You do give me a good homework, Spehro. Here goes my
reply.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capdis.html

from this equation I concluded that when

Vc = Vo[1-e(-t/RC)]

when Vc is 0.5 of Vo and after some magic moving
t = -ln(0.5)RC
which comes to 0.69RC.

Now to your question. Why 0.5Vo?  B'cos of this:
f = 1/T1 + 1/T2.
when you need a Frequency of x Hz for example
and C1R1 == C2R2
T1 == T2
then 
f = 1/(2T)
T = 1/2f

1/2f =xRC
x = 1/2f * 1/RC
since we know from the statement that f is known and
RC is known we can use the x. Okay I cheated..... I
work backwards...... Don know how to proof the
equation forwards........ Can you help?


John







--- Spehro Pefhany <speff@interlog.com> wrote:

> At 01:01 PM 12/10/2006, you wrote:
> >Reference
> >http://www.4qdtec.com/mvibs.html.
> >
> >I have been pondering how does the author claims
> the
> >period for each state in the multivibrator comes to
> >0.69CR. So for the entire cycle for on - off
> >
> >f = 1/T. Which is understandable.
> >T = t1 + t2
> >
> >t1 = 0.69CR
> >t2 = 0.69CR....... I have no clue how the author
> comes
> >out which this magic value. Shouldn't it be 5CR
> >instead of 0.69CR? Not sure here....
> >
> >Thanks,
> >John
> 
> We know from calculus that charging a capacitor from
> 0 to V0,
> v(t) = V0*(1-exp(-t/tau)) where tau = RC
> 
> if v(t) = 0.5 V0, t = -ln(0.5) = ln(2) = 0.693...
> 
> I'll leave it to you to figure out why it's ~0.5 of
> the charging voltage...
> 
> >Best regards,
> 
> Spehro Pefhany --"it's the network..."           
> "The Journey is the reward"
> speff@interlog.com             Info for
> manufacturers: http://www.trexon.com
> Embedded software/hardware/analog  Info for
> designers:  http://www.speff.com
> 
> 
> 
> 
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