Electronic – Converting variable capacitance value to variable DC voltage level

capacitancecapacitormeasurementsensorvoltage measurement

I need a simple method to convert a small variable capacitance value into considerable variable DC voltage.

  • Capacitance value range = 0nF ~ 14nF
  • DC value range needed = 0V ~ 6V

What I've tried is shown below. However final output should be a pure DC level without any ripples for a fixed Cx. But the following circuit seems to be still have ripples or noises.

  1. Is there any improvements to be made for this circuit to get a pure
    DC output?
  2. Are there any other methods/circuits to do this task?

enter image description here

  • Ro=100k
  • Oscillator frequency is about 71.78 Hz (t1=7mS, t2=6.93mS)
  • Cx is the variable capacitor (Range 0nF~14nF)

Best Answer

Maybe a linear ramp 555 circuit with an adjustable threshold and a timer chip would do. Here's the linear ramp using the 555: -

enter image description here

"C" in the circuit above could be the capacitor under test. The smaller the value the quicker the ramp. You can use the 555 output rising high to reset-trigger a counter and the counter value can be "frozen" when the ramp reaches a certain level. A precision comparator connected to the ramp would tell you the ramp has reached a certain level.

If you connect the output of your counter to a DAC and latch the DAC output when the comparator triggers (say 2/3 of way up the ramp) you have a linear, capacitor_to_voltage generator. Adjust R2 to give you the desired range of capacitance you need.

You could also use 7 segment displays to output the counter in hexadecimal.

The counter needs to be running a lot faster than the ramp to give you decent resolution. Anyway, the formulas are on the picture.

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