Capacitance Meter with Autorange Feature

Figure 1 

The circuit is a powerful capacitance meter with an autorange version in order to prevent the need to adjust the range settings of capacitors.

The capacitance meter design was based on a very simple circuit analysis of charging and discharging of capacitors in an RC circuit. The measuring range of the meter is quite large with the range from 5pF to 2600uF. The operation of the circuit is being handled by the PIC16F873A microcontroller.

Using the RC circuit, the charging voltage is 63.2% of 5V which will be about 3.16V. This value is derived from the equation where Tau is equal to RC where Tau is the Time constant of any given RC circuit. This will be the reference voltage for the comparator module on the board of the PIC16F873A microcontroller. The actual capacitor voltage itself is the other input to the comparator.

The operation of the capacitance meter starts by discharging the capacitor fully. It will then charge the capacitor and holds until the 0.632Vcc voltage across the capacitor is reached. The capacitance will be computed by capturing the time. The project is using a 16 bit division routine. The LCD displays the result.

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