SCR overcurrent protection circuit using Hall element

Here we introduce a thyristor overcurrent and overload protection circuit that uses Hall elements as sensors. In addition, it also has overheating protection and audible and visual alarm functions. The circuit schematic diagram is shown in Figure 1. Its basic working principle is as follows. It consists of two U-shaped magnets and two three-terminal integrated Hall elements to form a closed magnetic circuit with a certain gap. As a current detection system, through the load and thyristor current The wire passes through the center of the magnetic circuit plane, generating an alternating magnetic field in the magnetic circuit, as shown in Figure 2. When the current value through the thyristor is within the rated range, the magnetic field intensity it generates in the magnetic circuit is small, lower than the operating magnetic field intensity of the Hall element, and the protection circuit does not operate. Once an overcurrent situation occurs, the magnetic field intensity increases greatly and exceeds the action intensity of the Hall element. The Hall element will be triggered to flip, and the protection circuit will also be triggered, causing it to close the trigger circuit of the main circuit thyristor, and Causes the thyristor to turn off after zero crossing. IC1 and IC2 in the circuit are Hall integrated circuits ULN3020. During normal operation, their output pins are always low level, D2 and D3 are cut off, the thyristor T1 is also in the off state, and the light emitting tube L4 does not emit light. The positive power supply forms a loop through R6, L1, IC4, R5, and VT to the ground. If the base of the control switch transistor VT is at a high level, it will be in a conducting state, thus turning on the zero-crossing trigger IC4 and triggering the bidirectional silicon T3 When the load RL is turned on and the load RL is powered on, their current will generate an alternating magnetic field in the magnetic circuit. When the peak value of the magnetic field intensity does not reach the turn-on magnetic field intensity of the two Hall elements, both IC1 and IC2 remain unchanged. . Once the current increases abnormally due to abnormal load or other reasons, one of IC1 and IC2 will be in a short-term high-level output state, thereby triggering the thyristor T1 to conduct through D2 or D3, and D5 to conduct. The zero-crossing trigger IC4 is turned off, and the thyristor T3 will be turned off after the power supply reaches zero. At the same time, the luminous tube L4 lights up, D4 is also turned on, the buzzer BU sounds an alarm, and the thyristor T1 can remain on until the power supply of the control circuit is turned off. There is also a set of thyristor overheat protection circuit in this circuit. IC3 is a TO-220 packaged model 67L070 two-pin temperature sensing switching element. Its operating temperature is 70℃. At normal temperature, it is Normally closed state. It is installed on the radiator of the thyristor T3. When the power consumption of the thyristor increases greatly due to overload or other reasons, once the temperature of the radiator reaches 70°C, IC3 will turn from normally closed to normally open. At this time, T2 will trigger conduction through R10, and D6 will also conduct Turn on, causing IC4 to be powered off and cut off, and at the same time, thyristor T3 is cut off. At the same time, D7 is also turned on, the luminous tube L3 is lit, D6 is turned on, and the buzzer BU works and sounds an alarm. SCR T2 can also remain in the conductive state. Finally, until the control power is turned off. The size of the current that causes the Hall element to operate can be determined by adjusting the distance between the two U-shaped magnetic cores. For the 40A thyristor used in this circuit, the maximum current passing through it should be limited to less than 40A. Here, the The cross-section of the magnetic core used is 5mm×4mm, and the distance between the two cores is about 2.0mm. However, due to the differences in the permeability of different magnetic materials and the parameter differences of the Hall elements, the specific distance may be slightly different. The actual protection current The size is best determined by actual measurement and sealed on the printing plate with glue. The requirements for installing the two Hall elements are to align the core sections and all the printing to face one direction. Only in this way can the two Hall elements detect the positive and negative half cycles of the AC current respectively. For further protection, the main circuit is also equipped with a 32A air switch, which is also used to protect the thyristor and is also used as a control switch for the main circuit. The shortcoming of this protection circuit is that when a serious short-circuit problem occurs between the load and the circuit, the protection effect on the thyristor is slightly insufficient, because there have been cases where the thyristor was burned and short-circuited. This was mainly due to the failure of the protection circuit. After action, the thyristor can only be turned off naturally after the power supply reaches zero, and the breaking speed of the air switch is also limited. Therefore, other measures such as "quick melt" should be used for final short-circuit protection when necessary. This protection circuit has been widely used in the temperature controller of the self-designed 4000W box-type electric furnace. It has been working normally for four years and has successfully avoided overcurrent faults caused by abnormal loads many times.