Pure Capacitor Circuit Help

[kirrri]

Hi there,

A project I'm working on is the effect of resistance, inductance and capacitance on a single phase system. A 230Vrms supply is connected in series with a user-defined resistors, inductor and capacitor.

In the brief, we have been told that when simulating this circuit when looking at the effect of a pure resistor we must set inductance to 0H but the capacitance must be set to a value such as 1F. I understand this, as this capacitor creates a virtual short circuit across it and we cannot use a 0F capacitor as this would essentially be a break in the circuit. I also understand why the inductor can be set to 0H.

What I do not understand is the guidance given when investigating the effect of a pure capacitor circuit - inductance is again to be 0H but the resistance should be set to 1 Ohm? I would have expected to use 0 Ohms.

My initial thoughts are that this relates to the time constant equation, t=RC - hence if R was 0 then there would be no time constant and therefore the circuit would not function properly. Is this correct?

Many thanks,

Kyran Hill

 

[(*steve*)]

Possibly it is a limitation of the software something along the lines of what you suggest.

 

[Ratch]

Hi there,

A project I'm working on is the effect of resistance, inductance and capacitance on a single phase system. A 230Vrms supply is connected in series with a user-defined resistors, inductor and capacitor.

In the brief, we have been told that when simulating this circuit when looking at the effect of a pure resistor we must set inductance to 0H but the capacitance must be set to a value such as 1F. I understand this, as this capacitor creates a virtual short circuit across it and we cannot use a 0F capacitor as this would essentially be a break in the circuit. I also understand why the inductor can be set to 0H.

What I do not understand is the guidance given when investigating the effect of a pure capacitor circuit - inductance is again to be 0H but the resistance should be set to 1 Ohm? I would have expected to use 0 Ohms.

My initial thoughts are that this relates to the time constant equation, t=RC - hence if R was 0 then there would be no time constant and therefore the circuit would not function properly. Is this correct?

Many thanks,

Kyran Hill

If you are trying to energize a capacitor to any voltage and no resistance, it will reach the applied voltage in an infinitesimal amount of time and draw an infinite amount of current during that time. Of course, your voltage source must be able to supply that infinite amount of current for that small amount of time. The time constant of the circuit will be zero. When the capacitor reaches the applied voltage, the current draw of the voltage source will be zero. No simulation is necessary for that situation.

Ratch

 

[(*steve*)]

If you are trying to energize a capacitor to any voltage and no resistance, it will reach the applied voltage in an infinitesimal amount of time and draw an infinite amount of current during that time.

Doesn't i = C dv/dt?

 

[Ratch]

Doesn't i = C dv/dt?

Certainly. The derivative of the step voltage across the capacitor is plus infinity until it reaches the applied voltage, followed by a derivative of zero when the capacitor reaches the constant applied voltage.

Ratch

 

[(*steve*)]

Apart from any initial step (assuming initial conditions other than all zeros), the voltage applied is AC. The simulation should handle that without infinities.

 

[Ratch]

Apart from any initial step (assuming initial conditions other than all zeros), the voltage applied is AC. The simulation should handle that without infinities.

Correct. The derivative of a sinusoidal is another sinusoidal. No infinite derivatives will be involved, so the voltage source should able to supply the required current. The voltage across the cap will simply be the applied voltage.

Ratch