secondary side as the secondary voltage is 24V only.

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