# 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
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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?

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2. ### MrChips Moderator

Oct 2, 2009
12,652
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Put a known resistance in series between V1 and C1.

3. ### Bakez Thread Starter New Member

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

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4. ### MrChips Moderator

Oct 2, 2009
12,652
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Try R1 = 20Ω.

5. ### Bakez Thread Starter New Member

Aug 21, 2012
28
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ok

What am I supposed to be measuring?

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6. ### MrChips Moderator

Oct 2, 2009
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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.

Aug 21, 2012
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8. ### MrChips Moderator

Oct 2, 2009
12,652
3,461
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
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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
12,652
3,461
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