Practical constant current charger-----Constant current charger

Ni-Cd or Ni-MH batteries have strict requirements for charging. However, most chargers sold on the market are shoddy and cannot meet the battery requirements. I have analyzed several chargers, and their circuits are almost the same: a low-power 6V transformer (and it is of poor quality), a diode for wave rectification, and then a small resistor for current limiting charging. Some have a light-emitting diode connected to the current limiting resistor for status indication. The charging current varies from 20-70mA. And it is a pulse current, the size will also be affected by the mains voltage and battery status, and there is no constant current at all. The battery life is greatly shortened when charged under such conditions.
　

Solution:

This circuit is actually a constant current source. The core device is the integrated three-terminal adjustable voltage regulator LM317T. In the last lecture, we said that LM317T can keep its +Vout terminal 1.25V higher than its ADJ terminal voltage when the power supply voltage is sufficient. Please see the connection method in the figure. The ADJ terminal is directly connected to the battery to be charged. However, the internal resistance of the ADJ terminal is very large (the current at the ADJ terminal will not exceed 50μA under normal circumstances), which can be approximately regarded as an open circuit, but it can sample the voltage. The LM317T increases the voltage at the +Vout terminal to 1.25V higher than the ADJ terminal, then a current of 1.25V/25.5Ω=0.05A=50mA will flow through the resistor connected across the +Vout terminal and the ADJ terminal (25.5Ω is the total resistance of R1 and R2 in parallel when the switch is turned on). This current flows through the battery and charges the battery with a constant current. Think about it, why can't it flow somewhere else?
　

Extended Application:

According to the parameters in the figure, when the switch is open, the charging current is about 50mA; when it is closed, it is about 160mA. This is suitable for ordinary nickel-cadmium batteries with a capacity of 500mA. You can change the size of the charging current by changing the resistance value connected to the +Vout terminal and the ADJ terminal of the LM317T. Among them, the required resistance value (unit is Ω) = 1.25V/required charging current (unit is mA).