a resistor r6 is provided to sense the amount of current i as illustrated by fig2 a flowing through the fan f1 the resultant signal 10 is fed into the positive input of an operational amplifier s1 which is part of the current monitoring circuit x1 the negative input of the operational amplifier s1 is connected to a resistor r1 and to a feedback resistor r3 the resistors r1 r3 and the operational amplifier s1 work together to amplify the current i detected by the resistor r6 by a factor determined by the value of resistor r3 divided by the value of resistor r1 the resultant amplified signal 20 as illustrated by fig2 b is fed to a conditioning circuit x2 conditioning circuit x2 comprises a high pass filter network comprised of a capacitor c1 and a resistor r5 and a base clipping diode d1 the high pass filter is provided to eliminate a direct current signal which results if the fan f1 jams while operating the diode d1 is provided to clip the resulting alternating signal from the operational amplifier s1 to create a current pulse the resultant signal 30 as illustrated by fig2 c is fed into the base of.