What is a Ferroresonant Power Supply?

A ferroresonant power supply is a transformer-based power supply that uses nonlinear magnetic properties and a resonant circuit to provide a stable output voltage over a wide range of input voltage. Ferroresonant power supplies are widely used in applications requiring constant output voltages and especially when the input voltage is likely to vary due to instabilities and other power line disturbances. The power supplies can also absorb most of the power line induced transients.

The power supply is similar to an unregulated power supply except for the inclusion of the ferroresonant transformer, specially designed to maintain the voltage constant over a wide range of input voltage, and load current variations.

The ferroresonant transformer consists of an auxiliary secondary winding with a parallel capacitive tank to provide a resonant circuit at the supply voltage frequency. The transformer operation is based on the ferroresonance behavior associated with saturated iron cores in which further increase in the winding current results in negligible of no increase in the magnetic flux.

What is a Ferroresonant Power Supply

Figure 1: Ferroresonant transformer

The regulation is achieved through the saturation characteristics of the transformer core together with the capacitive tank circuit. The tank circuit stores energy in the form of AC oscillation, this energy sustains the output voltage for brief periods in the order of milliseconds in the event of a supply losses or variation. In addition, the tanks circuit acts s a filter to reject harmonics generated by the transformer core saturation.

Advantages of ferroresonant power supply

  1. Simple design, reduced components, robust and reliable
  2. Self- current-limiting and self regulating
  3. Regulated output voltage over a wide range of input voltages. The output remains constant even at ±40% input voltage or more of the nominal voltage
  4. Ideal for powering high crest factor or non-linear loads
  5. Provides isolation and protection from AC line voltage spikes, surges and noise. The primary and secondary windings of the transformer are physically isolated by a steel core. The shunting reduces the capacitive coupling of noise and spikes
  6. Parallel connection possible for high power equipment


The supply is very sensitive to line frequency changes, dissipates more heat than conventional transformers, produces more audible noise at resonance, and is heavier, than the linear regulated supplies.

Applications of ferroresonant power supply

This type of supply is used on applications requiring supply and isolation from mains disturbances. These include:

  • High Reliability Power Supplies
  • Inverter & UPS Systems
  • Battery Chargers
  • Line Voltage Stabilizers
  • Cable television, gas and other heavy industries

The ferroresonant power supply systems provide reliable regulation even under extreme operating conditions, and are not affected by surrounding electrical systems. The resonant regulators provide a constant average output voltage even with wide variations of the input voltage. However regulation is not as precisely regulated as in the switched mode or series regulated power supplies, but the output voltages are precise and constant enough for many applications.