Synchronized primary winding current shunting technique for controlling electro-stimulus level

An electronic apparatus ( 1 ) for training an animal is supported against the animal's skin, and includes stimulus electrodes ( 5 ) for electrically contacting the skin. A controller including output terminals producing aversive stimulus control signals, a first switch (Q 4 ) coupled to a winding to produce therein a burst of first current pulses in response to a first signal produced by the controller ( 33 ) and a second switch (Q 2 ) coupled to the first switch (Q 4 ) operative to synchronously shunt predetermined trailing portions of the first current pulses away from the winding in response to a second signal produced by the controller to reduce the amount of energy delivered to the winding by the switching transistor (Q 4 ) without substantially changing a peak value of a flyback voltage across the winding. The controller sets various values of time intervals during which portions of the first current pulses are shunted away from the winding in order to set various corresponding intensities of aversive stimulus.