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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