# charging capacitor partially

1. ## charging capacitor partially

I got a silly question to ask.
I am quite new to electronics and these kind of stuff.

I am trying to calculate the value of resistor use for charging 4700uF cap
from 1/3 to 2/3 of the supply voltage(6 volts) I had tried to figure out how to, but I can't find the way.

Is there any equations or ways to solve this problem?

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2. ## Re: charging capacitor partially

Vc=Vapplied(1-e^(-t/RC))

Vc is voltage across the capacitor
t is time.

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3. ## Re: charging capacitor partially

Hi,

There are missing informations.

Simply saying: Any resistor may charge a capacitor from 1/3 to 2/3 of the supply voltage.
Varying the resistor value will vary the time ... and the current.

Klaus

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4. ## Re: charging capacitor partially

if you want to stop the charging at 2/3 of the applied volts you will need a resistive divider, e.g. 10 ohm and 20 ohm with the 20 ohm across the grounded cap

this will charge from zero to 2/3 of applied V,

Are you trying to design a long time timer ckt for a 555 by any chance ...?

The time to charge from 1/3 to 2/3 is very roughly half a time constant, which is Rcharge x C ( then divide by 2 )

5. ## Re: charging capacitor partially

Hi,

Ln(1/3) = -1.099
Ln(2/3) = -0.405
The difference is 0.694.
Thus ideal charging from 1/3 to 2/3 takes 0.694 x R x C.

Discharging from 2/3 to 1/3 also takes 0.694 x R x C.

Klaus

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6. ## Re: charging capacitor partially

Thank you for all

"Are you trying to design a long time timer ckt for a 555 by any chance ...?"
I am working on different circuits I can do with 555 but I dont really understand the calculation for capacitor
so I searched for some information in my language but there is not much I could find so I try to search in english
but I can't find a good keyword for it

now I come up with this equation "ln(1-V1/Vs)-ln(1-V2/Vs) = t"
not sure if it works but it still work in all of my case

7. ## Re: charging capacitor partially

from 1/3 to 2/3 of the supply voltage(6 volts) I had tried to figure out how to, but I can't find the way....

If you want to know the time, you must specify the resistor and the capacitor; they decide together the time constant.

If you want to know the resistance value, you must specify the time.

Now for the details. RC (product of R and C) has the dimension of time. In this story, you will notice R and C will always appear together.

During charging, the voltage increases from 0 to V (the final value) but that takes infinite time. Both charging and discharging are exponential process.

33% charging will take -RC*log(0.66) s and 66% charging will take -RC*log(0.33) s. So the time to charge is RC*0.415s to RC*1.109s.

So you must say the R value or the time.

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