PWM battery charger

7-18 elektor july/august 1980 25 This circuit is designed to charge 6 V/3.5 Ah batteries similar to those often used in flash equipment. Of course, there are all kinds of methods to charge lead-acid batteries but what is special about this version is the fact that the charge current is continually corrected according to the state of the battery. Figure 1 shows the block diagram of the PWM battery charger (PWM stands for Pulse Width Modulation). Al is a squarewave oscillator which generates a frequency of around 2 kHz. A2 is a monostable multi- vibrator which is triggered by the negative-going pulses from Al. The pulse width from A2 depends on the control voltage which is derived from the differential amplifier A3. The latter constantly monitors the battery voltage. The output of A3 varies according to the difference in voltage between the preset reference level and the tested battery level. When both are equal the output voltage of A3 will be such that the duty-cycle of A2 will be...