In solid-state relays, the RC protection circuit mainly plays a role in suppressing transient voltage and current, absorbing spike pulses, and reducing RF interference.

The specific working process is to utilize the combined characteristics of resistance (R) and capacitance (C). When transient voltage or current spikes occur in the circuit, capacitors quickly charge and absorb some energy, thereby limiting the speed and amplitude of voltage rise. At the same time, resistors can act as current limiters, controlling the charging current of capacitors and avoiding excessive surge currents.

In this way, the RC protection circuit can protect the semiconductor switching elements (such as thyristors, bidirectional thyristors, etc.) inside the solid-state relay from damage caused by excessive transient voltage and current, enhance the stability and reliability of the solid-state relay, and extend its service life.

In communication solid-state relays, there may also be a "zero crossing control circuit". The so-called 'zero crossing' refers to the solid-state relay being in the on state when the AC voltage crosses zero when a control signal is added; And when the control signal is disconnected, the solid-state relay will wait for the junction point (zero potential) of the positive and negative half cycles of the AC power before it becomes disconnected. This design can prevent interference from high-order harmonics and pollution to the power grid.

In addition, some solid-state relays may also include absorption circuits, such as "R-C" series absorption circuits or nonlinear resistors (varistors), to further prevent spikes and surges (voltages) from the power supply from impacting and interfering with the bidirectional thyristor of the switching device (even causing misoperation).

Taking a typical AC type solid-state relay as an example, its working principle is roughly as follows: the control signal of the input circuit makes the photosensitive transistor in the optocoupler conduct, thereby triggering the subsequent circuit. The AC voltage detection transistor plays a role in detecting AC voltage in the circuit. When there is no signal input at the control end, the transistor saturates and conducts, causing the trigger voltage of the thyristor gate to be clamped at a low potential and in an off state, ultimately resulting in the bidirectional thyristor also being in an off state; When there is a control signal input, the working state of the thyristor is determined by the AC voltage zero point detection transistor. If the power supply voltage is divided and the voltage at a certain point is greater than the zero crossing voltage, the transistor is in a saturated conducting state and the thyristor is in an off state; On the contrary, when the voltage at that point is less than the zero crossing voltage, the thyristor can be triggered to conduct.

Solid state relays, due to the use of semiconductor switching elements and the absence of mechanical contacts, have advantages such as fast switching speed, no contact rebound, no arcing, and long lifespan. They can also switch "off" AC loads at zero load current points, completely eliminating problems such as arcing, electrical noise, and contact rebound associated with traditional mechanical relays and inductive loads. However, solid-state relays also have some limitations, such as large on state voltage drop, leakage current, and relatively small overload capacity. In practical applications, it is necessary to select and use solid-state relays reasonably according to specific needs and working conditions, and pay attention to their heat dissipation and protection measures to ensure the stable and reliable operation of the system.