Transformers step voltages up or down in proportion to their turns. Enter the primary voltage, the turns on each winding and the load current to get the secondary voltage, primary current and power.
Enter the primary voltage.
Enter the primary and secondary turns.
Enter the load current and read the results.
Voltage: Vs ÷ Vp = Ns ÷ Np
Current: Ip ÷ Is = Ns ÷ Np
A transformer changes an AC voltage up or down using two coils wound on a shared iron core. This calculator works with an ideal transformer: you enter the primary voltage, the number of turns on the primary and secondary windings and the current drawn by the load, and it returns the turns ratio, the secondary voltage, the primary current and the load in volt-amperes. It also labels the transformer as step-down, step-up or isolation.
Transformers are everywhere in India's power system, from the large units at generating stations and substations to the distribution transformers on poles and the small ones inside chargers, UPS units, doorbells and control panels. Class 12 physics covers the principle, and electrical and electronics students, ITI trainees and hobbyists use the same relations to choose or wind transformers.
1. Find the turns ratio a = Np ÷ Ns.
2. Find the secondary voltage: Vs = Vp ÷ a, which is the same as Vp × Ns ÷ Np.
3. Find the primary current in an ideal transformer: Ip = Is ÷ a, since power in equals power out.
4. Find the apparent power delivered: S = Vs × Is in VA. Divide by 1000 for kVA.
5. Label the transformer: a > 1 is step-down, a < 1 is step-up, a = 1 is isolation.
6. For a real transformer, choose a rating comfortably above S and allow for losses, which make the primary current slightly higher.
An alternating current in the primary sets up a changing magnetic flux in the core. By Faraday's law, each turn of wire around that core has an EMF equal to the rate of change of flux, dΦ/dt. Both windings share the same flux, so each turn on either side has the same voltage. The total voltage of a winding is therefore proportional to its number of turns: Vs ÷ Vp = Ns ÷ Np. This is also why a transformer does not work on steady DC; unchanging flux induces no voltage.
An ideal transformer has no losses, so the power drawn from the primary equals the power delivered by the secondary: VpIp = VsIs. If the voltage is stepped down by a factor a, the current is stepped up by the same factor. A 230 V to 12 V transformer supplying 5 A draws only about 0.26 A from the mains. This is why power grids transmit at high voltage: the same power at a higher voltage needs less current, and resistive losses fall with the square of current.
Real transformers lose power in winding resistance (copper loss) and in the core through hysteresis and eddy currents (iron loss), and they need a small magnetising current even with no load. Large distribution transformers are typically very efficient, around 97 to 99%, while small ones are less so. Output voltage also sags a little under load. Ratings are given in VA or kVA, not watts, because heating depends on current regardless of the load's power factor.
Ravi is winding a 230 V to 24 V control transformer for a machine panel with 920 turns on the primary and 96 on the secondary, and the panel draws 4 A.
Turns ratio: a = Np ÷ Ns = 920 ÷ 96 = 9.5833
Secondary voltage: Vs = Vp ÷ a = 230 ÷ 9.5833 = 24 V
Primary current (ideal): Ip = Is ÷ a = 4 ÷ 9.5833 = 0.4174 A
Rating: S = Vs × Is = 24 × 4 = 96 VA
Answer: Secondary voltage 24 V; Turns ratio 9.5833 : 1; Primary current 0.4174 A
Inverting the turns ratio and getting a step-up answer for a step-down transformer.
Assuming the current changes in the same direction as the voltage.
Choosing a transformer rated exactly at the load VA, leaving no margin for losses or inrush.
Rating a transformer in watts and forgetting the power factor of the load.
Expecting a transformer to work on DC from a battery.
Class 12 physics problems on transformers and electromagnetic induction.
Choosing control transformers for machine panels and doorbells.
Understanding power adaptors, UPS and inverter transformers.
Explaining why power is transmitted at high voltage across the grid.
Designing and winding small transformers in electronics hobby projects.
Is a real transformer ideal?
No. Real transformers are about 95–99% efficient, so primary current is slightly higher.
Does a transformer work on DC?
No, it needs a changing current to induce voltage.