RC Circuit problem...?????

[malli_1729]

Hi!

this is a basic doubt,.
here i am attaching image files of the circuits. it is just a RC Circuit with switches to PWR and GND.
test_WORes.jpg[/color]
This has basically R1(1k) and C1(22uf) conneted parallel to VCC(12) with on/off Switches.it simulates one part of our design.
I have simulated using multisim.
When switches ON output voltage across C1 is 12V OK.
when i make SW OFF voltage across the Cap is not fully discharging.it has path through R1 for discharging ..as time goes Voltage should be zero across C1, but it is 5V as shown in oscilloscope.How it is possible

test_Wres.jpg
Next circuit is i just added R2 in parallel with C2 to GND.
same as before i have simulated. when SW ON 12V ..OK.
when SW OFF Voltage now became zero.
actually as far as i know capacitor should not discharge through R2 because it doen't have return path for current to come back to Cap.it has only path to GND.

whether i am correct or else if there is no return path also Capacitor can able to discahrge to GND....

 

[keith1200rs]

You have the ground reference of the "oscilloscope" connected to ground. If you want to measure the voltage across the capacitor then you need to put it to the other end of the capacitor. When J1 turns off, the whole circuit "floats".

That is the cause of both your problems.

Keith.

 

[yangjinhua]

definely correct

 

[malli_1729]

Thanks for your reply....

i didn't explained what is the problem clearly in prev post i think,,,,,

upto i know in both circuits one with R2 to GND and one WO R2 when J1 and J3 are OFF the whole circuit floats. But there is path for the capacitor to discharge through R1.
So, what i thought was at the start(Just J1 OFF), the discharging current flows throgh R1 and as time goes the it will die out ===>there is no voltage across C1
But in the Circuit without R2 to GND scope is showing 5V. why ????

In circuit with R2 to GND scope is showing discharging to '0'.
but, in both circuits there is no path to GND
as per u told that we have GND path through oscilloscope. it is there in both circuits then both circuits should show same behavior if it is the case. but why both readings are incorrect.

one more confusing thing is ....you are teling that there is GND path for Capacitor to discharge to GND. as per the KCL we definitely need a closed path for capacitor current , so, if the capacitor if it dischrges to GND throgh Osciloscope or R2 there is no return path from GND to capacitor(as it is floating when J1 OFF) to make closed CKT... so, how it will discharge through oscilloscope or R2 to GND
....................................................................................................................

as per you told i have connected oscilloscope ends across the capacitor and found the voltage it is decaying as per we thought....i am attaching that circuit also....

 

[keith1200rs]

If you disconnect the ground from the circuit and don't connect the "oscilloscope" across the capacitor your results will be nonsense - as you have found out. The one you just posted with the oscilloscope across the capacitor looks correct to me.

Keith.

Added after 1 minutes:

By the way - what are you trying to achieve with your simulations? If you want to analyse charge/discharge of a capacitor then you should be switching resistors in series or parallel with the capacitor, not switching the ground.

Keith.

 

[malli_1729]

OK....

thank you for patient reply.....

SO, if the circuit is floating we can't determine it's behavior according to the laws.......
.....................................................................................................................

actually it is one of our appliaction scenario...in acual circuit 12V is from regulator, Capacitor is 22uf only..but the load R1 is Sonalert(Piezoelectric buzzer). i don't think Sonalert is a pure resistive load but for now i just make it as a rsistive load to know it's behavior.
actually when power is ON the sonalert have to ring....it is working ....but when suddenly power OFF sonalert is giving sound for some time....which not required for our application. SO, we thought capacitor may be having some residual Charge which is discharging through sonalert momentarily ....
So, we need to discharge the capacitor in other means when Power OFF suddenly....
for that we are simulating......

Hope you understand my english....if i am not clear please let me kow...

thank you...

 

[keith1200rs]

SO, if the circuit is floating we can't determine it's behavior according to the laws.......

You can, but if you are trying to analyse what happens to the capacitor you need to monitor the voltage across the capacitor. Measuring the voltage with respect to something that isn't connected [the ground - after the switch has been turned off] doesn't make any sense.

Keith.