# Capacitor Discharging Time

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Joined: Sep 24, 2018

Replies: 0
Capacitor Discharging Time
September 24, 2018 - 6:24pm

Hi i have designed a circuit for electricity generation using Piezoelectric crystal. As you can see in the below attached circuit diagram, the working concept is like that, the capacitor first charges by pressing the piezo crystal. And when the switch is pressed the capacitor transferred all its stored energy into the LED, and we get a flash of LED. it seems that the ON time of LED depends on the value of capacitor..  so can anyone explain how to calculate the discharging time of the capacitor so i can increase the ON time of the LED..  Offline

Joined: May 19, 2015

Replies: 208
Calculating charging and discharging time for a capacitor
September 25, 2018 - 11:10am

Calculating Energy stored in Capacitor and time taken:

The amount of energy that can be stored in a capacitor can be calculated by using the formulae below

E = CV/ 2

where,

E = Energy stored in capacitor in joules

C = Capacitance of the capacitor in farads

V = Voltage applied across the capacitor in Volts

In your case for a 47uF capacitor assuming your crystal produces 3V constantly (which is not possible) the energy stored will be

E = 0.21mJ

The time taken for the capacitor to charge will be

τ = RC

τ = Time constant in seconds

R = Resistance in ohms

Calculating the discharge time and energy:

Once we know how much energy is stored in the capacitor we can assume it to be a voltage source (battery) and then based on the current consumed by the load we can calculate the discharge time of the capacitor.

Remember these calculations will only give a rough figure and will not be accurate since we are not considering the nature of the circuit and the impedance associated with capacitor