Dielectric absorption

An ideal capacitor would lose its stored charge completely when shorted, but real capacitors can show a small voltage recovery after a charge–short–open-circuit sequence. During charging, the dielectric's response is not always instantaneous: different polarization and charge-redistribution processes can respond on different timescales. A brief discharge removes much of the immediately accessible charge, while slower internal processes continue to relax. Their delayed response produces the observed recovery voltage. The recovery is not energy created from nothing; it reflects charge and energy stored in the capacitor's dielectric during its prior electrical history.

The dielectric inside a capacitor becomes polarized in an applied electric field. In real materials, dipoles, domains, and other charge distributions do not all reorient or relax at the same rate. Their delayed response makes the effective electrical behaviour depend on the history and timescale of the applied voltage. This effect is called dielectric absorption; dielectric relaxation and soakage are also used as names for the phenomenon. A simple circuit model represents the behaviour with additional resistive-capacitive branches alongside an ideal capacitance. More detailed models use a range of relaxation times because a single time constant often cannot reproduce a material's measured response across all relevant timescales.

Dielectric absorption matters when a circuit must hold an accurate voltage or charge. In a sample-and-hold circuit, a capacitor stores a sampled signal while the circuit is not measuring it. If the dielectric slowly redistributes charge, the stored voltage can drift and cause measurement error. Similar memory effects can influence switched-capacitor circuits and other precision analog systems. The effect can also contribute to capacitor losses and affect the quality factor of high-Q circuits. Its practical significance depends on the circuit's required accuracy, operating conditions, capacitor construction, and the time and frequency range of interest.

Dielectric absorption is not the same for every capacitor type. Manufacturer data and engineering references compare dielectric materials and report that values can differ substantially; therefore, a capacitor's dielectric and its datasheet specifications matter in precision applications. A commonly described time-domain test charges a capacitor, briefly shorts it, and measures the subsequent recovery voltage. The result depends on details such as the charging duration, the shorting interval, leakage, and measurement input currents, so it is not a universal constant independent of test conditions. For design work, engineers use material-specific specifications or characterize the component under conditions representative of its intended circuit.