Enter constant source voltage, positive series resistance and capacitance, and elapsed time to find instantaneous charging current. Choose a result unit if needed; open Calculation details for substitutions. The capacitor is initially uncharged.

Find instantaneous current at an elapsed time after an initially uncharged ideal capacitor is connected to a constant DC source through a resistor.

Signed DC step voltage, measured from the chosen reference. The initial capacitor voltage is zero.

Total positive series resistance, including source resistance where relevant.

Positive ideal constant capacitance. Changing units converts the entered quantity.

Zero gives the initial current. Negative time is outside this charging model.

Result unit

Amperes are the default. The selected unit applies even when this section is collapsed.

Capacitor Charge Current Formula

For an initially uncharged ideal capacitor connected through a positive series resistance to a constant DC source, the instantaneous charging current is:

I = V / R ร— e - (t / (RC))
  • Where I is the Capacitor Charge Current (amps)
  • V is the constant source voltage (volts), with initial capacitor voltage zero 
  • R is the resistance (ohms) 
  • C is the capacitance (Farads) 
  • t is elapsed time since connection (seconds), with t โ‰ฅ 0

Divide the source voltage by the series resistance, then multiply by e raised to the negative elapsed time divided by R ร— C. The RC time constant has units of seconds.

How to Calculate Capacitor Charge Current?

The following example problems outline how to calculate Capacitor Charge Current.

Example Problem #1

  1. First, determine the voltage (volts). In this example, the voltage (volts) is determined to be 15 .
  2. Next, determine the resistance (ohms). For this problem, the resistance (ohms) is measured to be 10 .
  3. Next, determine the capacitance (Farads). In this case, the capacitance (Farads) is found to be 6.
  4. Next, determine the time. For this problem, the time is 5 seconds.
  5. Finally, calculate the Capacitor Charge Current using the formula above: 

I = (V / R) ร— e ^ (โˆ’t / (R ร— C))

Inserting the values from above and solving yields: 

I = 15 / 10 * e ^- (5/(10*6)) = 1.380 (amps), rounded to three decimals

Example Problem #2

Using the same method as above, determine the variables required by the formula. For this example problem, these are:

voltage (volts) = 20

resistance (ohms) = 15

capacitance (Farads) = 4

time (seconds) = 2

Enter these given values into the calculator or above yields: 

I = 20 / 15 * e ^ – (2/(15*4)) = 1.290 (amps), rounded to three decimals