Zero cross circuit
Encyclopedia
A zero cross 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 triac
s 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 thyristor
s 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.
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
load voltage at close to zero-phase. This is in relation to solid state relays, such as triac
Triac
Triac may refer to:* TRIAC , an electronics component* Triac , a green vehicle* Tiratricol, a common thyroid hormone analogue used for treating thyroid hormone resistance syndrome...
s 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 thyristor
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
s 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
Phase angle
In the context of vectors and phasors, the term phase angle refers to the angular component of the polar coordinate representation. The notation A\ang \!\ \theta, for a vector with magnitude A and phase angle θ, is called angle notation.In the context of periodic phenomena, such as a wave,...
circuits are more sophisticated and more expensive than zero cross circuits.