- 22nd February 2008, 03:09 #1

- Join Date
- Jun 2005
- Posts
- 599
- Helped
- 1 / 1
- Points
- 4,902
- Level
- 16

## parallel rc time constant

when i put a resistor and capacitor do i get a time constant? or what do i get when i put a resistor and capacitor in parallel its not a time constant but what?

Added after 9 minutes:

when i put a resistor and capacitor IN PARALLEL do i get a time constant?

- 22nd February 2008, 03:09

- 22nd February 2008, 06:12 #2

- Join Date
- Nov 2007
- Posts
- 33
- Helped
- 1 / 1
- Points
- 1,155
- Level
- 7

## rc parallel circuit

I think you what you get is time constant for the discharge time of the capacitor.

- 22nd February 2008, 06:28 #3

- Join Date
- Jan 2008
- Location
- Bochum, Germany
- Posts
- 27,009
- Helped
- 8373 / 8373
- Points
- 170,159
- Level
- 98

## time constant of parallel rc circuit

With both circuits, you have to consider external connected impedance which may be part of the time constant. Series circuit would be unchanged only when connected to a voltage source, parallel circuit with a current source.

- 22nd February 2008, 08:15 #4

- Join Date
- Jun 2005
- Posts
- 599
- Helped
- 1 / 1
- Points
- 4,902
- Level
- 16

## rc parallel

Series circuit would be unchanged only when connected to a voltage source, parallel circuit with a current source.

What do u mean by this?

How do u get a time constant with parallel RC? because of the LOAD resistance?

- 22nd February 2008, 17:06 #5
## parallel rc circuit time constant

There is a time constant with parallel RC, and it is equal to τ=RC, the same as for the series combination. The difference is that instead of charging up the cap with this time constant, now you discharge it. But it's the same thing: the voltage across the cap varies exponentially, with the time constant τ. It decreases this time, though.

- 22nd February 2008, 20:56 #6

- Join Date
- Jul 2002
- Location
- Windsor ON Canada
- Posts
- 387
- Helped
- 83 / 83
- Points
- 5,834
- Level
- 18

## time constant parallel rc

- 22nd February 2008, 23:39 #7

- Join Date
- Sep 2007
- Location
- USA
- Posts
- 201
- Helped
- 41 / 41
- Points
- 2,291
- Level
- 11

## rc time constant parallel

Time constant by definition is the time taken for the voltage to reach a certain level in a series RC combination but in a parallel the voltage will remain constant and hence you will not have a time constant.

even when u have charged the parallel RC and then allowed it to discharge through the resistor, it becones series RC ( same current flows).

- 22nd February 2008, 23:39

- 23rd February 2008, 00:04 #8

- Join Date
- Jan 2008
- Location
- Bochum, Germany
- Posts
- 27,009
- Helped
- 8373 / 8373
- Points
- 170,159
- Level
- 98

## rc parallel time constant

but in a parallel the voltage will remain constant and hence you will not have a time constant

Finally, you have a lot, that are neither clearly series nor parallel circuits, among them all circuits with two resistors and a capacitor.

- 23rd February 2008, 00:04

- 23rd February 2008, 01:39 #9

- Join Date
- Jun 2005
- Posts
- 599
- Helped
- 1 / 1
- Points
- 4,902
- Level
- 16

## parallel rc network

((""Time constant by definition is the time taken for the voltage to reach a certain level in a series RC combination but in a parallel the voltage will remain constant and hence you will not have a time constant""))

Thats what i thought that the parallel RC will NOT have a time constant , but the RC lOAD will make it series

But why doesn't a parallel RC network have a time constant? isn't it now a current time constant instead of voltage?

Now the current has a time constants in parallel RC?

- 23rd February 2008, 16:37 #10

- Join Date
- Aug 2005
- Location
- Salvador-BA-Brazil
- Posts
- 226
- Helped
- 28 / 28
- Points
- 2,906
- Level
- 12

## rc parallel circuit time constant

In order to answer this question, first we must define time constant.

What is the time constant in a RC series circuit ? It is the time necessary to capacitor reaches XX% of the supply, Right ?

So, lets do other question: What is the time constant in a thermo measurement system ? It is the time necessary to the system respond with xx% when a step change in temperature is applied.

What we must do to determine time constant ? is a good idea to Analize the transfer function ? it is a good idea to determine which critria should be observed ?

So, When asking about time constante in a RC serie circuit we build a transfer function that has the form 1/ (τS + 1) wich is a first order system with a time constant τ = RC.

What to do in a RC parallel circuit? Determine wich criteria you are analising, if the criteria is the time the voltage in capacitor takes to reache the supply voltage, the time constant is 0 if you have an ideal supply voltage and RinC if Rin is the resistance in serie with the capacitor. Anything wrong?

- 23rd February 2008, 21:00 #11

- Join Date
- Jun 2005
- Posts
- 599
- Helped
- 1 / 1
- Points
- 4,902
- Level
- 16

## rc parallel circuits

So whats the difference between a series RC time constants VS a parallel RC time constants?

