Energy is stored in an electric field.

A capacitor has conducting electrodes separated by an insulating dielectric. Applying a voltage causes charge separation and stores energy in the electric field.

Capacitance describes charge stored per volt, measured in farads (F). Microfarads (µF) are commonly used because one farad is a large unit for many ordinary electronic circuits.

Charging and supplying a load.

  1. ChargeCurrent changes the charge on the electrodes.
  2. StoreThe electric field holds energy.
  3. DischargeEnergy is transferred to a connected circuit.
Conceptual capacitor behaviour. No discharge or testing procedure is shown.

For an ideal capacitor, current flows while its voltage changes. With steady DC voltage, ideal capacitor current falls to zero; real devices can still have leakage.

Voltage matters as well as capacitance.

An ideal 1,000 µF capacitor at 24 V

1,000 µF = 0.001 F. Stored energy is ½ × C × V², so ½ × 0.001 × 24² = 0.288 joules.

Doubling the voltage gives four times the stored energy for the same capacitance. The voltage rating is a limit for the component, not a target that every circuit must reach.

Smoothing supplies and supporting a drive’s DC link.

In a power supply, a capacitor can support the load between rectified pulses. In a VFD, DC-link capacitors help manage the intermediate DC supply. Capacitors are also used in filtering and timing circuits.

A capacitor does not make an unlimited backup supply. Its voltage changes as energy is transferred, and its size and circuit conditions affect how long it can support a load.

Real components have limits.

Capacitors have resistance, temperature limits and ageing behaviour. Some types are polarised, while others are not. Matching capacitance alone is not enough to select a replacement.

Stored energy may remain after external power is removed. A stopped or disconnected drive is not evidence that its internal capacitors are discharged.

A closer look

Equivalent series resistance (ESR) models part of a real capacitor’s losses. Ripple current through those losses produces heat. In a simple resistor-capacitor charging circuit, the time constant is R × C; this is a mathematical model, not a safe waiting-time rule for equipment.

Check your understanding

If voltage doubles on the same ideal capacitor, does stored energy double?

Reveal the explanation

It quadruples, because stored energy is proportional to voltage squared: E = ½CV².

Further reading.

TDK: capacitor basics