what is solid state relay?

Solid State Relay (SSR) is a fully electronic circuit combination component that relies on the electrical, magnetic, and optical characteristics of semiconductor devices and electronic components to complete its isolation and relay switching functions.

Compared with traditional electromagnetic relays (EMR), solid-state relays are relays that have no mechanical or moving components, but have essentially the same functions as electromagnetic relays.

Solid state relays are divided into DC input AC output type, DC input branch output type, AC input AC output type, and AC input DC output type according to their working properties.

SSR solid-state relays can be divided into zero voltage type (Z) and phase modulation type (P) in the form of triggering. When a suitable control signal VIN is applied to the input terminal, the P-type SSR is immediately turned on. When the VIN is cancelled and the load current is lower than the bidirectional thyristor maintenance current (AC commutation), the SSR is turned off.

The Z-type SSR internally includes a zero crossing detection circuit. When applying the input signal VIN, the SSR can only be turned on when the load power supply voltage reaches the zero crossing zone, which may cause a maximum delay of half a cycle of the power supply. The Z-type SSR has the same turn-off condition as the P-type SSR, but is widely used due to its approximate sinusoidal load operating current and low harmonic interference.

Due to the different output devices used by some companies, SSR can be divided into ordinary type (S, using bidirectional thyristor elements) and enhanced type (HS, using unidirectional thyristor elements). When an inductive load is applied, the bidirectional thyristor is turned on before the input signal cutoff t1, and the current lags behind the power supply voltage of 90O (pure induction). At t1, the input control signal is canceled, and the bidirectional thyristor is turned off when it is less than the maintenance current (t2). The thyristor will withstand a reverse voltage with a high voltage rise rate of dv/dt. This voltage will be fed back to the gate electrode through the junction capacitance inside the bidirectional thyristor. If the bidirectional thyristor commutation dv/dt index (typical value of 10V/s) is exceeded, it will cause a long commutation recovery time or even failure.

Unidirectional silicon controlled rectifier (enhanced SSR) operates in a unipolar state and is only limited by the static voltage rise rate (typical value: 200V/s). Therefore, the commutation dv/dt index of the enhanced solid-state relay HS series is 520 times higher than that of the conventional SSR. Due to the use of two high-power unidirectional thyristor in reverse parallel, the current distribution and heat conduction conditions are changed, and the output power of the SSR is increased.

Application of Solid State Relay

Solid state relay is a contactless switch composed of solid state components, featuring safe and reliable operation, long service life, contactless, spark free, pollution-free, high insulation, high voltage withstand (over 2.5 kV), low trigger current, fast switching speed, and compatibility with digital circuits. It uses flame retardant epoxy resin as raw material, and adopts potting technology to isolate it from the outside. It has good performance in voltage resistance, moisture resistance, corrosion prevention, and vibration resistance. The internal characteristics of solid-state relays are to turn on when the voltage crosses zero and turn off when the load crosses zero. A complete sinusoidal waveform can be obtained on the load. Therefore, the RF interference of the circuit is very small, which can reduce the back EMF of inductive loads (such as fans, three-phase motors, etc.) and significantly reduce surge current when driving resistive loads (such as incandescent lamps, heating wires, etc.).