Investigating the Output of a Transformer

The relationship between the primary and secondary voltages ($V_p$ and $V_s$) and the number of turns ($N_p$ and $N_s$) is defined by the ratio formula:

$\frac{V_p}{V_s} = \frac{N_p}{N_s}$

This equation implies that the voltage per turn is constant across both coils in an ideal scenario, meaning $V_s = V_p \times (\frac{N_s}{N_p})$.

When investigating the output, if $V_p$ and $N_p$ are held constant, the secondary voltage $V_s$ becomes directly proportional to the number of secondary turns $N_s$.

• Independent Variable: The number of turns on the secondary coil ($N_s$), which is varied systematically (e.g., in increments of 20 turns).

• Dependent Variable: The secondary voltage ($V_s$), measured using an a.c. voltmeter across the secondary terminals.

• Control Variables: The primary voltage ($V_p$), the number of primary turns ($N_p$), and the frequency of the a.c. supply must remain constant to ensure a fair test.

• Procedure: Wrap a fixed number of turns for the primary, then measure the output voltage for different counts of secondary turns, ensuring the iron core pieces (C-cores) are in tight contact to maximize flux linkage.

A graph of secondary voltage ($V_s$) on the y-axis against the number of secondary turns ($N_s$) on the x-axis should yield a straight line passing through the origin.

The gradient of this line represents the constant ratio $\frac{V_p}{N_p}$, which is the voltage supplied per turn of the primary coil.

Linearity in the graph confirms the direct proportionality predicted by the transformer equation, assuming minimal energy losses.

In a step-up transformer, the output voltage is higher than the input, whereas in a step-down transformer, the output voltage is lower than the input.