# [Explained] How Long Does It Take a Capacitor to Discharge?

The time required to discharge a capacitor depends upon the period of the capacitor. After 5 time periods, a capacitor will be discharged almost completely. Practically a capacitor never discharges completely but goes very close to 0% of the initial voltage, which is practically assumed to be completely discharged.

The time period of a capacitor is the product of its capacitance (in farads) and the value of the discharging resistor (in ohms). It is denoted by **tau (τ)**. So, **τ= RC**. The unit of time constant is seconds. And, the discharge time, **t = 5τ = 5RC seconds**.

## How Much Time Does It Take a Capacitor to Discharge?

The discharge time of a capacitor is related to the period of the capacitor. After one time period, a fully charged capacitor will discharge to 63% of its original voltage. So, after 5 time periods, it can be safely assumed that the capacitor is completely discharged for any practical purposes.

The period of a capacitor, **τ=RC**. Where **C is in farads** and **R is in ohms**. And, the discharge time, **t = 5τ = 5RC**. In the following circuit, the resistor is connected to discharge the capacitor, the discharging time can be figured out by using the formula, discharge time, **t = 5RC**.

For the above circuit **C= 1000µF = 1×10 ^{-3}F** and

**R= 1KΩ= 1000Ω**. So, the time,

**τ= 1×10**. And, the discharging time of the capacitor is

^{-3}F × 1000Ω= 1 second**5τ = 5 × 1 second = 5 seconds**. The capacitor will take approximately 5 seconds to fully discharge itself.

### Will A Capacitor Discharge On Its Own?

Yes, a capacitor can discharge on its own. This happens due to the leakage current. A capacitor can be assumed to have a very high resistance resistor (mega ohms) connected in parallel to it. Multiplying this resistance with the capacitance will give the period (τ) and 5τ will be the self-discharge time of the capacitor.

### How Long Do Capacitors Retain Charge?

The self-discharge time of a charged capacitor depends upon the capacitance, it can be days or even years. The self-discharge time, **t = 5RC**. So, discharge time, **t ∝ C**. For large capacitors, the stored charge can stay for months or even years. So, for every capacitor with different capacitances, the discharge time will be different.

### Do Capacitors Discharge Quickly?

Small capacitors discharge quickly, but large-value capacitors can hold a charge for up to months or years. The discharge time is directly proportional to C and R. So, for larger value capacitors the discharge time is more than the smaller capacitors. This is true for both self-discharging and discharging using a bleeder resistor.

### How to safely discharge a capacitor?

A capacitor can be safely discharged by shorting its terminals through a 1KΩ bleeder resistor of sufficiently high power rating to handle the dissipated power safely. If such a resistor is not available, for small-value capacitors a screwdriver or any conductive material can be used to short its terminals to discharge it safely.

## Conclusion

The time required for a capacitor to be fully discharged is defined as five times the time constant of the capacitor. This relation is applicable for discharging a capacitor through a bleeder resistor and for the self-discharging of the capacitor. The approximate discharging time can be obtained by using the formula, **t = 5RC**.

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