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Capacitance Meter


The phenomenon appers until state of the circuit changes steady-state after state change, is called Transient. It is one of the fundamental operations of pulse circuit. When the switch in Figure 1a is opend, the capacitor C will be charged through the register R and voltage Vc will vary like shown in Figure 1b. To change state of the circuit, changing the value of EMF E instead can also be thought that equivalent. The relation between past time t and voltage VC is expressed in following formure.

---------- (1)

Each units are: t seconds, R ohms, C farad and epsilon is a Napier's number (approx. 2.72). When VC reaches VC1, the time t1 can be expressed in following formure.

----- (2)

This means that the t1 is proportional to C. Thus the capacitance can be calcurated from charge time and any other fixed parameters.


The schematic shown below measures capacitance from 2.2pF to 1000pF in the L (low) range, and from 1000pF to 2.2uF in the H (high) range. ICD of the 74HC132 (pin 11) produces a 300 Hz square-wave clock. On the rising edge CX rapidly charges through D1. On the falling edge CX slowly discharges through R5 on the L (low) range and through R3-R4 on the H (high) range. This produces an asymmetrical waveform at pin 8 of ICC with a duty cycle proportional to the unknown capacitance; CX. This signal is integrated by R8-R9-C2 producing a dc voltage at the negative meter terminal proportional to the unknown capacitance. A constant reference voltage is produced at the positive meter terminal by integrating the square-wave at ICA, pin 3. R6 alters the symmetry of this square-wave producing a small change in the reference voltage at the positive meter terminal. This feature provides a zero adjustment on the L (low) range. The DVM measures the difference between the positive and negative meter terminals. This difference is proportional to the unknown capacitance.

Test and Calibration...

Without a capacitor connected to the input terminals, set SW2 to the L (low range) and attach a DVM to the output terminals. Set the DVM to the 2-volt range and adjust R6 for a zero meter reading. Now connect a 1000pF 'calibration' capacitor to the input and adjust R1 for a reading of 1.00 volt. Next, switch SW2 to the H (high) range and connect a 1.00uF 'calibration' capacitor to the input. Adjust R3 for a meter reading of 1.00 volt. The 'calibration' capacitors do not have to be exactly 1000pF or 1.00uF, as long as you know their exact value. Fro instance, if the 'calibration' capacitor is known to be .940uF, adjust the output for a reading of 940mV.

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