Discussion in 'General Electronics Chat' started by gani, May 26, 2005.

1. ### gani Thread Starter Active Member

May 15, 2005
46
0
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!

2. ### David Bridgen Senior Member

Feb 10, 2005
278
0
No.
A capacitor connected across a d.c. source without any series R will charge immediately to the source voltage.

3. ### gani Thread Starter Active Member

May 15, 2005
46
0

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.

4. ### David Bridgen Senior Member

Feb 10, 2005
278
0
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.

5. ### gani Thread Starter Active Member

May 15, 2005
46
0
hmm....ok but i think i am a bit confused...
it is an R C circuit parallel to dc supply.....so extremely small resistance..?

6. ### David Bridgen Senior Member

Feb 10, 2005
278
0
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.

7. ### gani Thread Starter Active Member

May 15, 2005
46
0
Thanx a lot for ur timely help.
ok so i guess the RL parallel is similar to this case too.

once again,thankyou!