Battery discharge protector

　　The battery discharge protector introduced in this example can disconnect the battery from the load circuit when the battery voltage drops to 9V to prevent the battery from over-discharging; it can also issue a battery undervoltage indication.

　　How the Circuit Works

　　The battery discharge protector circuit consists of an undervoltage protection circuit and an indication circuit, as shown in Figure 5-131.

　　The undervoltage protection circuit is composed of transistor V2, voltage stabilizing diode VS, thyristors VT1, VT2, capacitors C1, C2 and resistors R1~R3.

　　The indication circuit is composed of a normal battery voltage indication light emitting diode VL1, a battery undervoltage indication light emitting diode VL2, a transistor V1, a voltage stabilizing diode VS and resistors R1, R2, R4 and the like.

　　When the terminal voltage of battery GB is above 9V, VS breaks down and turns on, making V1 reverse biased and cut off, V2 turns on, and the high level output by the emitter of V2 provides a trigger level for the gate of VT1 through C1, turning on VT1, and the battery supplies power to the load. At this time, VL1 lights up, and VL2 does not light up.

　　When the terminal voltage of battery GB drops to 9V, VS is cut off, V2 is reverse biased and cut off, V1 is turned on, and the high level output by the collector of V1 provides a trigger level for the gate of VL2 through C2, turning on VT2. The charge on C1 is added to the cathode of VT1 through VL2, so that VL1 is cut off due to the instantaneous reverse voltage on the cathode, and the power supply path between GB and the load circuit is cut off. At the same time, VL1 is extinguished and VL2 is lit, indicating that the battery voltage is insufficient.

　　Component Selection

　　R1~R4 are made of 1/4W carbon film resistors or metal film resistors.

　　Both C1 and C2 are tantalum electrolytic capacitors with a withstand voltage greater than 25V.

　　VL1 and VL2 both use φ5mm light-emitting diodes.

　　VS uses a 1W, 9V silicon Zener diode.

　　V1 uses S8550 or C8550 silicon PNP transistor; V2 uses S8050 or C8050 silicon NPN transistor.

　　VT1 uses 3~5A, 100V thyristor; VT2 uses 1A, 100V thyristor.