Members of my family still owned a couple of old battery chargers manufactured by my late godfather's (has to do with religion, not criminal activities, I was young and still believed, back then) factory, SILMA di Silvano Magrin, based on old AC power transformers, not up to todays energy efficiency standards, which I also had to fix already more than once.
Yet another fault, made me rebuild the charging circuit from scratch, employing a more efficient PWM method, managed by a PIC12F675, driving a IRF9540 P-mosfet. While on the old chargers I left the power transformer in place, which supplies about 14V and more than 5A, I employed a 3A switching at 14.4V, for the few new ones I have built.
Since the PIC microcontroller, programmed in Hitech C, works at 5V and does a A/D conversion of battery voltage, I had to divide it by three and also add a 5.6V zener, in order to protect the input from any overvoltages. It also computes various charging PWM levels, so as to limit power dissipation at around 1W. Charging state is indicated by two LED, a red and a green one. Red indicates charging process in place, while green, also by means of a flashing sequence, indicates battery voltage, between 11.5 and 13.85V. When battery is finally charged, green led stays on and red off, then processor waits, doing nothing, until voltage goes down again, starting then a new charging cycle. I have also implemented, in software, a short circuit protection and battery fault indication.
Here is the schematic, drawn in DipTrace.
And dual layer circuit.
And 31x47mm PCBs, which I had made out of my files. There are minor differences with drawings, due to successive minor modifications, mainly because I liked it better that way.
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