A brief discussion on capacitor charging, discharging and charge discharge

The capacitor charging and discharging circuit is shown in the figure. When the switch is in the position shown in the figure, the power supply charges the capacitor, and the voltage across the capacitor gradually increases. In the end, the capacitor is equivalent to a short circuit to DC, so the voltage across the capacitor is equal to the power supply voltage. At this time, switch the switch to the other end, the voltage across the capacitor cannot change suddenly, and it will discharge in the circuit with resistance until the charge disappears completely. This is the charging and discharging process of the capacitor.

The time it takes to charge and discharge a capacitor is only related to the value of the resistor and capacitor, and has nothing to do with the power supply voltage. This time can be expressed as the product of RC, which is the time constant τ.

The formula for RC is derived as follows:

During the capacitor charging process, the change rules of voltage and current in the circuit are as follows:

The time constant represents the time of charge and discharge. From the existing experimental results, the charge and discharge of capacitors is an exponential change. In the time of τ, the voltage across the capacitor reaches 63.2%; in the time of 3τ, the voltage across the capacitor reaches 95%; in the time of 5τ, the voltage across the capacitor reaches 99.3%. The discharge process is the opposite of the charging process.

Electrostatic discharge will have an impact on small components, and this transient interference pulse may cause damage. RC filtering, current limiting resistors, and shunt circuits can be used for protection. If you want to discharge the charge, you can form a loop for the already charged capacitor. According to the time constant τ, if the capacitor is not easy to change, reducing the resistance will significantly reduce the time constant and speed up the charge discharge. Using an analog switch, when the charge is too large, control the analog switch to close, which is equivalent to connecting a very small resistor in parallel to the capacitor to achieve charge discharge. If the charge is discharged, the switch is disconnected again to restore the original circuit. The drive switch closing action can generally be achieved with an external button, or it can be achieved by selecting a landmark voltage or current based on the specific circuit conditions and combining it with a comparator built with an op amp.