| 摘要 |
<p>Electronic control circuit 10 controls the application of power from a motor 14 to a shearing hand piece 12. The circuit 10 includes a capacitive bridge circuit 16 having a first arm 18 electrically coupled with the shearing hand piece 12 via a drive shaft 13; and, a second arm 20 coupled to ground through an adjustable capacitor 22. A signal is tapped from point 24 between the first and second arms 18, 20 which is indicative of a capacitive imbalance between arms 18 and 20. The signal on line 26 is provided as an input to a comparator 28. Provided the voltage on line 26 is above a threshold voltage, comparator 28 energises an indicator LED 30 and an on-delay timer 32. The timer 32 provides an output signal to close relay 34 provided the on-delay timer 32 is maintained in an active state by the comparator 28 for a predetermined continuous period of time. After expiration of this predetermined continuous time, the timer 32 maintains the relay 34 in a closed state provided the comparator 28 maintains the timer 32 in an active state. The relay 34 closes a circuit to energise the motor 14 thus providing drive to the hand piece 12.</p> |
