Capacitor charge redistribution

[aislam]

Hi guys!

I am trying to verify a simple charge re-distribution scenario between two capacitors using Multisim. I have a 220 nF cap initially charged to 5V and it is connected to a 5 pF cap through a 35 ohm resistance and a switch that is normally open but closes at some time To.

Theoretically, after the switch closes at time To, I should see the 5 pF capacitor charge to almost 5 V, while the 220 nF capacitor drops ever so slightly below 5 V. I expect the charging time constant to be very small, since the RC is basically 35*5*10^-12.

This is not what happens in simulation. The output actually shows the voltage of both caps continuing to rise beyond 5 V after the switch closes. Any ideas what went wrong? (circuit file attached)

Thanks,

Anik

[Tien_P]

Hi Anik,

I want to isolate your RC circuit so I removed the ground.  The hanging resistor is there because SPICE will complain if you don't have a ground somewhere in your circuit.  Run the Transient analysis and you should see the voltage across the capacitor.

[aislam]

Hi Tien,

Thanks for helping! I just noticed that the time constant in the simulation is huge! It takes about 199 uS for the 200 nf to charge up the 5 pf. Changing the value of the 35 ohm resistor has no impact on this time constant, which is somewhat counterintuitive. Is that what you would expect?

Anik

[Tien_P · Accepted solution]

It looks like the simulation is not taking enough samples.  In the Transient analysis, you should see "Tmax" radio button under the "Analysis parameters" tab, make this number smaller will force Multisim to take more samples, try 1e-9sec since you have a small time constant.