Thyristor protection, how to judge the thyristor pins and whether they are good or bad

The overload capacity of thyristors is poor, and short-term overvoltage or overcurrent may cause them to be damaged. Appropriate protective measures should be taken.

Overcurrent protection

Insert a fast fuse FU in series at an appropriate position in the circuit.

The current selection principle of fast fuse: the effective value of the thyristor allowable current is greater than the rated current of the fast fuse and greater than the effective value of the actual current of the circuit.

Overvoltage protection

Transient overvoltage can also cause thyristor breakdown or mis-conduction, often due to lightning, the device where the thyristor is located or nearby electrical equipment being switched off, or when the conducting tube is switched on, an induced electromotive force is generated due to the inductance in the circuit.

Overvoltage protection measures include:

The RC absorption circuit connects the resistor and capacitor in series and in parallel at the appropriate position of the circuit (using the principle that the capacitor voltage cannot change suddenly, the resistor absorbs short-term overvoltage and limits the excessive discharge current of the capacitor to impact the thyristor).

Selenium stack protection, selenium rectifier is commonly known as selenium stack. When the voltage of the selenium stack exceeds a certain value, the resistance value decreases rapidly, allowing large current to flow without burning out, and dissipating the overvoltage energy in the nonlinear resistor. The connection method is to connect two groups of selenium stacks in series with opposite polarity and then connect them in parallel to the input end of the AC circuit.

Transient voltage suppressor (VTP) is a new type of protection component, similar to an ordinary small diode, with the same symbol as an ordinary voltage regulator. Under reverse voltage, when subjected to a high-energy instantaneous large pulse, the working impedance immediately drops to a very low value, allowing large current to pass and clamping the voltage at a predetermined value.

How to judge the thyristor pins and whether they are good or bad

Judgment of unidirectional thyristor:

Use the ohmmeter to measure the resistance of any two pins. If one group of resistance is about tens of ohms (forward resistance from control electrode G to cathode K), another group of resistance is 2~3 times tens of ohms (reverse resistance between GK electrodes), and the other four groups are high resistance, it means that the tube is good and the pins are also determined. On the contrary, if there are multiple groups of low resistance or multiple groups of high resistance, the tube is broken.

After measuring the three electrodes, connect the black pen of the multimeter (the black pen of the pointer multimeter is connected to the positive pole of the battery inside the multimeter) to the anode of A, and the red pen to the cathode of K, and read infinitely. Then, short-circuit the G and A poles to reduce the meter reading to tens of ohms. When G and A are disconnected, the meter reading is still tens of ohms. At this time, swap the pen. If the multimeter reading changes from infinity to small when the G pole and A pole are short-circuited, and the reading returns to infinity after the short-circuit is canceled (the thyristor cannot maintain conduction), it means it is a unidirectional thyristor.

Note: If the difference between the forward and reverse resistance of the control electrode G and the cathode is not large, and the measurement result is that the thyristor cannot be turned on according to Figures c and d, then the test lead is not connected to the cathode. High-current thyristors require a 1.5V battery to be connected in series with the multimeter to provide a larger holding current.

Judgment of Bidirectional Thyristor

Abroad: main electrodes T1 and T2, potential reference electrode T1, control electrode G

Domestic: main electrodes A1 and A2, potential reference electrode A2, control electrode G

The forward and reverse resistances between T1 and G poles are both tens of ohms, and the others are infinite. After repeated measurement and judgment 6 times, the T2 pole can be obtained.

Connect the black test lead to the T2 pin and the red test lead to the assumed T1 pin. Use the assumed G pin to short-circuit the T2 pin momentarily. If the multimeter reading changes from infinity to tens of ohms and can be maintained, it means that the assumption is correct. If the multimeter reading changes from infinity to tens of ohms but cannot be maintained, it means that the assumption is wrong. The judgment is completed.

Figure b is set up to judge unidirectional and bidirectional thyristors, and the thyristor shown in Figure d cannot be turned on.