Cómo modificar una fuente conmutada de tensión fija a variable, para poder cargar baterías

This source comes from a discarded printer and provides two 40 Vdc outputs. Unlike other switched power supplies, it does not have the TL431 IC known as a variable Zener, but instead has an ALY type BJT transistor in SMD format that feeds a zener diode. This transistor is biased by a set of resistors at the optocoupler output. The modification consists of removing the set of SMD resistors that bias the base of the ALY transistor and replacing it with a potentiometer. In this case, the resistive value of said potentiometer is 50K. Potentiometers of 100K or more can be used to obtain a greater voltage range. I have obtained an interval of 7 to 22 Vdc. The maximum intensity it gives is 2.7 A so it is perfect for charging my small Pb / Acid and lithium batteries. In this way we can reuse recycled circuits, which would otherwise go to waste, which is a good way to do your bit in conserving our planet.