To clarify about polyfuses

[xEnOnn]

A while ago I was introduced to a component called polyfuse and I thought it's really a clever thing albeit I'm still a little confuse about how it is actually used. I understand that it is usually used to protect the USB port.

Are the polyfuses in the USB ports on computers there to protect the port or to protect the mainboard of the computer?

I read that the Arduino Uno has a polyfuse too. Does it mean if I ever have a short circuit or a bad connection, my Arduino Uno will remain safe, ie, not hurt or blown?

I'm quite surprised with how I accidentally messed up the wires on my Uno earlier and nothing actually happened. I did the same (by accident of course) on my Nano and it was blown immediately the other day. So I started to wonder if it was the polyfuse.

[CrossRoads]

Polyfuse is to protect the USB port, to keep your board from drawing too much power.
The Uno has a polyfuse between the USB connector and the rest of the board.
I don't recall if a Nano does, haven't looked at its schematic in a while.

Not sure what this means: "I accidentally messed up the wires on my Uno earlier and nothing actually happened" so I won't comment until you clarify.

[xEnOnn]

Basically what happened earlier was that I accidentally contacted the 5v wire with a ground wire. The other time when I did the same thing on my Nano, it popped with smoke immediately. I believe the short circuited blown it. But this seems okay on my Uno for some reason and I don't quite understand.

[CrossRoads]

Ok, just looked, Nano has no polyfuse to protect USB.
If you shorted 5V to Gnd, you may have caused too much current to flow in the 5V regulator and smoked it.

[xEnOnn]

But when I did the same short circuit on my Uno, it isn't blown. Was it because of the polyfuse that saved it from being damaged?

[CrossRoads]

Was it powered from USB at the time? Then maybe.

[xEnOnn]

Yes, my Uno was powered by the USB at the time.

[CrossRoads]

Then likely the polyfuse saved you.

[SirNickity]

I've shorted my Uno a few times with no ill effects either.  Usually, the device goes offline (the computer plays the device-disconnected sound) until I remove the short, and then it comes back.  I always assumed it was my hub or port's over-current protection kicking in.  I guess it could be either.

Always use a decent powered hub.    Don't let your computer suffer for your mistakes.

[cjdelphi]

It also depends on where/what you shorted...

The 5v to GND for example, would only be lethal if you did that with a pin out/in from the micri.

Problem with poly fuses is that most are rated > 500ma which is plenty to kill your arduino before it stops passing current!

[fungus]

Problem with poly fuses is that most are rated > 500ma which is plenty to kill your arduino before it stops passing current!

No Arduino pin will allow 500mA to pass and the onboard regulator on Arduino Uno has thermal shutdown. It's hard to kill an Uno with a short-circuit.

I don't know about the regulator on the Nanos.

[mrbossnl]

Just for info's; I'm replacing the blown B2 fuse with a thermal fuse. Those things cost about 2 euro's for a package of 50(!) so you can use the other for other projects. Next time I short the nano, it will become alive after a few seconds