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Consider this circuit, where the 5uH inductor represents the parasitic inductance of a 3m long, 24AWG wire. At steady state, when the switch is closed, 120mA are of current is expected to flow through R1. However, because of the inductance, the change in current is not instantaneous. For a period of time right after the switch is closed, the current slowly ramps up to 120mA.
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The red curve represents the voltage at the top right node (at R1). Note that the voltage on the power line slowly ramps to its final value at steady state as voltage across a resistor is proportional to the current through it. With real devices, this effect may be compounded with yet more parasitics/non-linearities, and cause further noise injection into the power line. This effect is phenomenon reduces the result performance of the wire/device because the power supply inductancevoltage is no longer constant.
The solution to this is to add bulk capacitors, which serve as a large local "tank" of power located closer to the load (i.e. after the long 3m wire).
C1 is a large capacitor that is constantly being charged by the 12V source. At times when current from the source is too slow to arrive, the local bulk capacitor can act as a "power supply" located much closer to the device. The voltage on the line now no longer dips and the device can access the fast change in current that it needs.
This makes the power line cleaner and prevents possible issues with low voltage dropout of the downstream devices.
Bulk capacitors are typically implemented by large electrolytic/aluminum capacitors, with values up to ~1000uF. Ensure that bulk capacitors are present on each individual unit of PCBs.