of each of comparators 27 29 31 and 33 another input of comparator 27 receives the selected portion of the voltage across the motor 11 as set by potentiometer 35 which is normally higher than the signal applied to the one input of comparator 27 by the amplifier circuit 25 the resulting low output 37 enables transistor 39 to activate potentiometer 41 as the control for setting motor current under light load this signal from potentiometer 41 is applied via amplifier 45 to the regulating transistors 43 which assure that the current through the motor is sufficient to maintain substantially constant flow of air through the chamber 19 as the differential pressure thereacross increases and as battery power decreases over long periods of operation as motor current i increases to maintain constant air flow against increasing back pressure the amplified signal at the one input to comparator 27 exceeds the portion of signal from potentiometer 35 provided by the setting thereof and the resulting low output 47 from comparator 29 enables transistor 49 to activate potentiometer 51 as the control for setting motor current under heavier load this signal is then amplified by amplifier 45 and is applied to the regulating transistors 43 which assure that adequate current is supplied to the motor 11 to maintain substantially constant air flow rate through the chamber 19 under conditions of increased back pressure i e heavier motor load the resulting current profile as a function of motor load 62 is shown in fig2 an integrating capacitor 68 is connected across the motor 11 to smooth the transition in drive current attributable to the switching over from one current setting 41 to the other current setting 51 the reference signal.