Physic on Time Constant

[Jenz08]

So in calculating the time constant of a timing circuit using a capacitor C of value 4.7�F and a resistor R of value 8.2kΩ, you get...

Time constant = 4.7/1000000 x 8200 = 38 milliseconds

Now you exchange the resistor for one with a different value so that the charging time is approximately 15 seconds, What would be an appropriate value for the resistor?! explaining the reasoning?!

Is it Time constant = RC?
equalling = 15/5
= 3 secs??

[Jenz08]

European time constant

[heathfield]

You are correct, Time Constant = R x C

(A handy hint here is that where R is in megohms, and C is in microfarads, then TC is in whole seconds).

You want 15 seconds...

Transpose to get TC/C = R

Thus, (using the handy hint) R = !5/ 4.7 = 3.19 megohms.

[mibn2cweus]

t / C = R

The "time constant" is the time required for the capacitor to charge up to 63.2% of the applied voltage or to discharge to 36.8%, (= 1 - 63.2%) of the initial charged voltage. This time increases in proportion to the resistance and capacitance of the RC (series) circuit. The greater the resistance to current flow the longer it will take for a given value of capacitance to charge up (or discharge) through the circuit.

To solve for R in
t = R C
divide both sides by C to get

t / C = R

15 / 0.000004.7 = R

[chakka35]

Jenz08, I answered this, and gave you the formula, when you asked the question just a few posts ago. Did you not see it?

[Jenz08]

Sorry chakka35, i didnt read the answer fully.... thought i would check if i was along the right line :)

[Jenz08]

Lol Zacsmaster ive never seen monty python