Resistor Calculation for Alarm Clock Circuit

[Masafi]

In an alarm clock, there needs to be a delay of 1 minute between closing switch and circuit being active. A 2200microF capacitor is charged through a resistor from a 5V supply. The pd across capictor must rise to 4.3V for alarm to become active.

Calculate size of resistor.

The answer is 14000ohms. I need to use the equation: V=Vo * e^(-t/RC) with t = 60s

but I am not sure what to use for the value of the 2 V's.

 

[ideasrule]

V is the voltage across the capacitor and Vo is the voltage across the power source.

 

[Masafi]

V is the voltage across the capacitor and Vo is the voltage across the power source.

So:

4.3 = 5 e^(-t/RC)

but this doesn't give R=14000 ohms?

 

[tiny-tim]

I need to use the equation: V=Vo * e^(-t/RC)

Hi Masafi

No, for a capacitor being charged from zero, it's …

I(t) = (V₀/R)e^-t/RC

V(t) = V₀(1 - e^-t/RC)

(so the current becomes effectively zero, and the voltage across the capacitor becomes effectively V₀, as t -> ∞)