flak88,
Couple of beginner questions about RC circuits:
OK, go ahead.
How do you calculate what value capacity + charging resistor you need at 10v dc supply to take 30 sec from 0v to reach the supply voltage 10v and vice versa for discharge.
You first of all determine what time constant you need. For a RC circuit, the time constant TC=RC. Thereafter, choose the product of R and C that will give you the desired time constant.
To find the second time constant do you find 63% of the first time constant ?
No, time constants are measured in seconds, not percent. Two time constants are 2*TC .
Can someone explain and give an example of how to use the vc = v(1 -e -t/RC) to get the voltage across the capacitor at any time constant ?
Yes I can. The general formula for energizing and de-energizing a capacitor is:
V = Vf + (Vo-Vf)*exp(-t/RC) , where Vf is the final voltage and Vo is the initial voltage. As everyone has been telling you, 5 or 6 time constants will get you very close to the final voltage. Assuming 6 time-constants, then the time-constant will be 5 seconds. Further assuming that R=1 Meg and C = 5 uF, then TC will be RC = 5 seconds. Plugging and chugging into the formula I stated above, we get after 5 secs, 30+(-30*exp(5/5)) = 18.96 volts, 10 secs = 30+(-30*exp(10/5)) = 25.94 volts, 30 secs is 30+(-30*exp(30/5)) = 29.93 volts.
So to calculate the required rc and c value for 30 sec it would be RC 3k x 2uf ?
The time-constant for the RC values you gave is 3000*2uF = .006 secs. The is way too short compared to 5 or 6 secs. Did you do the arithmetic?
To calculate the value for 2 time constant do i just get 63% of the first constant ?
I assume you mean the capacitor voltage value. As shown above, after the first time-constant, the capacitor voltage will be 18.96 volts. The difference between 30 and 18.96 is 11.04 . 63% of 11.04 is 6.96 volts . Adding 18.96 + 6.96 is 25.92 volts which compares favorably with the 10 sec value calculated above.
To calculate the voltage at time 3sec of rc of 3k and c of 2uf i set Rc to 6 and divide by the time 3sec then what steps?
Your specified a time constant of 0.006 secs. 3.0/0.006 = 500 time-constants. The voltage will have long reached its final value within 3 secs with such a short time-constant.
To get the rc current at any time is it just a case of getting the voltage at that time then dividing it by rc resistor ?
I = C*(dE/dt) , but you will find it easier to subtract the voltage across C from the supply voltage to get the voltage across R. Then using the resistance formula I = v/R , the current is obtained.
Ratch