How a transformer works
A transformer changes the size of an alternating (a.c.) voltage. It has two coils, a primary and a secondary, wound on a soft-iron core. An alternating current in the primary sets up a continually changing magnetic flux in the core. This changing flux passes through the secondary coil and, by electromagnetic induction, induces an alternating e.m.f. in it. A transformer works only with a.c., not d.c.
Key formula
Np / Ns = Vp / Vs
For an ideal transformer: Vp Ip = Vs Is (power in = power out).
Np, Ns = turns on primary/secondary; V = voltage; I = current.
Step-up and step-down
If the secondary has more turns than the primary (Ns > Np), the output voltage is higher — a step-up transformer. If it has fewer turns (Ns < Np), the voltage is lowered — a step-down transformer. For an ideal (100% efficient) transformer, raising the voltage lowers the current in the same proportion, since power is conserved.
Worked example
A transformer has 1000 turns on the primary and 50 turns on the secondary. The primary voltage is 240 V. Find the secondary voltage, and the secondary current if the primary current is 0.5 A (assume ideal).
Vs = Vp × Ns/Np = 240 × 50/1000 = 12 V.
Vp Ip = Vs Is → Is = (240 × 0.5)/12 = 10 A. This is a step-down transformer.
Remember
- Transformers work on a.c. only.
- Step-up: more secondary turns, higher voltage, lower current.
- Real transformers lose energy as heat in the core and coils, so efficiency is below 100%.