Capacitor Charge/Discharge question - Page 2 - Electronic Circuits Projects Diagrams Free

Roff · 1st June 2007, 02:24 AM

Quote:

Originally Posted by lord loh.

Sorry Ron H, I do not follow. How do RC Oscillators like the wein's bridge, Phase Shift and Colpitt's work?

Sorry, I should have said that in a passive circuit (no amplifiers), there can be no resonance in the absence of inductance.

FRIED · 1st June 2007, 03:34 AM

For the time constant, R1 is in parallel with R2 so TC = .5mS

BeeBop · 1st June 2007, 04:14 AM

My guess is that Vth is 9v, Rth of 500 ohm, so tau will be 500 micro seconds, thus full charge of 9v at 2.5 milliseconds.

It would be good to build and measure....

lord loh. · 1st June 2007, 10:46 AM

Quote:

Originally Posted by Ron H

Sorry, I should have said that in a passive circuit (no amplifiers), there can be no resonance in the absence of inductance.

I guess the inductor, in practical cases has some losses and needs soem energy to be fed into the tank circuit to compensate these losses. So I believe that there is no way to create an osicllator without an amplifier. Tell me if I am wrong.

Do you mean sustained oscillation when you say resonance? or do you count transient responses as well?

Now that 3iMaJ, mentioned the Laplace Transform, I am recollecting a few things.

ThermalRunaway · 1st June 2007, 11:10 AM

Quote:

Originally Posted by FRIED

For the time constant, R1 is in parallel with R2 so TC = .5mS

R1 is not in parallel with R2, it is in series with R2. However, once the capacitor becomes resistive, it is then in parallel with R2 which means the "R" variable in the equation is no longer a constant.

Brian

ericgibbs · 1st June 2007, 12:10 PM

hi brian,

Couldn't stand the suspense!... Built the dam'n cct.

Time constant without R2, Tc= 1.1mSec

With R2, Tc= 0.5mSec, which ties in with 'eng1' graph.

Both curves are exponential which they should be!

Regards

eng1 · 1st June 2007, 12:42 PM

Quote:

Originally Posted by ThermalRunaway

R1 is not in parallel with R2, it is in series with R2. However, once the capacitor becomes resistive, it is then in parallel with R2 which means the "R" variable in the equation is no longer a constant.

No, R1 is not in parallel with R2 and it's not in series with R2. But the RC constant can be calculated with an equivalent resistance Req, that equals R1//R2. This is a consequence of Thevenin's theorem. See the attachment and use your formula with the second circuit. Veq=Es*R2/(R1+R2)=18/2=9 V. Req=500 ohms.

Optikon · 1st June 2007, 12:58 PM

Quote:

Originally Posted by lord loh.

I guess the inductor, in practical cases has some losses and needs soem energy to be fed into the tank circuit to compensate these losses. So I believe that there is no way to create an osicllator without an amplifier. Tell me if I am wrong.

Do you mean sustained oscillation when you say resonance? or do you count transient responses as well?

Now that 3iMaJ, mentioned the Laplace Transform, I am recollecting a few things.

In an ideal circuit (like a test problem) capacitors and inductors have NO losses - they are ideal.

Remember for oscillatory behavior, there needs to be both inductance and capacitance.. if the circuit contains only 1 type of energy storage element, then it is First order and oscillation CANNOT occur.

I would not go as far to say resonance since that requires a particular condition setup in the circuit (C & L values & FREQ of operation)

Things that oscillate are not necessarily resonant.

Roff · 1st June 2007, 01:20 PM

Quote:

Originally Posted by lord loh.

I guess the inductor, in practical cases has some losses and needs soem energy to be fed into the tank circuit to compensate these losses. So I believe that there is no way to create an osicllator without an amplifier. Tell me if I am wrong.

You are correct.

Quote:

Do you mean sustained oscillation when you say resonance? or do you count transient responses as well?

See the Wikipedia topic "RLC Circuit".

Quote:

Now that 3iMaJ, mentioned the Laplace Transform, I am recollecting a few things.

piper91765 · 12th August 2008, 03:44 PM

Laplace transforms? Oscillation? Guys, this is a DC circuit. No need for AC analysis, and there should be no oscillation, simply decay.

ericgibbs · 12th August 2008, 04:48 PM

Quote:

Originally Posted by piper91765

Laplace transforms? Oscillation? Guys, this is a DC circuit. No need for AC analysis, and there should be no oscillation, simply decay.

hi piper,
I dont think you are working in the 'time domain'..

The post is a year old.!

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1st June 2007, 03:34 AM	(permalink)
FRIED	For the time constant, R1 is in parallel with R2 so TC = .5mS

1st June 2007, 04:14 AM	(permalink)
BeeBop	My guess is that Vth is 9v, Rth of 500 ohm, so tau will be 500 micro seconds, thus full charge of 9v at 2.5 milliseconds. It would be good to build and measure....

1st June 2007, 12:10 PM	(permalink)
ericgibbs	hi brian, Couldn't stand the suspense!... Built the dam'n cct. Time constant without R2, Tc= 1.1mSec With R2, Tc= 0.5mSec, which ties in with 'eng1' graph. Both curves are exponential which they should be! Regards __________________ Eric "Good enough is Perfect" PIC tutorials: Gramo's: www.digital-diy.net/ Bill's: www.blueroomelectronics.com/

12th August 2008, 03:44 PM	(permalink)
piper91765	Laplace transforms? Oscillation? Guys, this is a DC circuit. No need for AC analysis, and there should be no oscillation, simply decay.