# Measure capacitance in softwire using Tina/Multisim/other simulators

Is there a way to measure capacitance of a capacitive network as shown below in Tina-TI, Multisim, or other simulators? I tried searching but I couldn't find any way to measure capacitance directly as compared to the DMM in the simulators. Our professor requires Tina-TI, but if it isn't possible, we'll just ask his permission if our group could use other simulators.

We are only limited to DC sources since it is the scope of our course. We also considered using frequency generator or oscilloscope but our professor did not allow us.

• Have you tried oscillating? Dec 6, 2013 at 1:41
• That was one of our option but we asked our professor if we can use that but he refused since the scope of our course is up to DC sources only, we cannot deal yet with AC. Dec 7, 2013 at 1:29
• DC sources can oscillate too. Dec 7, 2013 at 1:50
• Since you seem to be a student, I'll point out something for your future reference: those capacitors are huge. Multi-farad capacitors are relatively rare in electronics design. A four farad PCB-mount capacitor would probably be limited to a volt or two. Jan 6, 2014 at 2:12

Step a DC voltage with a fixed output resistance across the terminals. You should be able to do a transient analysis and estimate the time constant of the exponential voltage rise across the network. Time constant = RC, so if you know R, you know C.

If you can't do a step function, you're out of luck. You can't measure capacitance unless something is changing.

As far as I know, there is no direct way to measure capacitance in simulators (e.g. capacitance-meter). Please take note that for this project, we were only allowed DC sources.

We came to a solution in this problem by implementing what we learned about steady-state response of an RC circuit. As such, we included a resistor and a voltage source in our circuit. Here is one of the circuits we've designed:

In Tina-TI 9, there is a Steady-State Solver under the Analysis tab. It will generate a graph on the response of the capacitor during charging and until it reaches its steady-state. The x-axis is time, while y-axis is voltage.

Knowing that in each time constant τ (tau) there is a corresponding percentage of input voltage stored in the capacitor, we used the cursor tool to locate on the graph its corresponding time τ. We can then compute for the capacitance since $C = \fracτ{nR}.$

Here's one of the tables in our data sheet:

Here is the graph as simulated in Tina-TI 9.

You could add a sine wave source at the input terminals with a known amplitude and frequency. Run the simulation and find the voltages across and the currents flowing through each capacitor. You can then calculate the impedance of each capacitor by dividing the voltage across by the current through. Then the capacitance can be calculated from that impedance since the frequency is known. The total capacitance of the network can be calculated from the impedance found by dividing the generator voltage by the current through the generator.

• I am thinking of just using the impedance meter across a capacitor without any sources, and just compute for the capacitance using Xc = 1/(2*pifc), if setting up the frequency without any sources is possible. The scope of our project concerns only DC sources btw, we can't use AC sources. Dec 7, 2013 at 1:28