* Question
| What are the characteristics and principles of photoconductive relays? |
* Answer
| Working principle (illustrated by photo-coupled SSR) 1 No input signal: T3 is off, T4 is on, VT1 is off (control electrode is clamped at low potential) 2 Signal input: T3 is on, T4 is off.• When the power supply voltage is less than the zero-crossing voltage, Vbe5→T5 whose voltage is lower than T5 at point A is cut off, VT1 control pole is triggered by R5 and R6, and VT2 is turned on → B and C are turned on → load circuit is connected.The current is greater than the holding current) until the load current decreases with the supply voltage, drops below the triac holding current, and VT2 turns off, thereby cutting off the load current. |
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