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I am designing a PCB for controlling servos.

My voltage regulator can provide 3 amps to the servos. My Arduino Uno also draws power from the same line. Using an online calculator, I was able to determine that I need 53 mil traces for power and ground lines coming directly from the regulator, and from devices powered by the regulator, respectively. Since the Arduino can draw at most about 500mA, I know that I don't need any special width for traces into and out of the Arduino's inputs and outputs.

The Arduino has its own voltage regulator, and a few ICs are powered by its 5V and 3.3V supplies.

Do I need 53 mil ground lines everywhere from these chips? Since the Arduino can't pump 3A, I can't imagine any one of these chips needing such a huge trace for ground lines.

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If you have traces forking off the main trace, or separate traces running directly to the regulator then these only need to be wide enough to handle the 500mA for the Arduino.

You can think of the power distribution a bit like a river with tributaries. The main "river" needs to handle the total power for everything and the tributaries need to handle the power for whatever they lead to. Remember to think about the full loop (e.g power and ground traces) for everything.
If the Arduino was directly in line to the servos (i.e. the servos were downstream of the Arduino, so it goes Regulator->Arduino->Servos) then the traces would need to be rated to handle the Servo current too.

Usually it is best to keep the high current stuff on a separate trace, or nearer to the supply, as the more of the route they share with sensitive components the more they can interfere. With planes a similar logic applies, though positioning is more important as you have to assume the path of least impedance will be taken (not always the shortest path)
Edit - As Kortuk mentions a plane is preferable if possible. A dedicated solid layer (i.e. no routing, or if some is necessary try and make it in the same direction to current flow so it does not have to "take the long way round") is best but a fill is certainly useful also.

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    \$\begingroup\$ I would just use a ground plane, but I agree if that is not an option. \$\endgroup\$ – Kortuk Sep 17 '11 at 23:04
  • \$\begingroup\$ @Kortuk - I agree, have updated the answer with some more on planes. \$\endgroup\$ – Oli Glaser Sep 17 '11 at 23:21
  • \$\begingroup\$ Very enlightening, thank you! Makes perfect sense. \$\endgroup\$ – Chris Laplante Sep 18 '11 at 0:38
  • \$\begingroup\$ Now I just need to figure out how to do different trace widths on the same net in Eagle... \$\endgroup\$ – Chris Laplante Sep 18 '11 at 0:50
  • \$\begingroup\$ One way to do different trace widths on the GND is to give it several net names (power_GND, signal_GND, etc.) and use the techniques from "How do I facilitate keeping multiple grounds, (i.e. AGND, DGND, etc…) separated in the layout when using Eagle?" -- but there's probably a simpler way. \$\endgroup\$ – davidcary Feb 2 '12 at 20:02

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