1. Working process:
AC zero-crossing solid state relays have the characteristic of turning on when the voltage crosses zero and turns off when the load current crosses zero.
Circuit diagrams, dimensions and drawings of zero-crossing AC solid state relays
When the optocoupler OPT is off (i.e., there is no input signal at the control end of the OPT), M1 is saturated and turned on by obtaining a base current from R2, as a result, the gate trigger voltage (UGT) thyristor SCR is clamped to a low potential and turned off. As a result, the triac BCR is in a shutdown state because there is no trigger pulse on the gate control terminal R6.
When the input control signal is applied to the input of the solid state relay, the phototransistor OPT is turned on (i.e., the control end of the OPT has an input signal). After the supply voltage is divided by R2 and R3, if the voltage at point A is greater than the zero-crossing voltage of M1 (i.e., VA VBE1), M1 will be in saturation conduction and both SCR and BCR thyristors will be turned off. If the voltage at point A is less than the zero-crossing voltage of M1 (i.e. VA
With the above process, it can be understood that the M1 is used as an AC voltage detector to turn on the solid state relay when the load voltage crosses zero and disconnect the solid state relay when the load current crosses zero. And due to the function of the zero-crossing detector, the influence of the load circuit on the load is correspondingly reduced, and the RF interference generated in the control loop is also greatly reduced.
2. Definition of Zero Crossing:
Here it is necessary to explain what a zero cross is. In AC currents, the zero-crossing point is the instantaneous point at which voltage is not present, i.e., the junction between the positive and negative half-cycles of the AC waveform. In every AC cycle, there are usually two zero-crossing points. If the power supply switches instantaneously at zero crossing, there will be no electrical interference. When the input terminal is connected to the control signal and the output AC voltage crosses zero, the AC solid state relay (equipped with zero-crossing control circuit) will be in the ON state; Conversely, when the control signal is off, the SSR is in the OFF state until the next zero crossing.
In addition, it should be noted that the zero-crossing point of the solid-state relay does not actually mean zero volts of the supply voltage waveform. The diagram is a part of the AC voltage sine wave. According to the characteristics of the AC switching element, the AC voltage in the figure is divided into three regions corresponding to the three states of the SSR output circuit. U1 and U2 represent the threshold voltage and saturation voltage of the switching element, respectively.
The diagram shows the I/O waveform of a zero-crossing solid state relay. And due to the nature of the thyristor, the solid state relay will be turned on after the voltage of the output terminal reaches the threshold voltage (or the trigger voltage of the trigger circuit). The solid state relay will then be in a physically turned on state after reaching the saturation voltage, while producing a very low on-state voltage drop. If the input signal is turned off, the solid state relay will turn off when the load current falls below the thyristor's hold current or the next AC commutation point (i.e., the load current passes zero for the first time after the SSR relay is turned off).
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