Practical takeaway
A transformer transfers AC energy magnetically between windings. The turns ratio determines the ideal voltage ratio; the current ratio changes in the opposite direction.
Electricity & Electronics
Transformers
Use turns ratio to see how an AC transformer changes voltage and current relationships.
TECHNICIAN FOUNDATION13 of 16
Technician exam focus
Know the basic function of a transformer, recognize the schematic symbol, and understand step-up versus step-down action.
Primary, secondary, and turns ratio
AC in the primary creates a changing magnetic field.
That changing field links the secondary winding and induces an AC voltage there. Ordinary transformer action depends on changing current, not steady DC.
The turns ratio sets the voltage ratio.
In the ideal model, Vs / Vp = Ns / Np. More secondary turns means a higher secondary voltage; fewer secondary turns means a lower secondary voltage.
Current changes in the opposite direction.
Ignoring losses, power is approximately conserved. If voltage is stepped down, the secondary can supply proportionally more current; if voltage is stepped up, available current is proportionally lower.
Why hams care
Transformers appear in power supplies, audio circuits, impedance matching, RF coupling, baluns, and many other radio applications.
STEMulation
Change the turns ratio
Primary voltage
Secondary voltage
Example secondary current*
*Ideal teaching model assumes 1.00 A primary current and ignores transformer losses. Real transformers have ratings and losses that limit voltage, current, power, and frequency.