calculating f and t0

Thread Starter

circuits56

Joined Nov 16, 2021
5
Hey everyone, I want to calculate t0 and the frequency for the following circuit, but I keep getting zero for some reason, can anyone help me with this?
R=10K C=100nf Vcc= +15 , -15 Picture 1.png
 

ci139

Joined Jul 11, 2016
1,864
the charge formula for the capacitor (a series resistor)
works invariant from - wheater the polarity changes during the "charge up" or not
. . . and this is exactly what you have in hand here
(much likely a bipolar square wave voltage source) // EDIT : UPS! -- I looked at the initial post only . . .
// . . . -- too frequently the members here like to explode the topic to an irrelevant , pointless , near off-topic hassle . . . if there is one post solution - and there are more posts around - and idon't see the solution . . . i ignore the hassle

↑ the formula ↑ - is easy to be derived from the capacitor´s charging from 0V to a Const. Volts formula?

? what part of it you cannot puzzle-in to the picture
 
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MrAl

Joined Jun 17, 2014
9,195
thanks for responding! yes, I'll attach the circuit. I think I should be finding t0 using the transient formula but not really sure howView attachment 263506

The charging of a capacitor is exponential like 1-e^(-t/RC).
However with an initial voltage across the cap we have to modify this slightly:
vC=(vH-vC0)*(1-e^(-t/RC))+vC0
where
vH is the voltage the cap is being charged from,
vC0 is the initial cap voltage at t=0,
RC is R*C,
vC is the final capacitor voltage after t seconds has passed.
Units are volts, Ohms, Farads, seconds.

Just one more little detail.
I see your square wave is not actually perfectly square but 'ramps' up a bit. That ramp means the formula has to change again a little bit because the formula above assumes a perfect step change. The ramp does not last that long, but i would consider this an approximation then not an exact expression. To get the exact expression we would have to know the start and end times of the ramp also. That may not be necessary for this exercise though but worth looking into at some point.
 
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