Capacitors in Parallel

- capacitors are often connected in parallel to provide additional capacitance or for other reasons. Calculations of resistors in parallel are easy to handle.

When designing or building circuits it is often useful to be able to place capacitors in parallel.

There can be good electrical reasons for doing this, but it is normally necessary to calculate the overall capacitance of these capacitors in parallel.

Parallel capacitors formula

It is very easy to calculate the total capacitance of a set of capacitors in parallel. The total value is simply the sum of the capacitance values of the individual capacitors.

A diagram showing how capacitors are added in parallel.
Capacitors in parallel

In theory there is no limit to the number of capacitors that can be added in parallel. Obviously there can be practical limits dependent upon the application, space and other physical limitations.

The formula to determine the total capacitance of capacitors in parallel: Ctot = C1 + C2 + C3 +  . . .

Advantages of using capacitors in parallel

There are several reasons why it may be beneficial to place capacitors in parallel:

  • Increase value of capacitor:   The most obvious reason for having two or more capacitors in parallel is to increase the value of capacitance available. It may be convenient to use two or more capacitors of a smaller value than a single larger one.
  • Utilise different capacitors for decoupling different frequencies:   A single capacitor is not always able to remove all the frequencies that may be present on a voltage supply line etc. To fully achieve this, it is often the practice to use tow capacitors in parallel: one such as an electrolytic with a larger value to remove the low frequency components (the electrolytic capacitor is not good at passing high frequency signals); and one such as a ceramic capacitor with a smaller value for removing the high frequency components (the smaller ceramic capacitor will not have a low enough reactance to pass the low frequency components).

    When using this approach it is necessary to be aware of the effects of spurious series inductance because it is possible for the stray inductance from one capacitor to resonate with the capacitance of the second. These effects do not normally cause a problem, but running two capacitors in parallel can give rise to these combined effects.
  • Distributed decoupling:   On many logic boards where there are many logic ICs it is common practice to distribute the decoupling around the board, typically having a capacitor on each IC, or possibly every other IC, and larger decouplers placed strategically around the circuit. It is necessary in circumstances like these to be able to calculate the overall capacitance level.
  • To obtain non-preferred capacitance value:   Like many components, capacitors come in preferred values. For some applications, specific values may be required that may not coincide with the preferred values. Almost any value cna be made from combining two or more preferred values, although be aware that some capacitors wide tolerance levels so the final value will be subject to these figures.

These are some of the instances where it is useful to be able to use two or more capacitors in parallel. There may also be a host of other applications as well.

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