The transformer power equation

A transformer does not create energy. For a transformer that is 100% efficient, the electrical power put into the primary coil equals the electrical power taken out of the secondary coil. Electrical power is potential difference multiplied by current, so:

V_P × I_P = V_S × I_S

Here V_P is the potential difference across the primary coil in volts (V), I_P is the current in the primary coil in amperes (A), V_S is the potential difference across the secondary coil in V and I_S is the current in the secondary coil in A. Power is measured in watts (W).

Worked example: a step-down transformer has V_P = 230 V and I_P = 2 A, so the input power is 460 W. The secondary has V_S = 23 V, so I_S = 460 ÷ 23 = 20 A. The voltage went down by a factor of 10 and the current went up by a factor of 10. A step-up transformer does the opposite: when the voltage goes up, the current goes down. For example, a step-up transformer with V_P = 25 V and I_P = 4 A has an input power of 100 W. With V_S = 250 V, I_S = 100 ÷ 250 = 0.4 A.