The charging device charges four 2Ah nickel-cadmium batteries. The working principle circuit is shown in the figure. The diodes VD7, VD8, and the transistors VT2 and VT3 form a constant source to limit the charging power. FC connector Waterproof Fiber Pigtail,Waterproof Pigtail,Fiber Optic Component,Waterproof Fiber Optic Pigtail,Waterproof,Optic Fiber communication systems Shenzhen GL-COM Technology CO.,LTD. , https://www.szglcom.com
The three-terminal integrated block 7806 and the diodes VD9 and VD1O constitute an imaistor. In the early stage of charging, the voltage across the battery GB is relatively low, the charging current is large, and the charging current is limited by the constant current power supply, which is about <200 mA. As the battery terminal voltage rises, the charging current gradually decreases, the value genset loses its effect, and it is charged by the 7806 constant voltage. Transistor VT1 and LED VD5 form a charging indicating circuit. At this time, VD5 changes from bright to slightly bright. When the battery is fully charged, the collector and emitter junction resistance of the transistor VT3 increases, and the base potential drops, causing the base potential of the transistor VT2 to rise, thereby causing the base potential of the transistor VT1 to rise, and VT1 is turned off, and the light emitting diode VD5 is dim to off.
Among them, the diode VDIO is a reverse intercepting pipe to prevent the battery GB from being reversed. In order to ensure that the battery has a charging voltage of 6V, a diode VD9 is added to the ground of the 7806 to compensate for the voltage drop of the VD1O. 
FC is one of the most common connection devices in single-mode networks. It also uses a 2.5 mm ferrule, but some of the early FC connectors are designed with ceramics built into the stainless steel ferrule. FC has been replaced by SC and LC connectors in most applications.
FC is the abbreviation of Ferrule Connector, indicating that its external reinforcement is a metal sleeve, and the fastening method is a turnbuckle. The round threaded connector is a metal connector, and the metal connector can be plugged in more times than plastic. Generally used in telecommunications networks, a screw cap is screwed onto the adapter.
Advantages: Reliable and dustproof.
Disadvantage: Slightly longer installation time.
SC connector
SC also has a 2.5mm ferrule. Unlike ST/FC, it is a pluggable device and is widely used because of its excellent performance. It is a connector standardized by TIA-568-A, but it was not widely used in the early stage due to its high price (twice the price of ST).
The full English name of SC is sometimes referred to as "Square Connector", because the shape of SC is always square.
The shell is rectangular, and the structure size of the pin and coupling sleeve is exactly the same as that of the FC type. Among them, the end face of the pin mostly adopts the PC or APC grinding method; the fastening method is the plug-in bolt type, without rotation.
Advantages: Standard square connector, direct plugging and unplugging, easy to use. Using engineering plastics, high temperature resistance, not easy to oxidize.
Disadvantages: The connector is easy to fall out.