Calculate the parameters yourself, the principle is as follows:
1....When it is not fully charged, the voltage on R2 is low, not enough to conduct the base of ZD1 and Q2, therefore Q2 cut off.
Because Q2 is not conducting, the voltage on R4 is 0, the base Q1 has no voltage, Q1 is cut off< /p>
At this time point, D1 turns on and D2 turns off.
2....When fully charged, the battery voltage increases, the voltage on R2 increases , bases ZD1 and Q2 are on, Q2 is on, and voltage and current are generated on R4.
Q1 is on, D1 is off, and D2 is on.
Simple automatic shutdown circuit when the lithium battery is fully charged?
Rx affects constant voltage charging voltage and constant voltage charging voltage V REG = 4.2+8.996×10-6×Rx. The constant voltage charging voltage is 4.2V and Rx = 15 ohms ~ approx. When the battery voltage is higherOnce at the trickle charge threshold, the charger switches to constant current charging mode. The charging current is defined by the internal reference voltage of 120 mV and an external resistor R CS, i.e. the charging current is 120 mV/R CS. Suppose the load current is 3 A and R CS = 40 m (chip resistor). Moss can use P-channel field effect transistors with high current and low on-resistance.
There is a circuit for your reference.
1. Working principle: 12V regulated power supply. The positive electrode of the battery is divided by the potentiometer and then connected to the base of the PNP tube. The potentiometer is adjusted to the appropriate position. the voltage is low, the PNP is turned on and the relay is closed, the charging circuit works, the voltage reaches the set value, the PNP is turned off, the relay is disconnected, and the charging circuit stops working.
2. Like thisircuit is simple, it will switch frequently near the switching point. It can be improved to work reliably and stably.
3. Circuit improvement: The 12V regulated power supply is started with a button switch, and a pair of normally open contacts are added to the relay to self-lock the start button, that is, in parallel with the button start, so that when the relay is disconnected, the 12V regulated power supply is also cut off and the system is completely turned off.
If you are using 5 volts for charging, you can simply replace the relay with 5 volts.