*Published on: 2023/2 14:35:21 * Views: 9

If the thyristor is to trigger conduction, in addition to sufficient triggering pulse amplitude and correct polarity, there must be a common reference point between the triggering circuit and the thyristor cathode. Some circuits may appear to have a trigger pulse applied to the trigger electrode G of the thyristor, but the cathode of the thyristor and the trigger signal have no common reference point, and the trigger signal is not applied between G-K of the thyristor, making it impossible for the thyristor to be triggered.

Figure 1a shows an automatic water level control circuit composed of 555, used for automatically maintaining the water level in a water tower. The author of this article considers that the water in the well and water tower cannot carry mains electricity, so the 555 control system uses a transformer to isolate and reduce voltage for power supply. Connect the 5553 pin output pulse to the G point of the bidirectional thyristor. Due to the suspension of the bidirectional thyristor T1 in the control circuit, the output pulse of pin 5553 cannot form a trigger current, and the thyristor cannot conduct. Furthermore, although the circuit uses isolated low-voltage power supply from the mains, the control circuit is still connected to the mains through G and T1 poles. When the 220V input terminal B is the live wire, the well water and water tower water supply will be replaced by mains voltage, which is absolutely not allowed!

The correct way is shown in Figure 1b. The thyristor and the pumping motor form a pumping control switch, and the triggering of SCR is controlled by a resistor connected between T2 and G. When the water level drops, the control contact opens, and pin 5553 outputs a high level (this circuit part is omitted), causing Q to conduct, relay J to close, SCR to trigger conduction, and the motor to start running. When the water level reaches, the contact is connected through the water, pin 5553 outputs a low level, Q is turned off, SCR is turned off at zero crossing of AC power, and pumping stops.

The above circuit has encountered inappropriate low-level errors due to inadequate design considerations. However, designs such as water tower water supply control systems that are not isolated from the mains power often appear in electronic publications. Two examples of improper trigger circuit design are common in electronic production drafts, as shown in Figure 2, which simplifies the circuit. In fact, no matter what control the control system completes, whether it is a unidirectional or bidirectional thyristor, the trigger circuit in Figure 2 cannot work properly. The problem is that the control system sends a trigger signal UG, with its reference point being the common ground, while the reference point of thyristor T1 or T2 is the load hot end. In fact, the trigger voltage UG applied to the thyristor is connected in series with the load terminal voltage UZ. Whether T1 or T2 is the trigger reference point for a bidirectional thyristor depends on the relative polarity of the trigger signal. As indicated in Figure 2, if T1 is below and T2 is above, then UG must be positive relative to T1 and have the same voltage as T1 at the reference potential. But regardless of whether T1 or T2 is used as a reference point, according to the connection in Figure 2, when the thyristor is conducting, UZ is usually approximately equal to Uin. If such a high voltage is applied between the trigger electrode G and T1, it will immediately cause the trigger electrode to be broken down and the thyristor to be damaged.

One method to improve this circuit is to use a trigger transformer to isolate the reference point of the control system. The trigger signal can be composed of BT33 to form a sawtooth wave generator controlled by the control system (rectangular wave can also be used). In this way, it is not affected by the primary reference point, and the secondary of the trigger transformer can be directly connected between G and T1, independent of the voltage on the load.

Another simple improvement method is to move the load circuit Z between T2 and Uin in Figure 2. However, this usage is limited as neither end of the load can access any reference point.

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