Contact relay output control interface circuit

When the switching output terminal PAO output is high level, it changes to low level through the inverting driver 7406, causing the light-emitting diode to emit light, thereby
turning on the photosensitive transistor and at the same time turning on the transistor VT, thus energizing the coil of the relay K , the relay contact is closed, turning on
the AC 220V power supply. On the contrary, when the switching output terminal PAO outputs low level, the relay contact is disconnected. In the figure, the resistor RL represents the load
, and the diode VD functions to protect the transistor VT. When relay K is closed, diode VI) is cut off, which does not affect the normal operation of the circuit
. When the relay is released, due to the existence of the inductance of the relay coil, electric energy is stored. At this time, the transistor VT has been cut off, so a
higher induced voltage will be generated at both ends of the coil. The polarity of this voltage is up and negative. Positive, the positive terminal is added to the collector of transistor VT. When the sum of the induced
voltage and Voc is greater than the collector reverse voltage of the transistor VT, the transistor VT will be broken down and damaged. After adding the diode VD
, the induced current generated by the relay coil flows through the diode VD, clamping the extreme voltage of the transistor VT collector, thus
protecting the transistor VT.