Capacitance for a parallel RC circuit

[envanyatar]

Homework Statement

A circuit consists of a Zener diode, capacitor and resistor in parallel. Voltage to be maintained across the components is 4.7V, power 5mW. Find the values for resistor and capacitor

Homework Equations

(Below)

The Attempt at a Solution

I found the resistance by taking P=V^2/R, and using the given values for P and V, R=4.4k(Ohms). I have a problem with finding the capacitance value.
I found the current for the resistor. But there're 2 unknowns now, the capacitor value and the Ic. I've come up to the point where two equations are obtained: Ic=CdV/dt; Ir=V/R. How do I proceed from here to get C? Is this the correct way of approaching this?

Thanks.

 

[CEL]

The problem is incomplete. Is this circuit connected to the output of a rectifier? If so, is it full wave or half wave? What is the peak voltage of the rectified voltage. (I assume the frequency is 60 Hz).

 

[piercas]

Yes, you are on the right track. To find the capacitance value, you can use the formula for the time constant of a parallel RC circuit, which is T = RC. In this case, we know the time constant (T) is equal to 1/f, where f is the frequency of the circuit. Since we are given the voltage and power, we can also calculate the current (I) using the formula P = IV. Once you have the current value, you can plug it into the equation I = C(dV/dt) to solve for the capacitance (C). Alternatively, you can also use the formula for the impedance of a parallel RC circuit, which is Z = 1/√(1/R^2 + (1/ωC)^2), where ω is the angular frequency of the circuit. You can rearrange this formula to solve for C. I hope this helps.