Resetable ptc fuse

Using Arduino Project Guidance
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I am working on a custom board that is close to a Nano, but it will do some specific tasks. Not a development board.

Question is:
I would like to add a fuse on 5V line to a USB connector. But which one?
Those markings are confusing. I will use a MBR0520LT diode but what fuse to put between diode and USB? Whole components are almost all from Nano schematics. That include voltage regulator, capacitors, resistors, everything, but headers. I will use only a few pins.

Board i am working on will be used for a small project which will hardly go over 400mA limit.

Thanks in advance.

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Why use a fuse at all? The USB has a current limit built in.

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Take a look at some of these

https://www.digikey.com/products/en/circuit-protection/ptc-resettable-fuses/150?FV=-8%7C150%2Cmu500mA%7C682%2C1989%7C0&quantity=0&ColumnSort=2073&page=1&k=ptc&pageSize=25&pkeyword=ptc

"what fuse to put between diode and USB"
Diode goes between your other sources, such as between barrel jack and regulator - and why aren't you just designing for a 5V wallwart anyway and skipping the regulator?

Altho I do put low Vf schottky diodes between Vcc and Vusb to keep external power from back feeding into Vusb when I want external power but an FTDI module connected for debugging, keeps the PC from complaining about current surges and shutting the USB port down.

@Railroader: "Why use a fuse at all? The USB has a current limit built in." Only in terms of shutting your port down, if you are lucky. Never tried a direct short from Vusb to Gnd to see if my PC could survive that. Have you?

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@Railroader
Thank you.

That's why i ask here. New revision of Nano has it. I did some research and get that it is PSMF050X or similar with 0.5A 5V.

It is ok to add it, but i would like to here is it worth it. Do i really need it?

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Why prepare for such shorts, less than 10 ohm connection between 5 volt and GND?
?

6

Building a lot of small computers during 40 years I've never had any issue that would call for such a fuse. Maybe the Arduino creators apply it because they know that sometimes very ignorant people will use the devices.

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I would add it. Lots of folks have accidentally shorted 5V to Gnd, this provides a little protection for your USB port.

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The rule number one is: Power down the circuitry before making any connections. One difficult task could be electrical measurements in an active, powered, board. Choose probes that can't short circuit. If that is not possible, switch power off and solder cable stubs to the measuring points...
Ounce the task was faultfinding a circuit board intended for flying in space. You bet that board was more than expencive to damage. Why be less caring during home built electronics? Making unplanned shorts often give unexpected damages and a lot more to faultfind and repair. Work in a safe way if You want the project to ever get functional and ready.

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So, if i decide to put it anyway (it's cheap), i guess the one i mention would be ok. It's 0.5mA 6V.

10

Looking at the schematics on https://content.arduino.cc/assets/NanoV3.3_sch.pdf i noticed 1K RP2C resistor which is tied to gnd by both side. Can someone explain me what is the purpose of it.

11

"RP" is a resistor pack of four resistors.

There are seven active 1k resistors in that circuit comprising two RPs; so one resistor is superfluous.

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I see. Thank you.

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