capacitor charging

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You can't have a capacitor time-constant by itself, it also requires a resistor. A time-constant is simply a convenient engineering definition which, for a resistor and capacitor, is the resistor value multiplied by the capacitor value (R X C). If you apply a step voltage to a resistor connected to a capacitor (other cap lead to ground), then the capacitor voltage will rise to approximately 63.2% (actually 1 - 1/e) of the step voltage in one time-constant of time.
 
It's considered 63.2% because that's the 'peak' of the exponential rise, after that point the voltage rises much slower, and before the rise is too fast. It's not ALWAYS considered the constant, just a common one. You can set a voltage comparator to any percentage of line voltage you want using different resistors. 63.2% is also the best point to avoid noise.
 
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There is no "peak" to the exponential rise. The exponential rise is a smooth curve that starts at zero and asymptotically approaches the final value, with the rate of rise continually slowing. The 63.2% level is just a point on that curve which occurs at the one time-constant (RC) time.
 
If I remember correctly the 62.5% is only when the capacitor is being charged thru a resistor. I you charge the capacitor with a constant current then the charge voltage increase linearly.
 
Yes, if you charge the capacitor with a current source the voltage ramps up linearly. If you charge it with a voltage source through a resistor the voltage goes up as 1 - e^-t/RC.
 
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Sceadwian said:
Crutschow, it's the mid point at which the rate of change goes from fast to slow rising?
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The RC rate of rise is continually slowing in a smooth fashion. There is no point of abrupt change in an exponential rise.
 
Ahh thank wikipedia, this explains what I was trying to say (rather poorly) a whole lot better.
e is derived from the point on an exponential curve where the slope is equal to 1 so it's not just some arbitrary number someone thought up. That's what I was trying to say =)
 
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I think I understand what you are trying to say but it's still a little confusing.

e is not derived from the exponential curve, it is used to describe it. As speakerguy79 noted, the curve is 1 - e^-t/RC where e is the base for the natural logarithm. At the arbitrary time of one time constant (t = RC) that point on the curve equals 1-(1/e) which is 0.632 or 63.2%.
 
Sorry cruts, it's my math and descriptive prowess that's the problem, not my understanding =) I apologize, but you get the jist of what I was trying to say now at least.
 
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