I am fiddling with a simple circuit in falstad.
The voltage across the capacitor goes above the source voltage by a large margin.
Does it really happen or is it a simulation error ?

Falstad link:
https://bit.ly/2lhXmKM

 

No, the voltage cannot go above +12V in that circuit, so there is some sort of error in the simulation.

How do I know that it's looking at the capacitor voltage?

 

hi,
Running the Falstad link shows 10V drop across the diode, which it is adding to the 12V source voltage.?????
E

 

Hello,

I have tried several diodes, but all show the 10 volts voltage drop.
This is a simulation part failure.

Bertus

 

No, the voltage cannot go above +12V in that circuit, so there is some sort of error in the simulation.

How do I know that it's looking at the capacitor voltage?

right click on the capacitor and click > view in scope

 

Hello,

I have tried several diodes, but all show the 10 volts voltage drop.
This is a simulation part failure.

Bertus

technically a -10 volt drop, or a 10 volt rise

removing the diode does solve the problem partially, but then another interesting problem comes up, damped oscillations.

 

Why would you believe a simulator whose name stats with fals?

Bob

 

Falstad seems to be the "Simulator D'Jour" lately. Is this what is being taught in school this semester?

 

You cannot exceed the voltage level on the source side. If anything, it will be 11.7, or thereabouts, due to the diode. Your simulator is buggy. Try everycircuit.com instead, for simple things.



Plus, this way you can see the voltage behavior, showing how the capacitor is fluctuating.

 

I am fiddling with a simple circuit in falstad.
The voltage across the capacitor goes above the source voltage by a large margin.
Does it really happen or is it a simulation error ?

Falstad link:
https://bit.ly/2lhXmKM

If the +12V source is modeled with a series inductance, this is what I would expect.

 

Hi,
Tried with a 1R instead of the 1m, still 22v.
E

 

what was initial state of the capacitor? was it already at 22V? then of course diode will be reverse biased, have 10V accross and output will be steady.

 

i just deleted everything and started from scratch. looks good. i guess that capacitor was part of previous circuit and was charged above voltage source then the circuit was modified. maybe just voltage source was dropped from 22+ V to 12V.

then went to original link, changed R from 1 milliOhm to 500 Ohms, put jumper across C and as expected voltage across cap dropped to zero. then when jumper was removed, capacitor charged as expected.

 

Ditch the cheesy simulator and use a professional simulator based on SPICE. LTSpice is a popular variant and is well supported through the yahoo group - and best of all it's FREE!

https://www.analog.com/en/design-center/design-tools-and-calculators/ltspice-simulator.html

 

i just deleted everything and started from scratch. looks good. i guess that capacitor was part of previous circuit and was charged above voltage source then the circuit was modified. maybe just voltage source was dropped from 22+ V to 12V.

then went to original link, changed R from 1 milliOhm to 500 Ohms, put jumper across C and as expected voltage across cap dropped to zero. then when jumper was removed, capacitor charged as expected.

no, the capacitor was not having any charge previously.
could you share a link of your simulation, where the capacitor charges as expected.

 

i think it did, once it got discharged, it behaved the way you expected. as mentioned, i just increased resistor value and added switch in parallel to be able to force discharging. you can add another switch to charge capacitor to higher voltage (or you can increase 12V if like). when you reduce supply to 12V capacitor remains charged and simulator does not reset this even if you delete and add capacitor.

here you go, just close then open switch
https://tinyurl.com/yygup6pu

EDIT

simulator really does not like low resistor values. anything below 1Ohm will produce overshoot related to used resistance. if you set resistor value to 500mOhm, you will see vertical spike. the 1mOhm value is too small for practical circuits anyway. set few ohms or hindreds of ohms and this may work for you.

 

yes, increasing the resistance value does solve the problem.

 

RE: ""Does it really happen or is it a simulation error""
Check for parasythic inductance of resistor. If it creates the time constant larger than diode`s delay, the multiplication of voltage is reality.

Falstad link:
https://bit.ly/2lhXmKM[/QUOTE]

 

RE: ""Does it really happen or is it a simulation error""
Check for parasythic inductance of resistor. If it creates the time constant larger than diode`s delay, the multiplication of voltage is reality.

Falstad link:
https://bit.ly/2lhXmKM

[/QUOTE]

that's very interesting.
yes if there is an inductor it would cause the capacitor voltage to go above source in real.
and the diode would prevent it from discharging.

for the simulator too, there might be some inductance counted as a part of the resistor.

indeed the simulator is not wrong actually !!