I need help understanding!!

homework? is ok, been there, too

The best way to solve math problems is to simply start 'poking' at them.

When confronted with an exponential, e, or a power of 10,you've first got to get everything to be 'like' with 'like'

How to do that? First divide both sides by V, changing to Vc/V=(1-e^(-t/R*C))

oh, oh, still not got 'like' with 'like', so add the exponential to both sides AND subtract Vc/V from both sides, leaving: e^(-t/R*C)=1-Vc/V now like is with like, and you can take the natural log of BOTH sides and keep going.

Forget the Algebra... go golfing >:-} ...Jim Thompson

SNIP.

It's not an English problem it's a math problem.

Normally I don't do this however.. Let me take a crack at it. e^1 = 2.71828 as you have found. e^-1= 0.3678 so what does this mean? If you were to do this; 1 / 2.71828, it would equal 0.3678

do you see the correlation here? When e^ has a "- sign" it returns the reciprocal (1/result) of the answer. That is, what ever math is going on for the ^??? part, that returns a -sign number, will return the inverse of the results. P.S. use 2 secs instead of 2000 msec in the calculations.

with out giving you the answer, I can tell you there isn't much of a charge in the cap after 2 secs. So little that you'd need a high end DMM to see it.

Jamie

Hi Jerry, Well maybe you need to review some basic algebra. The equation describes the charging of a capacitor through a resistor. So at time = 0, Vc = 0 (no voltage on C) and at time = infinity the cap should be charge fully. Vc=V.

One of the first things I like to do is check these initial and final conditions and make sure the equation works correctly. So as the first part to answering your question I'll ask you to find Vc(t=0) and Vc(t=infinity). Can you repeat my results?

George H.

Are you folks really reading the "mF" as "millifarads"? I think even the East Germans stopped using nonsense units like that, decades ago.

For the OP, take a look at a RC time constant chart. All that matters is 0 to 5 time constants. My math shows that in this case, in 2.178 seconds (one time constant) that cap will reach about 63% of 13.8 volts. The rate of charge decreases over time.

That's what your function looks like, I inserted some numerical values.

w.

I just remember the basic time constant equation...

from which all other forms can be derived. ...Jim Thompson

really? I still use it... yes, I read as millifarards. If he meant something else then he better properly specify it.

Maybe you're thinking of the "mmF"?

Jamie

In the old days it would be micro farad and mmF would be micro micro Farad or picoFarad in today's terms. By his calculation I assumed he meant milli Farad. I agree also that the term milli Farad is obscure.

the terms F, mF, uF, nF, pF are all alive and well, being used in PSpice and LTspice.

I think fF is interpreted as Farad, though.

Nope. fF = femto-Farad = 10^(-15) Farad

If you want Farads in Spice, it's just a number, no text. ...Jim Thompson

Do you use kilofeet and decimeters too? Goofy units should be avoided.

Oh gee, brand me a odd ball, I still use Webers..

I bet you still use stones!

Jamie

High again guys, Sorry now I realized I made an error in the info I posted after reading some of the replies...... the correct info would be: The numbers are V= 13.8V R= 33K ohms ***C= 66uF*** **t= 2 seconds** e= 2.71828 What I'm expecting is the cap will reach 63% of full charge(13.8V)at around 2 seconds this will be used within a circuit to turn on a relay so there is at least 2-3 seconds delay of the relay closing turning on a circuit to charge a battery. So I guess I need to know if this R/C combo will give me the result I hope for

Thanks for the replies and help, It is greatly appreciated. Jerry

Your first mistake was indicating that you were looking for

May I suggest that you work a little harder on your home work.

Jamie

Yup, tau is the RC time constant, an Ohm times a Farad is a second. George H.