Circuit for measuring a capacitance during constant charging and discharging periods

A measuring circuit for measuring a capacitance according to the discharge method, comprising:

• a measuring circuit input;
• a constant voltage source for charging a capacitor to be measured at said measuring circuit input;
• a discharge resistor with a known resistance being connected to the capacitor and through which the capacitor discharges;
• at least one switch connected between the capacitor and said constant voltage source;
• flow control means in the form of a microprocessor connected to said at least one switch for controlling said at least one switch to determine charging and discharging phases;
• a comparator connected to the capacitor, said comparator issuing an output signal;
• a counter associated with said microprocessor and connected to said comparator for counting clock pulses during a discharge until said comparator signals that a discharge voltage at the capacitor has dropped below a given reference voltage and therefore a discharge time is ended;
• current limiting resistor means for protecting against error voltages connected to said measuring circuit input in series with the capacitor, a charging current passing through said current limiting resistor means, for limiting a time constant for the charging, being increased through said current limiting resistor means, to a maximum value by a restriction of an allowable capacitance range, and for maintaining a charging time of the capacitor within a scope of typical measuring times;
• and
• said comparator having means for preventing incorrect displays and for detecting if, within a fixed given charging time, a charge voltage does not attain a given voltage value because the capacitor exceeds the maximum value of the allowable capacitance range, and for detecting if a given minimum value of the charge voltage is attained at the capacitor and for delivering a corresponding output signal to said flow control means, said flow control means being programmed for detecting, from the output signal of said comparator, if the given minimum value of the charge voltage was exceeded during the fixed given charging time.