Brief introduction of principle:
1, when QA is pressed, after QA is energized, the ZJ coil of the intermediate relay is energized, which makes the intermediate relay attract. At this time, the normally open contact of the intermediate relay is also attracted. At this time, although QA has been disconnected, the current flows to the intermediate relay coil through ZJ's closed normally open point, so that the intermediate relay is still attracted, thus playing the role of self-locking.
2. When it is necessary to stop using, as long as the TA button is pressed, the intermediate relay coil will immediately lose voltage and be released. At this time, the normally open contact ZJ of the intermediate relay is disconnected, and the self-locking effect is eliminated.