what's a Polyfuse on the Pi and what's ZigBee?

[lilzz]

It says there are 2 140mA Polyfuses F1 & F2 limit the current drawn by USB devices.
What's a Polyfuse? It's not a resistor, is it? what value if it is?


Secondly some system(not Pi) use ZigBee, but why? Can't a regular wireless adapter works?

[ShiftPlusOne]

http://elinux.org/Polyfuses_explained
https://en.wikipedia.org/wiki/Resettable_fuse

It's 1.1A, according to the schematic.
http://www.raspberrypi.org/wp-content/u ... .2_027.pdf
I'm not sure if it's trip or hold current though. It's important to note that they don't act instantly, so don't rely on it to protect you from short circuits.

ZigBee modules are for wireless connectivity between multiple devices. They use less power, are less complicated and less expensive than wifi devices. I would say they are probably more reliable too.

[rpdom]

Only the early Rev1.0 Raspis had the F1 & F2 140mA fuses. They were shorted out on later Rev1 boards and removed completely in the Rev2.0 design, leaving just F3, the main supply poly fuse. In the recent Rev2.1(Is it officially called that?) the parts were relabelled and the lone F3 is now called F1.

[ShiftPlusOne]

Sorry, I didn't notice that in the original question. Yeah, what he said.

[klricks]

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It's 1.1A, according to the schematic.
http://www.raspberrypi.org/wp-content/u ... .2_027.pdf
I'm not sure if it's trip or hold current though. It's important to note that they don't act instantly, so don't rely on it to protect you from short circuits.
..
...

The 1.1A is the trip current. The hold current is 750mA as printed on the part. If you attempt to draw more than that the resistance of the polyfuse begins to rise causing unstable operation... Reboots, USB devices dropping out etc. Of course going above the trip current will shut down the power in a few seconds.

[ShiftPlusOne]

Thanks for clearing that one up. That's what I've always suspected, but never was 100% sure.

[jojopi]

The 1.1A is the trip current. The hold current is 750mA as printed on the part.

If the hold current is 750mA (which I agree), then the trip current is 1.5A. See for instance http://www.te.com/catalog/pn/en/RF1404-000, or any of the other manufacturers.

It has never been clear why the schematic said 1A1, when 750mA is what was actually fitted.

[klricks]

The 1.1A is the trip current. The hold current is 750mA as printed on the part.

If the hold current is 750mA (which I agree), then the trip current is 1.5A. See for instance http://www.te.com/catalog/pn/en/RF1404-000, or any of the other manufacturers.

It has never been clear why the schematic said 1A1, when 750mA is what was actually fitted.

Yes I had also noticed that there are no parts available with a 750mA hold AND the 1A1 trip rating. I think I checked several manufactures but it's been awhile ago. Most if not all print the hold current on the part.