Capacitor Discharge Current Calculator

This tool calculates the discharge current, peak current and charge of a capacitor.

Calculator

Enter

  • Initial voltage Vo
  • Resistor value R
  • Capacitor value C
  • Time t at which the discharge current and charge are to be calculated

Formula

Vo is the initial voltage. The capacitor voltage Vc at time t is given by the formula:

Vc = Vo*e-t/RC

where R and C are values of the resistor and capacitor, respectively.

The discharge current is given by

I = Vc/R

The peak current is given by

Io = Vo/R

The peak charge is

Qo = C*Vo

The charge at time ‘t’ is

Qc = C*Vc

Example Calculations

For a resistor value of 10 ohm, Capacitor value of 2 µF, initial voltage Vo = 5 volt, the peak charge is 10 µC (micro Coulomb) and the peak current is 0.5 A.

At time t = 10 µs, the charge drops to 6 µC; discharge voltage is approximately 3V and discharge current is 0.303 A.

Background

Capacitor discharge refers to the process by which a capacitor, a device that stores electrical energy in an electric field, releases its stored energy. Here’s a more detailed explanation and a step-by-step process:

  • Capacitor Charging: Initially, a capacitor is charged by connecting it to a voltage source. During this phase, electrons accumulate on one plate, creating a negative charge, while the other plate loses electrons and becomes positively charged. The energy is stored in the electric field created between these two plates.
  • Storing Energy: The energy is held in the capacitor as long as the voltage source is connected and there’s no path for the charge to flow out. The amount of energy stored depends on the capacitor’s capacitance (a measure of its ability to store charge) and the voltage applied to it.
  • Discharge Process: When the capacitor is disconnected from the charging source and connected to a circuit with a resistor, the stored energy begins to flow out of the capacitor. The electrons move from the negatively charged plate to the positively charged plate, trying to neutralize the charge imbalance.
  • Voltage Decay: The voltage across the capacitor’s plates, which was initially equal to the charging voltage, starts to decrease as the charge flows out. The rate of this voltage decay depends on the capacitance of the capacitor and the resistance of the circuit it’s connected to. This relationship is described by the time constant, τ (tau), given by τ = RC, where R is the resistance and C is the capacitance.
  • Exponential Decay: The voltage decay during discharge follows an exponential pattern. It decreases rapidly at first and then more slowly as the charge reduces. Theoretically, it takes an infinite amount of time for the voltage to reach zero, but practically, it’s considered discharged when it drops to a very low value.
  • Applications: Capacitor discharge is used in many applications, such as in the flash of a camera, where the rapid release of energy creates a bright light, or in electronic circuits to smooth out voltage fluctuations.

This process is a fundamental concept in electronics and illustrates how capacitors function as temporary energy storage devices.

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