I need a circuit to convert variable capacitance value to considerable variable DC voltage.

Capacitance value range = 0pF ~ 20pF DC value range needed = 0V ~ 6V

Please explain in detail as i am newbie. Thanks in advance.

  • \$\begingroup\$ You mean like a frequency-to-voltage converter? \$\endgroup\$ Sep 22, 2015 at 10:58
  • \$\begingroup\$ capacitance to voltage converter \$\endgroup\$
    – Punith R
    Sep 22, 2015 at 10:59
  • \$\begingroup\$ Which is implemented as a frequency-to-voltage converter. \$\endgroup\$ Sep 22, 2015 at 11:00
  • \$\begingroup\$ I made a circuit using 555 generating 1.3KHz PWM with 96% duty cycle and fed that output to the capacitance under test and then amplified using opamp.DC voltage is varied from 0.5V to 12V for value of capacitance 1nF-15nF. \$\endgroup\$
    – Punith R
    Sep 22, 2015 at 11:02
  • \$\begingroup\$ now i need DC voltage change of 0-6V for 0pF-20pF. \$\endgroup\$
    – Punith R
    Sep 22, 2015 at 11:03

1 Answer 1


There are a number possible approaches.

  1. Use the fact that impedance at a fixed frequency varies inversely with capacitance. You basically make a resistance meter, but with AC. You put in a signal at known voltage and frequency, then measure the voltage of what you get out.

  2. Use the fact that a capacitor together with a inductor define a frequency. Use the capacitor under test with a known inductor as the frequency-setting part of a oscillator, then measure the frequency.

  3. Dump a fixed charge on the cap and measure voltage.

  4. Charge the cap to a fixed voltage, then discharge thru a known resistor and measure how long it takes to get to a known point on the discharge curve, like 2/3 discharged for example.

  5. Use a known capacitor to see how the two share voltage after being connected with one charged and the other discharged.

  6. Set up a camera with a machine vision system feeding optical character recognition and read the label on the cap.

  7. Use your own brain and get creative.

  • \$\begingroup\$ Please elaborate on 3 and 4 \$\endgroup\$
    – Punith R
    Sep 22, 2015 at 12:09

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