dvdr
Posts: 15
Joined: Mon Aug 26, 2013 12:58 pm

RemotePi board & polyfuse - sorry, I don't understand that, please explain

Sun Aug 12, 2018 12:22 pm

Hello
sorry for that newbie question: I do understand, what a polyfuse is. But I don't know, whether I can use the RemotePi-Board I bought for my RPi Model 2 B V1.1. I am using the official Raspberry Pi power supply (it's rated 2.5A, as far as I know).

I want to know, whether I can safely use that board on my RPi Model 2 B V1.1 and what they mean by "limits the short circuit current" - what could go wrong, is this a fire hazard issue or could I fry my USB-Harddrive, that is directly attached to the USB ports with max_usb_current=1

On their compatibility website, they say about the RemotePi board (I only could by the Board for the PI3, the PI2-Version was not available)
The RemotePi Board for Pi2 and B+ is almost identical to the RemotePi Board for Pi 3, the only difference being that the built-in polyfuse has a different value to mirror the value of the polyfuse on the Raspberry Pi. The RemotePi Board for Pi2 and B+ has a 2.0 A polyfuse while the RemotePi Board for Pi3 has a 2.6 A polyfuse.
Despite the different polyfuse value, it is possible to use the RemotePi Board for Pi 3 and Pi 3 B+ on a Raspberry Pi 2, Pi 1 B+ or A+ as well, but this is only recommended, if the power supply limits the short circuit current. In case of a short circuit the 2.6 A polyfuse on the RemotePi Board would allow for a higher current draw than the Raspberry Pi 2, Pi 1 B+ or A+ were designed for.
If the power supply already limits the current (most power supplies have a built-in overcurrent protection), then it would be no issue to use the RemotePi Board for Pi 3 and Pi 3 B+ on a Raspberry Pi 2, Pi 1 B+ or A+.
So: can you kindly tell me in layman terms, if this is in any way a fire hazard or frying my HDD issue - or what are the dangers here?
Thanks!

drgeoff
Posts: 8343
Joined: Wed Jan 25, 2012 6:39 pm

Re: RemotePi board & polyfuse - sorry, I don't understand that, please explain

Sun Aug 12, 2018 1:23 pm

Don't worry, The difference is not really important. There is no added danger to your HDD. The increased fire risk is insignificant.

Like UK plug fuses ar readily available in 3A and 13A ratings. If you have an appliance that is 1kWatt in theory you would want a 5A or 10A fuse. In practice you will find a 13A one in the plug.

And remember that RPi Zeros have no polyfuse at all.

alphanumeric
Posts: 1515
Joined: Tue Jan 19, 2016 2:17 pm
Location: Sydney, Nova Scotia, Canada

Re: RemotePi board & polyfuse - sorry, I don't understand that, please explain

Mon Aug 13, 2018 11:22 am

They use a poly fuse because in most cases, if you give it enough time, it will recover from a current overload. If you do manage to exceed its current rating it overheats and basically becomes a high impedance. That restricts your current flow to almost nothing. Let it cool off over night and it returns to its low impedance state, basically a short circuit.
A normal fuse once tripped is an open circuit and stays an open circuit and has to be replaced. The poly fuse is soldered to the board and not easily replaceable, thus they made it self healing. They are also referred to as resettable fuses, I've used some of these in my projects.
https://shop.pimoroni.com/products/rese ... 373571455

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