Thyristor self-locking switch circuit

1. Figure 1, the control pole current is provided by an isolation transformer through rectification and capacitor filtering. TV1 is a 3~5 watt small transformer with a primary rated voltage of 220V and a secondary rated voltage of 6~10V.

When S1 is pressed, the thyristor is turned on, and the DC voltage output from the secondary of the transformer provides the control current. Even if S1 is released, the thyristor remains on. After the power is cut off, the thyristor is cut off, and S1 must be pressed again to turn it on again, so it has a power-off self-locking function. The pressed S2 (contact closure) completely shunts the control current and disconnects the switch. At the same time, S2 releases the voltage of C1 (to ensure that the charge of C1 is released to a low enough level, the time for pressing S2 is less than 1 second). When S2 is released again, the thyristor can no longer be triggered, and the thyristor can be reliably turned off. R2 is used to reduce the voltage and prevent the secondary of the transformer from being short-circuited when S2 is pressed. This circuit can control any type of load.

2. Figure 2, press S1, the thyristor is turned on, R2, C1 obtain current, the voltage at the C1 end is out of phase with the voltage at the load end, the thyristor is turned off when the anode current passes through zero, and at this time the capacitor voltage is not zero, which can generate a control current to automatically trigger the thyristor to turn on, so the load will not be powered off when S1 is released. If the power supply is accidentally cut off, C1 will discharge through R1, and when the power supply voltage is automatically restored, as long as the capacitor voltage has been discharged to a low enough level, the thyristor cannot be triggered again, and the power-off self-locking is achieved. When the power is manually cut off, press S2 to cut off the control trigger current. S2 must be pressed for at least 1 second to ensure that C1 is discharged through R1 to a residual voltage that is small enough, and the thyristor is not triggered and is reliably turned off. When this circuit carries a large capacitive load, the thyristor is difficult to trigger (such as an induction cooker, the input end of the induction cooker generally has a 2~5uF capacitor to resist differential mode interference). If the power supply of all loads is converted by AC/DC (such as computers and televisions), R4 needs to be connected to provide a basic path for the control current. If there are inductive or resistive loads, R4 can be omitted.