Working Principle of Photocoupler Solid State Relay (SSR)

Assembled with solid semiconductor components - contactless switch (no mechanical contact parts for connection and disconnection)
Advantages: fast switching speed, high operating frequency, long service life, low noise, reliable
operation Application occasions: replace conventional electromagnetic relays, widely used in: digital programmable devices
Data processing systems
Computer terminal interface circuits
Especially: frequent action, explosion-proof, moisture-resistant, corrosion-resistant occasions
Disadvantages: large leakage current, high contact voltage, single contact, narrow operating temperature range, poor overload capacity, high price
1. Basic features
① Small control power: it can work normally with a very small input control current, and the output uses a high-power tube thyristor device with power amplification
② Reliability: cast with insulating waterproof materials, no moving parts
③ Strong anti-interference ability: no contact action, no electromagnetic interference such as sparks, and isolation between input/output
④ Fast action: DC SSR - response time < tens of μS
zero-crossing AC SSR - conversion time ≤ 10Ms (1/2f s f = 50Hz)
⑤ Long life: 1012~1013 times (105~106 times for ordinary electromagnetic relays)
⑥ Large surge current: 6~10 times rated value
⑦ Wide adaptability to power supply voltage: AC SSR - 30~220VAC, any choice
⑧ High withstand voltage level: input/output medium withstand voltage of 2.5kV or more
2. Classification
According to the nature of the switching load: DC solid-state relay AC solid-state relay

According to the isolation between input/output: Photoelectric isolation Magnetic isolation

According to the control trigger signal mode: Zero-crossing type Non-zero-crossing type Active trigger type Passive trigger type
3. Working principle (take photoelectric coupling SSR as an example)
① No input signal: T3 is cut off, T4 is turned on, VT1 is turned off (the control electrode is clamped at a low potential)
② Signal input: T3 is turned on, T4 is cut off. When the power supply voltage is greater than the zero-crossing voltage (about ±25V), the voltage at point A is greater than the Vbe5 of T5 → T5 is turned on, the VT1 control electrode is at a low voltage and turned off, and the VT2 control electrode is turned off without a trigger signal.
• When the power supply voltage is less than the zero-crossing voltage, the voltage at point A is less than the Vbe5 of T5 → T5 is cut off, and the VT1 control electrode obtains a trigger signal through the voltage divider of R5 and R6 → VT2 is turned on → B and C are connected → the load circuit is connected.
•VT2 conduction process: power supply voltage "+": power supply → R8 → D6 → VT1 → D9 → R9 → load → VT2 control electrode obtains trigger pulse
Power supply voltage "-": power supply → load → R9 → D8 → VT1 → D7 → R8 → power supply VT2 control electrode
obtains trigger pulse
•When the input signal is cancelled: T4 is turned on → VT1 is turned off → but VT2 is still turned on (load current is greater than holding current), until the load current decreases with the power supply voltage and drops below the holding current of the bidirectional thyristor, VT2 is turned off, thereby cutting off the load current.