Patent ID: 7057309

Claim:
A power supply for an electrical apparatus, said power supply comprising: an input for receiving power from a commercial power source; an output for delivering power to the electrical apparatus; a switch for selectively coupling said input to said output; a removable battery coupled to said output; a thermistor coupled to said removable battery, said thermistor having a thermistor output that is coupled to a thermistor terminal when said removable battery is coupled to said power supply; a resistor, said resistor having a first end connected to a voltage source set at a first voltage level, and said resistor having a second end connected to said thermistor terminal, wherein a thermistor terminal voltage at said thermistor terminal equals said first voltage level when said removable battery is removed from said power supply, and wherein said thermistor terminal voltage is changed by said thermistor when said removable battery is coupled to said power supply; a circuit for detecting removal of said removable battery and for activating said switch to couple said input to said output upon detection of the removal of said removable battery, wherein said detecting removal of said removable battery is performed by a comparator in said circuit for detecting removal of said removable battery, said comparator being capable of comparing said thermistor terminal voltage to a reference voltage that is set to said first voltage level, and wherein said comparator outputs a “no battery” signal when said thermistor terminal voltage and said reference voltage are equal due to said removable battery being removed; and a capacitive circuit for storing electrical energy, said capacitive circuit being coupled to said output, said capacitive circuit temporarily supplying power to said output in response to the removal of said removable battery, and wherein said capacitive circuit is charged only while said removable battery is being refreshed by a process that includes completely discharging said removable battery to avoid memory effect in said removable battery.