Self-locking switch circuit

Ordinary transfer switches or pull switches are prone to wear and tear due to long-term use. At the same time, the contacts are also easily ablated due to the impact of large currents. Here is a simple circuit that can replace this switch. The circuit is as shown in the figure above. When AN is not pressed, SCR1 does not conduct, and the relay or contactor K does not act, which is equivalent to the "off" state. When AN is pressed, the 12V voltage provides trigger current to SCR1 through R1, AN, and VD1 to turn it on, K is powered, and the contact is connected to the load. Since the voltage across the capacitor cannot change suddenly, SCR2 does not conduct when pressing AN, and the current charges C through R3 and VD2. If AN is pressed again, the voltage on capacitor C is used as the trigger voltage of SCR2, causing SCR2 to conduct when AN is closed, the anode voltage of SCR1 decreases, SCR1 is cut off, and K is released. Capacitor C is discharged through R2, the control electrode and cathode of SCR2, and the circuit returns to its original state. Thereby achieving self-locking effect. Component selection: Relay K uses JRC-5M, its electrical parameters are DC12V, winding resistance 850Ω, contact capacity is DC27V 1A (can also be selected according to actual load); AN uses inching buttons, such as LA19-11J for electrical operation etc.; other components are as shown in the attached drawings. During debugging, the values ​​of R2, R3, and C should be adjusted repeatedly so that the circuit does not malfunction when AN is pressed, and if AN is pressed a moment after K is released, K can be powered immediately. Note: Xiaodou believes that this circuit should connect a small load from the cathode of SCR2 to the ground, such as a 12V small light bulb, etc., so that SCR2 will not be damaged when the current is large (12V voltage is directly applied to both ends of SCR2). R1 ​​can also be removed and not used. .