0
\$\begingroup\$

Consider the following circuit:

enter image description here

I want to acquire Q-V characteristics of the capacitor C using that circuit, with LTSpice. How can I do this? Does LTSpice provide a straightforward method for measuring Q, or should I find an expression involving electrical laws and that can be evaluated using LTSpice?

(I am using LTSpice v4.23)

\$\endgroup\$
5
  • \$\begingroup\$ What "Q-V" characteristics? What is it you want to measure in electrical terms? The capacitor in LTSpice is ideal as long as you don't supply parameters. \$\endgroup\$
    – Felix S
    Commented Nov 6, 2016 at 18:42
  • \$\begingroup\$ You can measure dQ/dt in Spice. \$\endgroup\$
    – user16324
    Commented Nov 6, 2016 at 18:49
  • \$\begingroup\$ Q=CV initial would apply for a step charge with DC input = CV output on big Cap except conservation of energy factors loss of I^2R*t in Ri or as Bri says for I=dQ/dt, in the end you have a partial Capacitive voltage divider like a resistive divider if Ri is low Cs/(Ci+Cs)*Vin=Vout \$\endgroup\$
    – D.A.S.
    Commented Nov 6, 2016 at 18:49
  • \$\begingroup\$ Of course the capacitor being used in LTSpice is ideal, but I want to see the expected characteristics using the other measurable quantities of the simulation, as one may use the methodology used in LTSpice in real oscilloscopes. \$\endgroup\$
    – jnbrq -Canberk Sönmez
    Commented Nov 6, 2016 at 18:53
  • \$\begingroup\$ The integrator is useful in DSO measurements where fo<< ripple frequency then AC ripple is across R and pure DC is across integrator cap, which is why example shows 100Hz the frequency of a rectified 50Hz, then the integrator needs to be 8T or 80ms for 10% ripple or 50T for <<1% ripple \$\endgroup\$
    – D.A.S.
    Commented Nov 6, 2016 at 18:56

1 Answer 1

Reset to default
1
\$\begingroup\$

Since $$ Q_C = \int i_C dt = \int \frac{v_1}{R_{100}} dt $$ So if you put a perfect integrator on \$R_{100}\$ then you are measuring \$Q_C\$ (times a constant).

Now for example, if you have Spice tabulate \$v_C\$ and \$Q_c\$ at various time and scatter-plot them on a graph, you should see them all fall on a straight line for an ideal C. The slope of the line would be: $$ \frac{Q_c}{v_c} = C $$

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.