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# Re: Mot DC Ps

```Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Mddeming-at-aol-dot-com>

In a message dated 2/17/01 4:15:15 PM Eastern Standard Time, tesla-at-pupman-dot-com
writes:

>
> I have a question too when you discharge a cap though a resistance for a
> certain value of R you have an oscillating circuit (because of parasitic
> inductance ), if you increase the value of R to a certain point the cap
> will just discharge whit out oscillation how do you calculate this
> value.
>
> Tx
> Luc Benard

Hi Luc
The critical point is at R=2*sqrt(L/C). If R is greater than this
value, no oscillation takes place. The frequency equation f=1/((2pi*sqrt(LC)
is actually a simplifications of:
F=sqrt(1/LC-(R/2L)^2)/2pi

This follows from the circuit current as a function of time being:

I(t)=Vo/Lw * exp(-at) * Sin(wt), where a=R/2L and
w=sqrt((1/LC)-(R/2L)^2) and Vo is the initial voltage on the capacitor.

If you need a complete derivation of this, write me on or off list.

Matt D.

```