# calculating f and t0

#### ericgibbs

Joined Jan 29, 2010
15,544
hi c56,
Welcome to AAC,
Have you drawn a circuit using those components and Voltage values??
E

#### ericgibbs

Joined Jan 29, 2010
15,544
hi c56,
As this is homework, we can only give hints.
Can you calculate the Time Constant of R=10K C=100nf , then apply Vcc= +15 , -15

E

Update:
if you don't know how, study this link and post your calculations here.

https://www.electronics-tutorials.ws/rc/rc_1.html

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#### circuits56

Joined Nov 16, 2021
5
hi c56,
As this is homework, we can only give hints.
Can you calculate the Time Constant of R=10K C=100nf , then apply Vcc= +15 , -15

E

Update:
if you don't know how, study this link and post your calculations here.

https://www.electronics-tutorials.ws/rc/rc_1.html
so this is what I did
since vc=+1/3 vs at t0 then
vc=-4/3vs*e^(-t*10^3)+vs
so t0= 690 micro s
is this correct?

#### ericgibbs

Joined Jan 29, 2010
15,544
hi,
So what do you make the frequency.?
E

#### circuits56

Joined Nov 16, 2021
5
hi,
So what do you make the frequency.?
E
frequency would be 1/T which is 2t0

#### ericgibbs

Joined Jan 29, 2010
15,544
hi c56,
Looks good to me. E

#### circuits56

Joined Nov 16, 2021
5
Thank you for your help! much appreciated ^^^

#### ci139

Joined Jul 11, 2016
1,822
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
9,001
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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