Battery over-discharge protection circuit principle and component selection (with circuit diagram)

　　OFweek Electronic Engineering Network: Over-discharge of batteries will cause acidification of the plates, affecting the normal service life of the batteries. To solve this problem, a micro-power battery over-discharge protection circuit is specially designed, as shown in the figure above. The circuit has the advantages of extremely low detection current (detection current ≤ l.lmA), no more power consumption after the protection circuit is activated, and strong anti-interference ability.

　　1. Circuit Principle

　　After the trip switch K1 is manually closed, the circuit is powered and enters the working state. First, because the voltage across C1 cannot change suddenly, it remains near 0V, and Q2 is cut off. When the battery voltage is >10.7V, the voltage regulator D1 is turned on, Q1 is saturated and turned on, and the control electrode of Q2 is cut off because it cannot get the trigger voltage. K1 does not act, and the battery and the subsequent circuit are in a closed working state. When the battery voltage is <10.7V, the voltage regulator D1 is cut off, Q1 turns to cut off, and the resistor R3 charges the capacitor C1. When the voltage across C1 reaches about 0.6V, the thyristor Q2 is triggered to turn on, the trip switch acts, and the subsequent load circuit is disconnected.

　　2. Component Selection

　　The trip switch can be a power switch with a relay for color TV, model KDC-A04-8Y, the switch current is dual 5A, can be connected in parallel to a maximum working current of 10A, the DC resistance of the electromagnetic coil is about 25Ω, and the measured coil voltage ≥7V can be attracted. Ql uses MCR100-6, MCR100-6 has extremely sensitive control pole trigger current (trigger current is 10μA~30μA), low on-state voltage drop (the measured voltage drop in this circuit is 0.8V). The value of the voltage regulator Dl is the protection circuit action voltage minus 0.7V. If there is no suitable voltage regulator, two voltage regulators can be used in series (when the circuit is in action, the Rl voltage is below 0.2V, and its voltage drop can be ignored).

　　3. Notes

　　The circuit detects current ≤1.1mA, mainly because the current through the R1 branch is 0.35mA~0.1mA, and the current through the R2 branch is 0.5mA~0.75mA. The operating current depends on the DC impedance of the coil of the selected trip switch K1. The operating current of KDC-A04-8Y used in this test is about 400mA, and the operating time is no more than 0.2 seconds.

　　4. Two other micro-power battery over-discharge protection circuits

　　The circuit is shown in the middle and lower figures. The front-stage voltage detection is completed by the precision voltage regulator TL431, and the control shutdown voltage point is precisely adjusted by VR1.

　　The TL431 is selected due to its high control accuracy, typical equivalent full-range temperature coefficient of 50ppm/℃, low current consumption in this circuit and many other advantages.

　　Because the cathode voltage of TL431 in this circuit varies from 1.9V to 2.7V, in order to satisfy the triggering of Q2 by the cathode voltage of TL431 of about 2.7V, it provides two connection methods, the middle figure and the lower figure. The middle figure is to divide the voltage through R5 (and the subsequent R4), and the lower figure is to divide the voltage through the forward voltage drop of LED (the forward voltage drop of ordinary red light-emitting diode with micro-current is about 1.7V) (the actual measured control electrode to cathode voltage of 02 can trigger conduction at 0.65V~0.7V).