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RC Time Constant

How do you figure out the RC time constant of a battery in series with a resistor that is in series with the parallel combination of a capacitor and resistor? (Basically a low pass RC filter with a resistor in parallel across the output cap)

battery ---> R1 ----> (C1||R2)

I would think that the RC time constant would only involve the resistor in series with the battery (R1)... and if I were to disconnect the battery, then the capacitor would discharge through the resistor that is in parallel with it (R2).

so RC Time Constant = R1C1 ?

Does the resistor in parallel with the cap play a role in the RC time constant when it's charging?

Reply to
panfilero
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resistor that is in series with the parallel combination of a capacitor and resistor? (Basically a low pass RC filter with a resistor in parallel across the output cap)

series with the battery (R1)... and if I were to disconnect the battery, then the capacitor would discharge through the resistor that is in parallel with it (R2).

when it's charging?

Sounds like a question for M. Thévenin.

--
Rich Webb     Norfolk, VA
Reply to
Rich Webb

resistor that is in series with the parallel combination of a capacitor and resistor? (Basically a low pass RC filter with a resistor in parallel across the output cap)

series with the battery (R1)... and if I were to disconnect the battery, then the capacitor would discharge through the resistor that is in parallel with it (R2).

when it's charging?

Yes, the time constant is the parallel combination of the two resistors times the capacitor. Don't forget to use the voltage divider equation to calculate the terminal voltage (and voltages at the times inbetween).

Reply to
krw

resistor that is in series with the parallel combination of a capacitor an= d resistor? (Basically a low pass RC filter with a resistor in parallel acr= oss the output cap)

in series with the battery (R1)... and if I were to disconnect the battery,= then the capacitor would discharge through the resistor that is in paralle= l with it (R2).

nstant when it's charging?

Doh! Absolutely right! thanx.

which leads to RC =3D (R1+R2)*C

Reply to
panfilero

----- Original Message ----- From: "panfilero" Newsgroups: sci.electronics.basics Sent: Friday, July 27, 2012 9:37 PM Subject: Re: RC Time Constant

Doh! Absolutely right! thanx.

which leads to RC = (R1+R2)*C

Hello,

Sorry, your given formula is wrong.

Take a look from the capacitor's view into the circuit while assuming the battery has zero Ohm resistance. Now you will see R2 parallel R1.

tau = C*R1*R2/(R1+R2)

Best regards, Helmut

Reply to
Helmut Sennewald

a=20

r=20

lel=20

r=20

is=20

=20

=20

agreed. thanx

Reply to
panfilero

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