Constant current charger using light emitting diode as voltage regulator

　　I have several 2.4V/280mAh NiMH batteries and plan to make a simple constant current charger. I can't find a voltage regulator diode, so I consider using a light-emitting diode instead of a voltage regulator diode. Generally speaking, within a certain current range, the voltage drop across the two ends of a light-emitting diode is relatively stable.

　　From the volt-ampere characteristics of 3mA to 6mA, the dynamic resistance of the red light-emitting diode is about 16n, which is similar to the general voltage-stabilizing diode. Therefore, when the requirements are not high, it is possible to consider using light-emitting diodes instead of voltage-stabilizing diodes.

　　The circuit diagram of the constant current charger is shown in the attached figure.

　In order to stabilize the constant current circuit, the following three measures are taken:

　　1. To keep the voltage on LED2 stable, a constant current source consisting of BG1, LED1, R1 and R2 is used to power LED2.

　　The current is (Uledl-0.7)R2, which is about 4.8mA. In the range of 4-6 mA, the dynamic resistance of LED2 is small, which can improve the voltage stability across LED2.

　　2. In order to absorb as little current as possible from the constant current source, a Darlington transistor is used as the output tube, which can keep the voltage drop of LED2 slightly unchanged, thereby increasing the voltage stability across LED2.

　　The second constant current source composed of LED2, BG2 and R3 charges the battery connected in series between the positive electrode of the power supply and the collector of JE3300.

　　In fact, this is an emitter output stage. Since the voltage drop on LED2 is stable, its emitter current is also stable.
　　3. JE3300 consists of two emitter junctions in series, and the temperature coefficient of each emitter junction (PN junction) is about -2mV/℃.

　　In this way, when the temperature rises, the output current will increase.

　　For example, if the temperature rises by 10 degrees, the two PN junctions will drop by 40 mV (may be smaller in reality, because the current of the first PN junction is very small), and the output current will increase by about 40 mV ÷ 13 ohms = 3.08 mA. Therefore, a small heat sink is added to the JE3300, so that the temperature change of the JE3300 will be smaller and its output current will be more stable. The output current is (Uled2-1.3) ÷ R3. Among them, 1.3V is the approximate voltage drop of the emitter junction (two P-N junctions) of the JE3300.

　　To test the performance of the circuit, connect the output of the regulated power supply to both ends of C1. When the voltage is 8.5V~10.5V, a 2.4V/280mAh NiMH battery is charged, and the output current is 27.4mA-28mA. This circuit can charge 1~2 batteries (in series), and the charging time should be 12-14 hours. It has been practical and the effect is good. It is a simple and practical circuit. It is especially recommended to enthusiasts.