Transformer Calculator: Turns Ratio, Voltage, and Current Calculations
Calculate transformer turns ratio, secondary voltage, secondary current, and power for step-up and step-down transformers. Essential for power electronics and circuit design.
Transformers are fundamental to power electronics — they step voltage up or down while conserving power (with some losses). Whether you're designing a power supply, calculating secondary winding requirements, or figuring out current limits, transformer math follows directly from the turns ratio.
The CalcHub Transformer Calculator calculates turns ratio, secondary voltage and current, and apparent power for ideal and real-world transformers.
The Transformer Equations
For an ideal transformer:
Turns ratio: N = Np ÷ Ns = Vp ÷ Vs = Is ÷ Ip Secondary voltage: Vs = Vp × (Ns ÷ Np) Secondary current: Is = Ip × (Np ÷ Ns) Power (ideal): Pp = Ps → Vp × Ip = Vs × IsWhere:
- Np, Ns = number of primary, secondary turns
- Vp, Vs = primary, secondary voltages
- Ip, Is = primary, secondary currents
Step-Up vs. Step-Down
| Type | Turns Ratio (Np:Ns) | Voltage | Current |
|---|---|---|---|
| Step-down | >1 (e.g., 10:1) | Decreases | Increases |
| Step-up | <1 (e.g., 1:10) | Increases | Decreases |
| Isolation | 1:1 | Same | Same |
Worked Example: 120V to 12V Power Supply Transformer
Primary: 120V AC, 2A input
Target secondary: 12V
If the transformer is rated at 100VA instead:
- Maximum secondary current: 100VA ÷ 12V = 8.33A
- Maximum primary current: 100VA ÷ 120V = 0.83A
Real-World Efficiency
Real transformers have losses — core losses (eddy currents and hysteresis) and copper losses (I²R heating in windings). Typical efficiencies:
| Transformer Type | Efficiency |
|---|---|
| Large utility transformer | 95–99% |
| Medium power supply | 90–96% |
| Small wall-wart transformer | 70–90% |
| Toroidal transformer | 92–97% |
For a 240VA primary with 90% efficiency: secondary delivers 216W.
VA Rating vs. Wattage
Transformer power is rated in volt-amperes (VA), not watts, because transformers handle reactive power from inductive and capacitive loads. For resistive loads, VA = watts. For inductive loads (motors), actual watts = VA × power factor (typically 0.6–0.85).
Always size your transformer VA rating higher than the load's actual watt requirement when driving inductive loads.
Common Transformer Applications
| Application | Type | Notes |
|---|---|---|
| AC adapter/charger | Step-down | Followed by rectifier + regulator |
| Audio amplifier output | Impedance matching | Maximizes power transfer |
| Distribution grid | Step-up (generation) | 10kV–500kV transmission |
| Distribution (local) | Step-down | 11kV to 120/240V service |
| Isolation transformer | 1:1 | Safety, noise isolation |
| High-voltage igniter | Step-up | Pulse type, car ignition |
What is an autotransformer and how does it differ?
An autotransformer uses a single winding with a tap, rather than two separate isolated windings. It's more compact and efficient for small ratio changes (say, 120V to 100V) but provides no galvanic isolation. A Variac (variable autotransformer) is a common lab instrument.
How do I calculate the number of turns needed for a custom transformer?
You need to know the core material and cross-sectional area. The turns-per-volt formula is: Turns/V = 10⁸ ÷ (4.44 × F × Bmax × Ac), where F is frequency, Bmax is peak flux density for the core material, and Ac is core area in cm². This is core selection territory — the calculator handles voltage and current ratios once you have the core specified.
Why do transformers hum?
The characteristic 50/60Hz hum comes from magnetostriction — the core material physically expands and contracts with the alternating magnetic field. Loose laminations amplify this. A louder hum than usual can indicate an overloaded or partially saturated core.
Related Tools
- Power Consumption Calculator — load analysis for transformer sizing
- Series-Parallel Resistance Calculator — impedance matching analysis
- Electrical Load Calculator — total system load