Zero cross circuit

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A zero cross circuit (or zero crossing circuit) is an electrical circuit that starts operation with the AC load voltage at close to zero-phase. This is in relation to solid state relays, such as triacs and silicon controlled rectifiers. The purpose of the circuit is to start the controlled triac conducting as soon as possible, so that the input and output voltages and waveforms are as close as possible. This is useful when the triac is used to control outlets, motors, ballasted lights, or other loads where voltage drops or waveform clipping could cause ill effects.

The point where the line voltage is 0 V is the Zero Cross Point. When a triac is connected in its simplest form, it can clip the beginning of the voltage curve, due to the minimum gate voltage of the triac. A zero cross circuit works to correct this problem, so that the triac functions as well as possible. This is typically done with thyristors in two of the three phases.

Many opto-triacs come with zero cross circuits built in. They are often used to control larger, power triacs. In this setup triac turn-on delays will compound, so quick turn on times are important.

The corresponding phase angle circuits are more sophisticated and more expensive than zero cross circuits.