Inductorless analog relay circuit

This article introduces a non-inductive analog relay, the circuit principle of which is shown in the figure below. In the figure, the 220V power supply supplies bias to Q4 and Q3 in turns in the positive and negative half cycles through the load RL, R1, D1~D4, and ZD1; at the same time, it provides power to the photocoupler Q1 through R3, D5~D8. When the front-stage TTL circuit outputs a high-level signal, the photocoupler is turned on during the positive half cycle of the mains power, so a voltage drop occurs across R5, triggering the SCR to turn on, and the load RL is powered to work. The entire circuit functions like a relay, but does not generate reverse induced voltage, thus avoiding the possibility of the load being damaged by high back voltage breakdown. C1 and R6 are pulse absorption components, and R3 plays a current limiting role. In order to avoid the 90° phase between the voltage of the thyristor and the power supply of the photocoupler when RL is an inductive load, the power supply of the photocoupler in this circuit is taken from the anode of the SCR and not directly from the mains power supply.