techniques for controlling motor winding current by employing on / off duty cycle control switching between an upper and a lower current threshold to establish an average motor winding current approximately midway between the two thresholds . rising current during on time intervals is directly measured , and an upper threshold detector is employed to turn off a switching transistor when the upper current threshold is reached . rather than directly measuring decay or flyback current which circulates in the motor windings through free - wheeling diodes for comparison against the lower current threshold , the decay time is simulated . a lower current threshold detector and an upper current threshold detector are employed . following winding energization , the lower threshold current detector starts a timing device . the upper threshold current detector subsequently stops the timing device , and turns off the switching transistor . winding current then decays , and the decay time needed to reach the lower threshold is simulated by waiting for a decay time interval which is a function of the measured rise time interval .