The red part in the image represents the power applied
Phase cut is an AC switching principle that reduces the power that can be reduced to a load, a consumable device such as a light bulb or electric motor.
Each half-period of the sinusoidal voltage is only activated a given time after the zero crossing, so that only the last part of the curve is used.
The time elapsed between the zero crossing and the switching moment can be determined by a simple resistor capacitor combination, but can also be sent more advanced from, for example, digital circuits. To make sure that the switching element actually responds to this signal, it is often offered as very short, narrow pulses on the control input.
A disadvantage of phase cuts is that the waveform changes rather, resulting in higher harmonics, which in turn can cause interference in other equipment. This electronics requires additional electronics. For example, a poorly lit dimmer can cause a noise in nearby audio equipment.
An advantage of phase cuts is that hardly any energy is lost in the circuit itself, so that relatively large power can be controlled with a small circuit. In case of power reduction by means of a bias resistor, a part of the total power is absorbed by the resistor, making it hot and cooling.
Phase cutting occurs with thyristors and triacs, possibly accompanied by a diac. It can also be applied to single-sided rectified alternating voltage.
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