Capacitor Charging/Discharging

The experiment was to design a charge/discharge network using capacitors and resistors in series. The design required us to find a way to charge the capacitor in 20s, then abruptly discharging it in 2s. With a few calculations, our design was built in the following manner:

We determined the values of the capacitor, charge resistor, and discharge resistor to be 47 µF, 85 kΩ, and 8.5 kΩ, respectively. We ran the circuit through a 9 V power supply, charged the capacitor by closing the left half of the circuit and discharged it by immediately closing the right half. and recorded the data on Logger Pro. Our results were:

As expected, the capacitor charges at 20 s and discharges in 2 s. 

Follow-up Questions

1. During charging,

    R­_Th = R_charge * R_leak / (R_charge + R_leak)

           = 85 kΩ * 992.5 kΩ / (85 kΩ + 992.5 kΩ)
           = 78.3 kΩ

2. During discharging,

    R­_Th = R_discharge * R_leak / (R_discharge + R_leak)
           = 8.5 kΩ * 992.5 kΩ / (8.5 kΩ + 992.5 kΩ)
           = 8.4 kΩ

3. When V = 0.6321 * (final voltage), τ_c = 4.0 s. Thus, R = τ_c / C = 85.11 kΩ
    The value is indeed close to the value in question 1.

Practical Questions

1. C = 2E / V² 
        = 2 * 160 MJ / (15 kV)2
        = 1.42 F

2. If the capacitance is arranged in the schematic, the value of each capacitor, C, is 0.71 F