- Capacitor Equivalent Circuit -
[IC By-Pass Caps] [Equivalent Cap Circuit]
[Capacitor Decoupling Value] [Capacitor Terms]
[Surface Mount Chip Size] [MIL Specs]
The equivalent circuit for a capacitor is shown in the graphic below.
Each element is important to how the capacitor operates, starting with the capacitance 'C'.
Component 'C' represents the actual value of the capacitor.
Resistor 'Ri' is the insulation resistance of the capacitor and accounts for leakage current with in the device.
Resistor 'Rs' equates to the series resistance of the capacitor, and relates to how fast a capacitor will charge or discharge.
Inductor 'Ll' is the lead inductance of the component and is also in series with the capacitance.
The lead inductance also effects how fast the capacitor can react to circuit changes because inductance inhibits current change.
Always use chip capacitors [surface mount], never use leaded parts [through-hole], because of lower inductance.
Low Inductance Chip Capacitors [LICC Definition] have low values of 'Ll', which allow the cap to quickly charge and discharge.
When possible always use low ESR capacitors. Normally a capacitor below .01uF are by definition low ESR. ESR Definition.
Higher value ceramic capacitors, Tantalum and Electrolytic types need to be checked for its ESR value.
Note that ESR or Equivalent Series Resistance is the sum of the series resistance and insulation resistance.
The lead inductance changes as the package type changes, the larger the package, the larger the inductance.
Series resistance is effected by both the lead type and the material used to make up the capacitor.
Any component could be seen as a collection of resistors, capacitors and inductors.
In this case the capacitor in the equivalent circuit represents the capacitance of the device.
See a graph of Capacitor Insulation Resistance by capacitance [R sub I in the graphic].
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