Does changing the input frequency of a transformer rectifier change the output voltage, given that the input voltage is constant with no voltage regulator or protection circuit on the output?


2 Answers 2


Assuming all components are perfect, and there are no other components involved, the answer is no.

However, you will usually have an output filter/resevoir capacitor. If the input frequency is increased, this will droop less between half-cycles, and so increase output voltage slightly.

The transformer may have a low primary inductance, and be loading the input supply. Raising the frequency will reduce this effect, and raise the output voltage slightly.

The transformer may be close to saturation, and so be loading the input supply. Raising the frequency will reduce this effect.

The transformer may be operating somewhere near to resonance (unlikely if it's iron and mains, but possible if it's ferrite and driven at 10s to 100s of kHz). If so, increasing the input frequency can dramatically alter the output voltage, either up or down, as it swings through resonance.


Assuming a basic system consisting of transformer, rectifier and output filter, we can make the following considerations:

The transformer

Transformers made to operate at line frequency, having a magnetic core (an iron-silicon alloy) material, which can accept a magnetic flux density of about 15000 Gauss or less. This applies to line frequency (50/60 Hz).
When the frequency increases, so do the losses by hysteresis and eddy currents. The effect is that the core accepts less magnetic flux density and saturates quickly, increasing its temperature and losing performance.

The rectifier

Common rectifier diodes used to rectify power (from 1 A to ~ 300 A) have maximum operating frequency in the order of 1 kHz. When this level is exceeded, the response of the diode becomes critical (reverse recovery time), which drastically reduces its output due to the switching.

The output filter

The output filter may be capacitive or inductive-capacitive. In either case, the cutoff frequency is quite low, so input frequency increase implies that the output will be much attenuated by filtering.

In the case that the operation frequency is lower, the rectifier system critical component is the transformer which saturates quickly.


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