#### gani

Joined May 15, 2005
46
Hi everyone,

I was just reading Floyd,RC across dc supply. But the author hasn mentioned abt RC parallel across DC supply.It takes 1 timeconstant for the voltage to reach 63% of its full value.Is this applicable to RC parallel too?

Thanx a lot!

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by gani@May 26 2005, 07:03 AM
I was just reading Floyd, RC across dc supply. It takes 1 time constant for the voltage to reach 63% of its full value.Is this applicable to RC parallel too?
No.
A capacitor connected across a d.c. source without any series R will charge immediately to the source voltage.

#### gani

Joined May 15, 2005
46
Originally posted by David Bridgen@May 26 2005, 07:37 AM
No.
A capacitor connected across a d.c. source without any series R will charge immediately to the source voltage.
[post=7950]Quoted post[/post]​

The capacitor voltage and current still follow the exponential curve?
i suppose its the same with RL parallel to dc supply.L will
develop current.......am i right?

Thanx for all the help.

#### David Bridgen

Joined Feb 10, 2005
278
Originally posted by gani@May 26 2005, 03:29 PM
The capacitor voltage and current still follow the exponential curve?
Taking into account the extremely small resistance which will inevitably appear in series with the voltage source and the C, then, theoretically, yes.

But in practice you wont see it. The voltage across the C rises effectively instantaneously. The charging current will be a very narrow pulse.

#### gani

Joined May 15, 2005
46
hmm....ok but i think i am a bit confused...
it is an R C circuit parallel to dc supply.....so extremely small resistance..?

#### David Bridgen

Joined Feb 10, 2005
278
The left hand diagram shows a series RC across a d.c. source. The C charges up exponentially through the R, as you already know, reaching 60-odd percent in one CR period.

The right hand diagram shows a parallel RC across a d.c. source. The only resistance in series with the C is that of the wiring (very small) and that of the source (negligible.) The time constant is therefore very short and the C charges (almost) instantaneously.

#### gani

Joined May 15, 2005
46
Thanx a lot for ur timely help.
ok so i guess the RL parallel is similar to this case too.

once again,thankyou!