Question with RC Time Constant

You can only estimate a minimum charge time (must be greater than 5* R*C seconds) once the capacitor reaches essentially full charge voltage. In order to measure the charge time accurately from the voltage, the voltage has to still be measurably changing at the moment you open the switch.

The capacitor charges to 1-(1/e) or about 63.2% of full charge (with e being the constant, 2.71828) when the charge time has been one R*C.

The formula for voltage versus time, assuming the capacitor starts at zero, is, 1-e^(-time/R*C) times full charge voltage.

To solve that for time versus voltage, time capacitor has been charging is -ln(1-V)*R*C seconds, with V being the fraction of full charge voltage, and R in ohms, C in farads.

For other

Sorry about these questions:

Basically I've done a practical with an RC network and I've got 2 wires that touch to "complete the circuit" and provide current to the capacitor. I need to find out the amount of time they've touched by lookign tat the voltage in the capactiros. I have the capacitor charge/discharge curves.

my question is can I work out the time using simply time constants? 5TCs = time to charge a capacitor (almost 100%, irrelevant). So because I have the capacitor charge curve, say it only had half the voltage o fthe input voltage, and i look up half on the y axis and find wher eit meets, and look up the x-axis' time and work out how long that is.

or do i do this:

basically use that formula. if i do use it i will get more ducks.

are both these methods correct? thanks

Yes, you can do this, but you need to scale the output appropriately. A generic graph like that can have its time scale marked in time constants, and its voltage scale go from 0 to 1. In that case, using the y=0 axis as the initial voltage, you find the point where y = (end - initial)/(charging - initial). This allows you to determine how many time constants you've charged for. If a time constant in your circuit is equal to TC, then the answer is graphical time constants / TC.

You need this equation that's on that page.

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

As suggested though, a constant current source makes it easier since theer will be a linear rise in the cap voltage voltage with time ( like a scope timebase ).

Make sure the cap is fully discharged before commencing timing though.

Graham

I can't, I've already done the practical. Thanks anyway.

Thanks for the help!