- 23rd February 2008, 22:46 #12

- Join Date
- Jan 2003
- Location
- Ireland
- Posts
- 268
- Helped
- 19 / 19
- Points
- 5,720
- Level
- 17

## parallel rc circuit current source

R, C and V are series connected. Time constand T= RC

Now lets V=0. This is same as parallel RC circuit.

Time constand remain T=RC

- 24th February 2008, 21:26 #13

- Join Date
- Aug 2005
- Location
- Salvador-BA-Brazil
- Posts
- 226
- Helped
- 28 / 28
- Points
- 2,906
- Level
- 12

## time constant of rc parallel circuit

Originally Posted by**walters**

- 24th February 2008, 22:44 #14

- Join Date
- Jun 2005
- Posts
- 599
- Helped
- 1 / 1
- Points
- 4,902
- Level
- 16

## rc parallel to series

i don't konw what will happen thats why i'm asking

Series time constant is different than parallel time constant , how are they different and why ?

- 24th February 2008, 23:14 #15

- Join Date
- Jan 2003
- Location
- Ireland
- Posts
- 268
- Helped
- 19 / 19
- Points
- 5,720
- Level
- 17

## rc time constant calculator

Originally Posted by**walters**

Lets calculate the time constant of series RC. (V(t)=V constant)

V/s=Ri(s)+i(s)/sC

i(s)= V/R [1/ (S+1/RC)] ==> T=RC

Lets calculate the time constant of parallel RC. (Vc(0)=Vo)

Ri(s) + i(s)/SC - Vo/S =0

i(s)=Vo/R [1/(s+1/RC)] ==> T=RC

Time constants are same.

- 25th February 2008, 04:23 #16

- Join Date
- Jun 2005
- Posts
- 599
- Helped
- 1 / 1
- Points
- 4,902
- Level
- 16

## function of rc parallel circuit

so series RC time constant and parallel RC time constant have the SAME exponentially curves?

I thought the series RC time constant had a different exponential curve VS the parallel RC time constant exponential curve is this true and how are they different?

Isn't the Parallel RC a CURRENT time constant, a ""current exponential curve"" sinces the RC components are in parallel network??

- 25th February 2008, 13:14 #17

- Join Date
- Jan 2007
- Posts
- 943
- Helped
- 176 / 176
- Points
- 8,931
- Level
- 22

## parallel rc discharge

Hi,

Yes, there is a difference. In the calculation shown by Bunalmis the current i(s) is assumed to be same through R and C, which is not true for a parallel circuit. Intially, The capacitor hogs all the current, and Ir through R will be zero, but, as the capacitor gets charged as per Vc(t) = (1/C)∫ ic(t)dt, the the resistor starts diverting a current Ir(t) through it such that Ir(t) = Vc(t)÷R. and this process continues till the capacitor gets charged fully to the applied voltage Vi. Thus the capacitor charges at a slower and slower rate as times goes and this is how the time constant comes into picture.

Here we have to note that by assuming a finite i(t), we have implicitely included an internal resistance for our voltage source, as otherwise an infinite current would have charged the capacitor instantly to the applied voltage Vi. So to calculate the time constant, you have to consider this source resistance also. Once you include the source resistance Rs, you will notice that the current i(t) itself is time dependent and is given by i(t) = (Vi-Vc)÷Rs, which keeps on dimnishing as the capacitor voltage Vc increases. Thus the overall time constant in this case is increased. At t = 0, the output voltage Vo = 0, and at t = ∞ the output voltage is given by Vo = Vi*R÷(Rs+R).

Regards,

Laktronics

Added after 2 hours 40 minutes:

Hi,

There is a correction in what is written above, that is for a voltage source when an Rs is assumed, the overall time constant IS NOT INCREASED, IT IS ACTUALLY REDUCED since Rs appears in parallel with R and the time constant becomes parallel combination of R with Rs multiplied by C. The expression of output voltage Vo in this case is given by Vo = [Vi*R÷(Rs+R)](1-e^-t/Tc) where Tc = (RsR/Rs+R)C .

This is because, a voltage source with a series Rs is equivalent to a current source with a parallel Rs and a current value = V/Rs. So, Rs appears in parallel with R.

Now if you assume that the parallel R,C is connected to a current source of strength I, then the time contant will be just R*C. In this case, the current through the capacitor Ic = I*e^-t/RC and voltage Vc = IR*(1-e^-t/RC).

Regards,

Laktronics

- 26th February 2008, 07:14 #18

- Join Date
- Jan 2008
- Location
- Bangalore,India
- Posts
- 207
- Helped
- 35 / 35
- Points
- 2,684
- Level
- 12

## filtre rc parallĂ¨le

In parallel combination also you find a time constant which is dependant on the source impedence. Suppose you have a resistance R1 II to a capacitor C1 and the combination is driven with a source of resistance R0, the time constant of the combination will be (R1 II R2)C and the steady state vol;ltage across the capacitor will be V1(R1/(R1+R0)) where V1 is the source generated voltage( the voltage before the output resistance R0 of the source).