# graph of time constant rc network

Discussion in 'General Electronics Chat' started by bhuvanesh, Apr 20, 2014.

1. ### bhuvanesh Thread Starter Member

Aug 10, 2013
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in the below image u see that time constant is 0.37 of initial value and 63% of final value.if that so see the second graph in that image the point should be on 0.37 (from initial value) but actually the time constant point is on 0.63.
this is messing with my i dont know what i misunderstood.

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2. ### ericgibbs AAC Fanatic!

Jan 29, 2010
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You could say in the first graph the voltage has decayed by 63% of its initial value in one time constant or its decayed to 37% of its final value.

Last edited: Apr 20, 2014
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3. ### crutschow Expert

Mar 14, 2008
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Don't you mean "to within 37% of the final value"?

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4. ### AnalogKid AAC Fanatic!

Aug 1, 2013
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By definition, one time constant is 63%. 63% of what?

63% of way from the initial value to the final value. It doesn't matter if the values are positive or negative, and it doesn't matter if the direction of change is positive or negative. In terms of a simple RC circuit with a positive power supply, it doesn't matter whether you are charging the capacitor up from zero volts to Vcc (like the right-hand chart), or discharging a fully charged capacitor from Vcc to GND (like the left-hand chart). Either way, the voltage on the capacitor has changed by 63% of the absolute value of the difference between the max and min voltages in one time constant, 86% in two, 95% in three, etc.

ak

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Jan 29, 2010
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Hi Carl,
Woops.!