How do I calculate Rs and Cs from these waveforms?

Discussion in 'General Electronics Chat' started by Bakez, Jul 23, 2013.

  1. Bakez

    Thread Starter New Member

    Aug 21, 2012
    28
    0
    I am trying to work out how an LCR meter works as I want to build one of my own to measure small capacitors, nF range

    As far as I have read, an LCR meter applies a sinusoidal signal to a capacitor and calculates the parameters from the resulting waveform (using phase and magnitude)?

    So I put a capacitor of 2nF and ESR 2Ω in LTspice and a voltage source of 500mV, 10Mhz

    These are my resultant waveforms. How do I now calculate the 2nF and ESR from these waveforms?

    Am I missing something?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    Put a known resistance in series between V1 and C1.
     
  3. Bakez

    Thread Starter New Member

    Aug 21, 2012
    28
    0
    OK I put 2 ohm resistance between cap and voltage source
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    Try R1 = 20Ω.
     
  5. Bakez

    Thread Starter New Member

    Aug 21, 2012
    28
    0
    ok

    What am I supposed to be measuring?
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    Measure the voltage across R1 or C1.
    Do the calculation.

    btw, this is not how an LCR meter works unless you build a computerized LCR meter.
     
  7. Bakez

    Thread Starter New Member

    Aug 21, 2012
    28
    0
  8. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    Why did you choose 10MHz test frequency?
    What is the relationship between the 10MHz input signal and the measured quantity?
     
  9. Bakez

    Thread Starter New Member

    Aug 21, 2012
    28
    0
    Xc = 1 / (2 * pi * f * C)


    Dont i want to make Capacitive reactance small so I can measure the resistance properly, so I have to make a large frequency?
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,364
    Depends on what you want to measure.
    Do you want to measure capacitance or ESR?
     
  11. Bakez

    Thread Starter New Member

    Aug 21, 2012
    28
    0
    Rs, ESR predominantly
     
Loading...