Use the terms primary, secondary, step-up and step-down

4.5.6 The Transformer

What is a Transformer?

A transformer is an electrical device that changes the voltage of an alternating current (AC) supply. It does this without converting AC to direct current (DC). Think of it like a water pump that can increase or decrease the pressure of water flowing through a pipe. In a transformer, the “pressure” is voltage, and the “flow” is current.

Key Parts

• Primary winding (🔌): The coil where the input voltage is applied.
• Secondary winding (⚡): The coil that delivers the output voltage.
• Core (🧲): Made of magnetic material that guides the magnetic flux between the windings.

How It Works

When an AC voltage is applied to the primary winding, it creates a changing magnetic field in the core. This changing field induces a voltage in the secondary winding. The ratio of the number of turns in the secondary to the primary determines whether the transformer steps the voltage up or down.

Transformer Equation

The core relationship is:

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

Step‑Up vs Step‑Down

1. Step‑Up Transformer (📈): $N_s > N_p$ → $V_s > V_p$
2. Step‑Down Transformer (📉): $N_s < N_p$ → $V_s < V_p$

Practical Example

Suppose you have a 120 V supply (primary) and you need 12 V for a small appliance. Using the transformer equation:

$$\frac{V_s}{120\,\text{V}} = \frac{N_s}{N_p}$$ If we choose $N_s/N_p = 0.1$, then $V_s = 12\,\text{V}$. This is a step‑down transformer because the secondary voltage is lower than the primary.

Safety Note

Transformers are designed for AC only. Using them with DC can damage the core and create safety hazards. Always check the specifications before connecting a transformer to a circuit.

Quick Quiz

• What happens to the current in the secondary when the voltage is stepped up?
• Why is the core material important for transformer efficiency?
• Give an everyday device that uses a transformer.