Transformer, Turns Ratio & Power

A transformer works by mutual induction. Alternating current in the primary sets up an alternating flux in the laminated core, and because that flux is continually changing it induces an EMF in every turn of both windings. Each turn sees the same flux, so the induced voltage is simply proportional to the number of turns: V_s/V_p = N_s/N_p. There is no electrical connection between the windings at all, which is why a transformer also provides isolation.

An ideal transformer neither creates nor destroys energy, so V_pI_p = V_sI_s. Stepping the voltage up therefore steps the current down by the same ratio, and vice versa. That is the whole basis of power distribution: transmit at high voltage and low current to keep I²R losses small, then transform down for use.

Because induction depends on a changing flux, a transformer cannot work on DC. Apply DC and the flux rises once to a steady value and then stops changing, so the secondary voltage collapses to zero. Worse, only the winding's own small resistance now limits the current, so the primary can overheat and burn out. The core is laminated, with thin insulated sheets, precisely to break up the eddy currents that the changing flux would otherwise drive through a solid core.

• Try this: Set N_s greater than N_p to step up. What happens to the secondary current relative to the primary, and why must it?
• Try this: Press Apply DC instead. Why does the secondary voltage fall to zero even though a large voltage is still applied to the primary?
• Try this: Reduce the load resistance. Which current rises first, and how does the primary "know" that the secondary has been loaded?