Brief introduction of thyristor trigger circuit--composed of single junction transistor

The thyristor trigger circuit composed of a single junction transistor is shown in Figure 1, and the relevant voltage waveform of the trigger circuit is shown in Figure 2. Compared with the relaxation oscillation circuit composed of a single junction transistor, the oscillation part of the circuit is the same, and synchronization is achieved by improving the power supply circuit. The sinusoidal AC power taken from the main circuit is stepped down by the synchronous transformer T to a lower AC voltage, and then converted into pulsating DC through the diode rectifier bridge. The role of the voltage regulator VW and the resistor RW is "clipping". When the pulsating voltage is less than the voltage regulation value of the voltage regulator, VW is not turned on, and the voltage at both ends is equal to the rectified output voltage; if the pulsating voltage is greater than the voltage regulation value of the voltage regulator, VW will be broken down, and the voltage at both ends will maintain the voltage regulation value. The part of the output voltage of the rectifier bridge that is higher than the voltage regulation value drops on the resistor RW. In this way, the voltage waveform at both ends of VW is similar to a trapezoidal wave. This voltage is used to replace the DC power supply in the relaxation oscillation circuit to play a synchronization role.

Since the power supply of the oscillation circuit is a trapezoidal wave, the power supply voltage of the oscillation circuit is very small during the period between the end and the beginning of each half wave of the main circuit sine wave, the circuit does not oscillate, and the capacitor voltage is released to 0. When the power supply voltage approaches the top of the trapezoidal wave, the oscillation circuit starts to work. When the capacitor is charged and the voltage at both ends reaches the peak voltage, the single junction transistor is turned on to discharge the capacitor. The discharge current flows through the parallel circuit of R1 and the gate of the triggered thyristor to form an output, providing a trigger pulse for the thyristor to turn on the thyristor. Then the circuit enters the next oscillation cycle, but the thyristor loses control once the gate is turned on. The pulse output by the oscillation circuit in a half cycle of the power supply voltage is only the first one to play a triggering role, and the subsequent pulses are invalid. When the half cycle of the main circuit voltage is close to the end, the power supply voltage of the oscillation circuit enters the hypotenuse of the trapezoidal wave and drops rapidly, the oscillation circuit stops oscillating, and the capacitor voltage is released to 0. Therefore, in each half wave of the main circuit, the capacitor is always charged from 0, ensuring the synchronization of the trigger pulse and the main circuit voltage.