Silicon controlled rectifier automatic constant current charger circuit

In addition to charging various nickel-cadmium batteries, this charger can also charge dry batteries. Its charging current is adjustable. The charge termination voltage is predetermined by RP1. 　　

working principle  　　

The circuit principle is shown in Figure 1. When charging starts, the voltage across the battery pack is low and not enough to turn on the transistor VT. The phase shift circuit composed of RC provides trigger current to the thyristor. The phase shift angle is determined by RP2. The thyristor is cut off during the negative half cycle. The thyristor is therefore charged through the battery pack in a controlled half-wave rectification manner. The charging current can be adjusted by adjusting RP2, and the maximum charging current is determined by R1. Indicator lights are stringed in the circuit to indicate the charging status and the size of the charging current. R3 is used to adjust the brightness of the indicator light. When the voltage of the battery pack slowly rises and approaches the predetermined value, the transistor begins to conduct, the conduction angle of the thyristor decreases, and the charging current decreases until it is completely cut off. In this way, charging automatically stops and the battery pack is maintained at the predetermined value. on voltage. Because when the voltage drops, the transistor tends to cut off again, and the thyristor restarts. However, at this time, the conduction angle is very small and the current output is very small, which has a protective effect on the rechargeable battery and prevents overcharging. The component selection and production component list is shown in the table below. No. Name Model Quantity R1 Resistor 4.7K (needs adjustment) 1 R2 Resistor 1.8K 1 R3 Resistor is selected according to the indicator light 1 RP1 Potentiometer 100K 1 RP2 Potentiometer 2K 1 C Polyester capacitor 0.22u 1 VD1, VD2 Crystal diode 2CP type 2 VT Transistor 3DG class 1 VS Unidirectional thyristor 1A/25V 1 After the component connection is completed and checked, it can be adjusted. When adjusting, connect the ammeter in series to the input circuit, short-circuit R3 first, rotate RP2 to the minimum resistance, adjust R1 so that the ammeter indicates the required maximum charging current, and R1 is determined. Then remove the short-circuit line and adjust R3 to make the indicator light slightly brighter for indication. Turn RP2 and calibrate the current value on its scale (corresponding to the ammeter connected in the loop). Finally calibrate RP1. Find a regulated power supply or battery pack with variable output, connect the positive and negative poles to the charging terminal respectively, and adjust the RP1 indicator light to turn off the voltage to the predetermined value. Just mark 3.0V, 4.5V, 6V, etc. on the scale of RP1.