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I was looking over my raspberry pi model B and I noticed a resistor between the USB power connector shield and ground. I've noticed this on many PCB designs with much larger packaged resistors although I've never known the reason for them.

This is the part I'm curious about, R51.

enter image description here

On the board, it looks like its an 0805 package, although I've seen larger one.

So my questions are basically:

  1. Why do we need a resistor here? is it something to do with ESD when inserting a cable into the connector?
  2. Why do we use such a large package? I assume its to handle more power, but where would this power come from since they are both "GND"

Please correct me if I'm wrong in any of my thinking

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  • \$\begingroup\$ possible duplicate of Why is there a 0R resistor linking GND and AGND in analog voltage reference circuit? \$\endgroup\$ – Ilmari Karonen Nov 10 '14 at 19:45
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    \$\begingroup\$ @IlmariKaronen While the question is similar, there are a lot more reasons to have that setup between AGND and GND since they'll be mixed-signal devices and proper grounding is a much larger concern. With this question, I'm asking why you'd bother doing so between the shield of a connector and GND since you would generally not send any signal through a shield. Here, I'd be questioning if this is even necessary between a shield and GND \$\endgroup\$ – Funkyguy Nov 10 '14 at 20:59
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It's a jumper-- 0R0 is a "zero ohm" resistor.

The purpose is probably to allow the option of disconnecting the shield from the ground or replacing it with a resistor (or possibly a capacitor so it can be AC grounded and allow a DC voltage difference to exist).

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  • \$\begingroup\$ Interesting, so they just make the package large to accommodate any possible configuration a user might make to their board? As in they don't base the package size based off of power, but rather more flexibility? \$\endgroup\$ – Funkyguy Nov 10 '14 at 19:00
  • \$\begingroup\$ Something like that. 0805 is big enough just about anyone can solder it, big enough for a high voltage capacitor or a 50 ohm resistor that can withstand a couple volts across it. \$\endgroup\$ – Spehro Pefhany Nov 10 '14 at 19:03
  • \$\begingroup\$ @Funkyguy: The first time I used a 0-ohm resistor was for a customer who banned wire jumpers. Some standards and certifications also disallow wire jumpers. \$\endgroup\$ – slebetman Nov 11 '14 at 2:46
  • \$\begingroup\$ @slebetman interesting, I've only used them to jump over traces that I couldn't route around on a 2-layer board :D \$\endgroup\$ – Funkyguy Nov 11 '14 at 3:09

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