A Decoupling Capacitor is used in Electrical Engineering when your circuit contains a device that might need an extra current boost now and then, or when you want to improve AC noise performance. In any case, the basic purpose of a decoupling capacitor is to keep your circuit running smoothly and predictably. The term "decoupling capacitor" is often used interchangably with "bypass capacitor".

As an example, you might have a logic chip in your circuit. When some change is taking place in the circuit, the logic chip might suddenly try to draw additional current from the power supply. Figure 1 shows a very simple circuit consisting of a power supply connected across a device, with no additional components. In this scenario, if the device sporadically requires more current than the power supply can put out without dropping voltage, then the voltage will actually drop, resulting in unpredictable or unwanted behavior. In Figure 1 and Figure 2, Current1 is the power supply current.


Figure 1

---------------------------- | | | Current1 -> | + | | Power | | Supply O { } Device | | - | | | | ----------------------------
What we need is an energy storage device, something that will sit there ready and willing to release that energy on demand. By placing a capacitor in parallel with your device, you introduce an additional source for the device to draw current from. The power supply will charge the capacitor; if the device begins to draw a lot of current, the capacitor will release its stored energy. Figure 2 shows the placement of the decoupling capacitor, and the additional current from the capacitor, Current2. In times of higher current demand, the current received by our logic device is the combination of Current1 and Current2.

Figure 2

---------------------------- | | | Current1 -> | Current2 -> | + | | | Power | --- | Supply O --- C { } Device | | | - | | | | | | ----------------------------

The value of the decoupling capacitor C you need might vary; in general, values from 0.01 to 0.1 microfarads will suffice for logic circuits. If you want to decouple across the power supply for the entire circuit, you might require something larger, in the range of tens of microfarads.

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