Capacitor Fundamentals

Capacitor Charging

A capacitor does not charge instantly when connected to a voltage source. Instead, the voltage across the capacitor rises gradually as electrical charge accumulates on its plates. The charging rate depends mainly on the values of the resistor and capacitor in the circuit. This behaviour is fundamental to timing circuits, filters, oscillators and many other electronic applications.

Capacitor Charging

Basic Charging Circuit

A simple charging circuit consists of a resistor, a capacitor and a DC voltage source. 


      +V
       โ”‚
       โ”‚
       R
       โ”‚
       +------ VC
       โ”‚
      || C
       โ”‚
      GND

The resistor limits the charging current while the capacitor stores electrical energy.

How Charging Occurs

  1. The capacitor is initially uncharged.
  2. When power is applied, maximum current flows through the resistor.
  3. Electrical charge accumulates on the capacitor plates.
  4. The capacitor voltage gradually increases.
  5. The charging current gradually decreases.
  6. Eventually, the capacitor voltage approaches the supply voltage and the charging current becomes almost zero.

The RC Time Constant

The charging speed is determined by the RC time constant. 


ฯ„ = R ร— C
Symbol Description
ฯ„ Time constant (seconds)
R Resistance (ohms)
C Capacitance (farads)

A larger resistor or larger capacitor increases the charging time.

Charging Curve

Elapsed Time Capacitor Voltage
1ฯ„ Approximately 63%
2ฯ„ Approximately 86%
3ฯ„ Approximately 95%
4ฯ„ Approximately 98%
5ฯ„ More than 99%

Although the capacitor theoretically never reaches exactly 100% of the supply voltage, it is considered fully charged after about five time constants.

Charging Current

The charging current is highest when the capacitor is completely discharged.

Time Charging Current
0 Maximum
1ฯ„ About 37% of the initial current
2ฯ„ About 14%
3ฯ„ About 5%
5ฯ„ Almost zero

Worked Example

Given:

  • Resistance = 10 kฮฉ
  • Capacitance = 100 ยตF
  • Supply Voltage = 12 V

Time constant: 


ฯ„ = 10,000 ร— 0.0001

ฯ„ = 1 second

The capacitor voltage will be approximately:

Time Voltage
1 second โ‰ˆ7.6 V
2 seconds โ‰ˆ10.3 V
3 seconds โ‰ˆ11.4 V
5 seconds โ‰ˆ12 V

Factors Affecting Charging Time

  • Resistance value.
  • Capacitance value.
  • Supply voltage.
  • Leakage current.
  • Temperature.
  • Component tolerances.

Typical Applications

  • Power-on reset circuits.
  • 555 timer circuits.
  • LED delay circuits.
  • Microcontroller reset timing.
  • Audio muting circuits.
  • Soft-start power supplies.
  • Camera flash charging.
  • RC oscillators.

Common Problems

Problem Possible Cause
Charges too slowly. Resistance or capacitance too large.
Charges too quickly. Resistance too small.
Never reaches full voltage. Leakage current or circuit loading.
No charging. Open resistor or faulty capacitor.
Very high charging current. Shorted capacitor or very low resistance.

Safety Considerations

  • Large capacitors can store dangerous amounts of energy.
  • Always discharge high-voltage capacitors before handling them.
  • Never exceed the capacitor's voltage rating.
  • Observe polarity when using electrolytic capacitors.
  • Use an appropriate resistor to limit charging current.

Real-World Examples

Equipment Charging Function
Camera Flash Stores energy for the flash tube.
555 Timer Timing capacitor charging.
Arduino Reset Circuit Power-on delay.
Audio Amplifier Soft-start delay.
Switch-Mode Power Supply Soft-start and timing functions.

Key Points

  • A capacitor charges gradually rather than instantly.
  • The charging rate depends on the RC time constant.
  • Charging current decreases as the capacitor voltage increases.
  • After approximately five time constants, the capacitor is considered fully charged.
  • Capacitor charging is fundamental to timing, filtering and energy-storage circuits.

Next Lesson

Continue by learning about Capacitor Discharging, including discharge curves, current flow, RC time constants and practical applications.

Next Lesson โ†’ Capacitor Discharging