Relay pickup circuit under low voltage

Often the relay cannot work properly because the power supply voltage is lower than the pull-in voltage of the relay. In fact, once the relay pulls in, it can work reliably at about half of the rated voltage. Therefore, you can give the relay a starting voltage at the beginning to make it close, and then let it work at a lower power supply voltage. The circuit as shown in the figure can achieve this purpose. Working principle: As shown in the figure. V1 is a single-junction transistor BT33C, which forms a relaxation oscillator with R1, R2, R3 and C1. SCR is a one-way silicon controlled thyristor. After pressing the start button AN1, the circuit is energized. Because the SCR has no trigger voltage, it does not conduct electricity. On, relay J does not operate, and the power supply quickly charges capacitor C2 through R4 and VD1 to a level close to the power supply voltage (Vcc-VD1 voltage drop). At the same time, the power supply charges capacitor C1 through R1. After a few seconds, the voltage on C1 is charged to the trigger voltage of V1. C1 immediately discharges through V1, forming a positive pulse on R3. This pulse is added all the way to the base of V2, causing V2 to quickly saturate and conduct, and the collector of V2 is the capacitor C2. The positive terminal is close to ground. Since C2 is charged with a positive voltage of positive and negative polarity at this time, the negative electrode of C2, that is, one end of the J coil, has a negative potential. The positive pulse on R3 goes through VD2 and C3 to trigger the thyristor to conduct, and the SCR cathode, that is, the other end of the J coil, is close to the power supply voltage. At this time, the J coil actually withstands about twice the power supply voltage, so J1-1 is closed. After releasing AN1, J1-1 protects itself. J1-2 cuts off the power supply of V1 and V2, and the relay works at a voltage close to the power supply. In the figure, AN2 is the stop button. When AN2 is pressed, J loses power and is released, J1-1 is disconnected, and the entire control circuit loses power. When making this circuit, it is generally recommended that the rated voltage of the relay be about 1.5 times the power supply voltage. Under normal circumstances, any type of one-way thyristor (or two-way thyristor) can meet the needs of this circuit. The withstand voltage of V2, C1 and C3 depends on the selection of the power supply voltage. The withstand voltage of C2 is preferably no less than twice the power supply voltage.