A zero cross circuit is an electrical circuit that detects the instant when a sine wave, or the natural format of alternating current (AC), is at zero volts in amplitude and sends a signal to its controlled circuit. It is very useful in preventing high-surge currents for protecting resistive loads such as incandescent lamps and heaters and in preventing high-surge currents that generate electromagnetic interference to electronic circuits. The zero cross circuit detects the power line voltage two times during the cycle and makes sure the instantaneous power line voltage is zero before engaging the power switch. Without the zero cross circuit, the switch could engage at a peak voltage level that causes an abrupt high-surge current. Moreover, the zero cross circuit may also ensure that the AC load is switched on early enough in the voltage cycle to obtain full power from the AC supply.
Electromechanical relay circuits do not benefit from zero cross circuits because the relay contact cannot close fast enough to achieve low resistance while the AC power from the mains is at zero, which is why relay drivers do not detect zero-phase. Semiconductor switches, on the other hand, are able to switch very fast, so these devices benefit from the signal from a zero cross circuit. Silicon controlled rectifiers (SCRs) are electronic power switches that behave much like ordinary diodes, but unlike ordinary diodes, SCRs need a trigger signal before forward conduction will take place. When the trigger event occurs, the SCR latches “on” while the current is above its holding current. With a power diode bridge, an SCR may work in bidirectional mode and be able to switch full AC power into AC loads.
The triode for alternating current (TRIAC) is a three-terminal semiconductor switch for AC applications that is very much like an electromechanical relay because it conducts currents in both directions. It differs from the SCR control, as the TRIAC trigger is also bidirectional, which triggers the TRIAC every time the voltage is at zero-phase. There are optical isolator devices that are designed to simplify the provision of TRIAC trigger. Optical isolators promote safety by separating the main power circuit from the control circuit. There are even zero-cross-firing optical isolators that take care of zero cross detection.
Zero cross circuits become a bit complicated with reactive loads. Resistive loads will have voltages and currents that are in phase. Phase angle circuits are needed to process the trigger voltages for non-resistive loads, which may be inductive or capacitive. For instance, AC motors are inductive due to the windings used for field and rotor windings of these devices.
The AC load voltage in inductive loads leads the current. In a zero cross circuit, the zero cross of interest is the current that has to be delayed with reference to the input voltage. Most inductive load compensation circuits would provide a control voltage to the zero cross circuit that is delayed by an angle equal to the current lag across the load.