niga

hello there,

:roll: is there damping in first order circuits?i think there is but....wud like to know more...
hope someone can help.
thanx.
niga

Roff

No. One measure of damping is how quickly ringing dies out in response to a step input. First order systems cannot have ringing.

niga

Thanx for the reply,Ron.
Step input is dc input right?
Why cant the first order have ringing?
Can i get more info on that pls?
Thanx for the help.

niga

Roff

Do you have a math background in calculus and Laplace transforms? This would be a lot easier to explain if you do. For example, see this site.
In a passive circuit (no amplifiers), for ringing to occur in response to a step input, it is necessary but not sufficient that the circuit have an inductor and a capacitor. A circuit with one inductor and one capacitor (and possibly one or more resistors) is a second order system. A passive circuit with two capacitors and resistors is also a second order system, but it will not ring in response to a step input.

ljcox

A step input can be either voltage or current.

Len

niga

thanku Ron and Len.


i read that resistance causes damping in electric circuits.
so gee..i am confused....:mrgreen:
WHY???
1.am i to understand there is no damping in first order ckts or second order with R and C alone(or R and L alone)?BUT I REALLY DON GET IT...WHY??

2.Has the AC or DC anything to do with it?

I really thanku guys for ur responses.

ljcox

Ron's response is very good. But I'm not sure what you do not understand.

We are both assuming that you are talking about damped oscillations. Is this true?

As Ron said, you need to understand the maths. I'll try to explain it without maths.

Take a pure tuned circuit made from a perfect inductor and a perfect capacitor connected in parallel. Now apply a short current pulse to input some energy. The capacitor stores energy in the electric field between its plates and the inductor stores energy in the magnetic field that it creates.

If you start at an instant when all of the energy is stored in the inductor, the energy from the inductor is then transferred to the capacitor, then vice versa.

So the circuit will oscillate for ever with the energy being moved from L to C and then C to L, etc.

Now if you connect a large value resistor (say 1M Ohm) in parallel with the circuit, some of the energy is dissipated as heat in the resistor. Therefore, the system gradually looses energy until all the energy is dissipated. If you looked at the voltage across the circuit with an oscilloscope it would be a damped oscillation. Damping is the gradual loss of energy.

Other examples include a spring with a weight suspended from it. If you pull the weight down (ie. input some energy into the spring) then let it go it will oscillate up and down. The oscillation will be damped because of frictional losses. The energy is being transferred backwards and forwards between the spring and the potential energy of the weight in similar manner to the LC circuit I descrobed above.

Another example is a pendulum.

In all of these cases, there are 2 modes of energy storage eg. L and C, but with an RC circuit, there is only one storage mode, ie. the C.

In the case of the pendulum, the modes are - kinetic energy and potential energy. And energy is lost due to air friction and pivot friction.

I hope this helps. If not feel free to ask more questions.

len

niga

okay i get it....finally.
so damping takes place in second order ckts ..when there is l and c and inserting an r will determine the rate of damping.

so irrespective of dc or ac damping happens in these kinda ckts...am i right? :P

ljcox

Yes.

When you solve a second order differential equation, there are three possible outcomes.

1. Over damping

2. critical damping

3. under damping - this is where a damped oscillation will occur.

R & L or R & C circuits always lead to case 1. L, C & R circuits can lead to any of these cases, depending the value of the R.

I don't know what you mean by "dc or ac damping"

Damping is damping.

Len

niga

oh, sorry abt that.
i meant be it ac or dc supply, damping occurs in rlc circuits.
am i right?

ljcox

Damped oscillation occur with RLC circuits and it can also happen with circuits that have gain.

The supply does not make any difference.

Len

niga

hai len

u have been very helpful and i must add its very kind of u.
thanx a lot!