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 0° (or 180°) 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 triac conducting while the voltage is crossing zero volts (starting and middle of each Hertz cycle), so that the output voltage is in complete sine-wave half-cycles. This is useful when waveform clipping could cause ill effects like high frequency spikes for which the circuit or environment is not designed to handle gracefully.

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.