This simple charger was found in a 1980s documentation. The transistor may be the TIP42 with a heat sink. R1 adjusts the load current. The transformer has 1 A of secondary current.
This circuit is from the 1990 Motorola Linear Databook. The integrated circuit used is the LF355,...
This circuit, which in our times would be called a shield, can also be used with other loads. The...
This simulation shows how a transistor can be used as a switch turning on and off in the LED. The...