Power across voltage controlled capacitor

[rachelgl]

Hi,  I have the a basic enough circuit (attached). I am modellling a mechanically variable capacitor using the voltage controlled capacitor element.

I wish to measure the power across this capacitor as I am trying to calculate the electrical efficiency of the circuit, but it seems that this voltage controlled capacitor element is not really acting as an actual capacitor.

Can anyone help me on this?

Thanks

Rachel 

[Mahmoud_W]

Hi Rachel,

I looked at the circuit. The voltage controlled capacitor is not connected to a voltage source to act as a capacitor (unless you're controling it using LabVIEW which is another possibility). You need to connect a voltage source to the two bias terminals of the voltage controlled capacitor and according to the conversion ratio (how many Farads per volts) the other two terminals will act as a capacitor.

Please check the attached circuit and let me know if you need more help.

Mahmoud W

National Instruments

[rachelgl]

Hi,

Sorry, I actually do have it connected to a voltage source but it is in a sub-block which I havent included in the original attachement as I didn't want to confuse the situation. The actual capacitance varies perfectly with voltage, I just need to figure out how to find the power across the voltage controlled capacitor.

Regards

Rachel

[Mahmoud_W]

Hi,

You can try these 2 power measurements you can perform:

- Instantaneous power, you find this out by running a transient simulation and defining an output as the product of the current through the capacitor and the voltage accross it (you should write and expression for that). Or simply, it is the instantaneous reading of the Wattmeter (might not be fast enough

- RMS power, again in a transient simulation calculate the RMS V and I of the capacitor and multiply them

I am not sure if this is what you are looking for. Please let me know if you have other question.

Mahmoud W

National Instruments

[rachelgl]

Hi,

Unfortunately the problem I am having is in trying to get the current through the capacitor. As there is no I(C_var) variable in the simulation variables, I tried to take is as a difference in diode currents, but this isn't really working either. This is why the voltage controller capacitor is not acting like a regular capacitor component (where I(C) and P(C) both exist).

Regards

Rachel

[Mahmoud_W]

Hi Rachel,

If you add a probe to the net you can sample the voltage and current of it.

Check the attached file, what you're looking for in your transient simulation result are I(probe1) and V(probe1)

Mahmoud W

National Instruments

[rachelgl]

Hi,

Again, I'm not sure this probe is working correctly as I'm not sure the capacitor model is realistic enough. I want to determine electrical efficiency of my circuit (Pout/Pin). if i take Pin to be across my capacitor and Pout to be across my load resistor, i get a crazy value (in the range 10^3) when ideally i'd like a result in the range 10^-3 or 10^-2 so that when I multiply by 100, i get a percentage less than 100%.

Thanks for help so far!

Rachel

[Mahmoud_W]

Hi Rachel,

I verified that the probe and the Voltage Controlled Capacitors (VCC) are modeled correctly.

Just simply run a transient simulation with a VCC that give 10nF/V and apply 1V to it, so a total of 10nF and plot in the output the waveforms of V and I. Then replace the VCC and its bias with a regular 10nF capacitor and you will get the same waveforms.

About the efficiency, I think it is about how you define it. Usually Pin is the power from the supply not the capacitor power.

Hope this helps,

Mahmoud W

National Instruments

[rachelgl]

I initally considered the power from the supply, but if I take the power of the input supply to be Pin and the power across the load resistor to be Pout and try to simulate Pout/Pin, the result doesnt make sense. Also my multisim simulation has a few subblocks to model the mechanical action of the mechanically variable capacitor so my interface circuit doesnt really take into account the electrostatic forces etc. I thought I could simplify the efficiency by taking it across the capacitor or by using an expression (0.5*C*V^2)/t. This is what I had previously done when I was using a square wave input voltage, but now that I have changed it to a sine wave this doesn't seem to work so well.

Thanks for your help. I feel like I am a burden now 🙂 so I will try ask around the university for help.

Regards

Rachel

[johnsold]

Rachel,

The term "power across...capacitor" does not make any sense. Power is generated by or dissipated in a device, but not across it.  An ideal capacitor dissipates no power.  The expression you used, (0.5*C*V^2)/t, meets the definition of power, but if V varies with time you need to do an integration to get a realistic value.  Once you have the value, you need ot decide what it represents.  It appears that it may be some kind of average of the stored energy over time.  This makes it comparable to the VARs used in utility power distribution systems to measure or correct for power factor.

You talked about efficiency but were not clear how you were defining that.  Efficiency in a stored energy system needs to be carefully defined.

I do not have Multisim so I cannot look at your files.

What is your overall purpose, as opposed to capacitor power?

Lynn