The low cost charger schematic is very simple because the entire circuit must be fitted in a 5x5 cm printed circuit board. The charger/discharger is based on a variable current source/sink directly driven by a small PIC microprocessor (PIC12F675).


The current source is based on a very basic and simple circuit. The 5V PIC output is firstly reduced by a trimmer (that in fact sets the maximum charge current) and filtered low pass in order to allow the current reduction with a PWM modulation. This signal (that should be at the pin 12 of the operational at 0.5V maximum) enters in a block (U5D and Q5) that translate the 0.5V to the Power supply minus 0.5V. This referencing is useful for the current source that equalize the voltage drop caused by the charge current in 0.1Ohm resistor to this input signal. In practice this force the maximum charge current to 5A with a setting of 0.5V in input. The power mosfet is a very general purpose P channel capable of sourcing much more than the requested 5A. The selected operational amplifier is selected with two main features: the rail to rail input and output stage (necessary for current source driving) and the power supply (at least 12V if the charger must operate with 6 NiCd or NiMh cells).


The discharge block is very simular to the charge block, and in practice is a variable current sink. The trimmer R18 sets the maximum discharge current, that has a conversion factor equal to the charger: 0.5V in input are converted in 5A current.


The PIC processor is configured for a internal 4MHz operations and the key on a master reset act as "START" key. The other two keys are for operational mode change. The charge and discharge output also drives two leds for charge/discharge process identification. The power supply is a very basilar 5V, 100mA linear regulator for PIC purposes.