Protect optocoupler LED: Prevent overcurrent burnout of optocoupler LED due to high input voltage. Resistance value must ensure current does not exceed the optocoupler LED's maximum allowable value (typically 5-20mA) under input voltage.

Match control signals: Adapt to different level control signals (e.g., 3.3V, 5V, 24V).

Voltage division and stabilization: Share part of the input voltage to stabilize optocoupler operating conditions.

Application of Ohm's Law: Limit current (I = U/R) through resistance value (R).

Dynamic adjustment: Maintain current stability through voltage division when load changes.

Suppress transient interference: Work with TVS diodes to absorb voltage spikes or surges at the input.
Section 2: Optocoupler
An optocoupler is a semiconductor device that achieves electrical isolation through light signals as the medium. It integrates a light emitter (such as an infrared LED) and a photosensitive receiver (such as a phototransistor or photoresistor) in the same package. In SSRs, the core function of the optocoupler is to isolate the input control terminal from the output load terminal, ensuring that high-voltage load circuits do not interfere with low-voltage control circuits.

Electrical isolation: Achieve complete electrical isolation between input (low-voltage DC) and output (high-voltage AC/DC) through light signal transmission, with isolation voltage reaching thousands of volts.

Signal transmission: Convert electrical signals from the control terminal into light signals, then back into electrical signals at the output terminal, completing the "electric-optical-electric" conversion.

Anti-interference protection: Suppress electromagnetic noise (EMI) and voltage surges to enhance system stability.

Input signal activation: Control voltage drives LED to emit light.

Light signal transmission: LED-emitted light illuminates the photosensitive device.

Output conduction: Photosensitive device conducts after receiving light, triggering the SSR's power switch (such as TRIAC or MOSFET).

Load control: Power switch controls load current on/off.

Unidirectional transmission: Signals transmit only from input to output, preventing feedback interference.

Fast response: Light transmission delay is in microseconds, suitable for high-frequency switching.

High-voltage optocouplers: Some SSRs use photo-controlled thyristors (Photo-TRIAC) to directly drive AC loads, simplifying the circuit.

Zero-crossing trigger: AC SSRs often integrate zero-crossing detection circuits, allowing optocouplers to trigger at the AC zero-crossing point, reducing surges and EMI.
Section 3: Transient Voltage Suppressor Diode
Also known as TVS (Transient Voltage Suppressor), this is a high-speed overvoltage protection device that suppresses transient voltage spikes through avalanche breakdown characteristics. In SSRs, TVS diodes are typically connected in parallel at the input or output to absorb surge voltages and protect internal semiconductor components (such as optocouplers and thyristors) from damage.

Surge voltage clamping: When transient high voltage (such as lightning strikes or inductive load switching) occurs in the circuit, TVS quickly conducts and limits the voltage to a safe level (clamping voltage).

Electrostatic discharge (ESD) protection: Can absorb electrostatic pulses exceeding 10kV to prevent SSR control terminal failure due to ESD.

Electromagnetic interference (EMI) suppression: Reduce voltage oscillations during switching to lower noise impact on surrounding circuits.

Normal state: TVS is in high-impedance state with no impact on the circuit.

Abnormal overvoltage: When voltage exceeds TVS breakdown threshold (e.g., 24V), its PN junction undergoes avalanche breakdown, impedance drops sharply, forming a low-resistance path to divert surge current.

Recovery: TVS automatically returns to high-impedance state after overvoltage disappears.

Typical response time: Nanosecond level (10^-12 seconds).

Input protection: Connected in parallel across optocoupler LED sides to prevent control signal overvoltage.

Output protection: Connected in parallel with TRIAC/MOSFET to absorb back electromotive force generated by load switching.