We are all very familiar with the idea and concept of transformers. In particular the step down transformers that are used to power low power electronic devices, usually D.C from a mains A,C source of a much higher voltage. These generally consist of a transformer to reduce the voltage and a bridge rectifier and signal smoothing circuitry such as the LCR filters previously detailed, to create D.C from A.C. Rectification will be looked at briefly in a later chapter.
A pair of coils (called windings) wound around a core (often ferromagnetic, though other core materials are used).
The primary winding (input side): When AC flows through it, it generates a varying magnetic flux.
The secondary winding (output side): The magnetic flux from the primary induces an AC voltage in the secondary.
Air core: Simple but low-efficiency (poor flux coupling); used in low-power, high-frequency scenarios.
Laminated steel core: The most common type, made of stacked steel plates separated by a thin non-conductive layer (e.g., heat-resistant varnish). This design reduces electrical losses (eddy currents) and improves flux coupling between windings.
Solid core: Rare, as solid materials amplify eddy current losses; only used in specialized low-frequency applications.
Toroidal core: A cylindrical core with windings (typically copper wire) wrapped around its circumference. It minimizes both electrical and flux losses, enabling highly efficient flux coupling—ideal for high-performance scenarios.
The turn ratio (primary windings : secondary windings) determines the voltage/current transformation ratio. For an ideal transformer (no losses), the relationship is 
/
For example with a perfect transformer supplied with an input voltage of 110 volts A.C on the input and 100 coils on the primary and 50 coils on the secondary
we get : Vs = 110 * 50 / 100 = 55
This shows us that varying the number of turns on the coils will change the voltage at the secondary coil. Obviously if we have the same number on the primary and secondary coil there will be no change, other than of course a reduction at the secondary due to physical losses.
Oftentimes the secondary coil will have what are called taps at different points to essentially vary the number of turns that are used on the secondary coil and thus vary the output voltage. For example if we have a transformer with a primary coil of 100 turns, and then the secondary coil has 100 turns but we make a connection at 50 turns, what would be called a centre tapped transformer we will, given a perfect transformer, be able to tap off voltages at both the same value as the input voltage and also half of that.
This idea can be extended to any number of different tap points on the secondary and allow us to access a continuous spectrum of voltages between zero and the input voltage, remembering that losses will always mean that we can never get the same voltage out as we put in.
