Transformer (power)
Transformer
A device that transfers electrical energy between circuits through electromagnetic induction, changing AC voltage and current levels while maintaining frequency.
Schematic symbol
Transformer
How transformer (power) appears in a circuit diagram.
Types & variants
Key specs
Primary voltage
V (AC)Input voltage the transformer is designed to handle.
Secondary voltage
V (AC)Output voltage at no load. Actual voltage under load may be lower.
VA rating
VA (Volt-Amp)Maximum power the transformer can handle. For resistive loads, VA ≈ Watts.
Frequency
HzOperating frequency (typically 50/60Hz for power transformers, higher for SMPS).
Efficiency
%Power output vs power input. Higher = less energy wasted as heat.
Isolation voltage
VMaximum voltage primary and secondary can withstand without breakdown.
Markings
Usually printed with primary/secondary voltages, VA rating, frequency, and model number. Color dots may indicate phasing. Toroids often have no markings - refer to datasheet.
Standard values
Common primary voltages: 120V, 230V. Common secondary voltages: 6V, 9V, 12V, 24V. Common VA ratings: 5VA, 10VA, 25VA, 50VA, 100VA.
How to choose
1) Determine input voltage (primary) and required output voltage (secondary). 2) Calculate required VA rating: VA = Secondary Voltage × Max Current. 3) Choose frequency (50/60Hz for line power). 4) Consider efficiency for power-sensitive applications. 5) Verify isolation voltage if safety is critical.
Pinout & package
Primary and secondary windings (typically 2-4 pins). Common packages: enclosed metal or plastic case with wire leads, PCB-mount with pins, toroid with no leads. Pinout varies - always check datasheet.
Example circuits
- AC to AC voltage step-down for power supplies
- Isolation transformer for safety
- Multiple secondary outputs for different voltage needs
- Phase splitter in audio applications
Common failures
Open windings (no continuity), shorted windings (low resistance), insulation breakdown (visible burn marks), increased hum from core damage. Failure often caused by overheating or voltage spikes.
How to test
1) Check continuity between primary and secondary pins (should be infinite resistance). 2) Measure resistance of primary and secondary windings (should be low but finite). 3) Apply low AC voltage to primary and measure secondary voltage (should match turns ratio). 4) Check for hum or overheating when powered.
Substitutes
Use transformer with same primary voltage, secondary voltage within ±10%, and VA rating equal to or higher than required. For isolation, isolation transformer is essential. For step-down, autotransformer may be used but doesn't provide isolation.