How to test the F3 Polyfuse - Wiki Troubleshooting page

[bgirardot]

I added info on how to test the F3 Polyfuse to the troubleshooting page:

http://elinux.org/R-Pi_Troubleshooting# ... r_problems

If any of you could just give it a quick once over I would really appreciate it. Feel free to make any needed changes there or tell me here and I will update it.

2 things I was not 100% sure about:

What voltage drop across F3 should be considered probamatic, I put in 0.03v - 0.10v as about normal

How long a polyfuse takes to "reset" if it has been tripped, I said about 24 hours.

[thsBavR10]

F3 is something like this: TE CONNECTIVITY / RAYCHEM - MINISMDC075F/24 - POLYSWITCH, SMD, 1812, 0.75A
The product information from farnell says: Initial Resistance Max: 0.29 ohm / Initial Resistance Min: 0.09 ohm
The datasheet for the polypfuse F3: http://de.farnell.com/te-connectivity-r ... /1345924RL.
PDF --> http://www.te.com/commerce/DocumentDeli ... RF0323-000
There (page 7) I found for 'miniSMDC075F': Rmin: 0.11 Ohm; Rmax: 0.45 Ohm

Asuming the Pi itself draws a average current of 0.5 A: results in a voltage drop between 0.06 V and 0.23 V for a 'good' fuse.
Due to limited accuracy and resolution of many cheap multimeters i would say 'from 0.1V to 0.3V drop over F3'.

I didn't find anything about the typical reset time.

[bgirardot]

F3 is something like this: <snip>

Perfect, thank you very much, wiki updated.

[thsBavR10]

Regarding the reset time maybe this link helps you further:

Fundamentals of PolySwitch Overcurrent and Overtemperature Devices

http://www.te.com/content/dam/te/global ... entals.pdf
Here on page 5:

Device Reset Time
Returning to Figure 4, we note that after a trip event, the resistance recovery to a quasi-stable value is very rapid, with most of the recovery occurring within the first one-to-two minutes.

An article in wikipedia (http://en.wikipedia.org/wiki/Resettable_fuse) states

When the power and fault are removed, the PPTC device will cool. As the device cools, it regains its original crystalline structure and returns to a low resistance state where it can hold the current as specified for the device. This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, slowly approaching the initial resistance value.

In my first post a modified the conclusion a little:
"Due to limited accuracy and resolution of many cheap multimeters i would say 'from 0.1V to 0.3V drop over F3'."

[bgirardot]

I'll read through the PDF, thank you again, and I updated the value for the voltage drop.

I know I read on these forums somewhere that it can take a long time for the polyfuse to reset, I think maybe on the boards with that had F1 and F2 in place it was mentioned about a day or something for those to recover, but like I said, I'll read the pdf and get it clarified.

[thsBavR10]

I also often read about a very long recovery time in this forum, nevertheless i tried to find something more trustworthy about this.
Happy reading!
Thomas

[AshrafNabil]

hi guys ,,, i have the same problem in the power due to F3 polyfuse ,,, but i have left it for 6 hours but still it the voltage across the F3 is 0.5 V this then i used the usb ,,,, but when i used the pins of GPIO to operate the Raspberry it works but it gets very warm,,, any ideas ???

[Dilligaf]

The first line of the testing procedure is wrong IMO You want to test the fuse under load so you want the sd card and peripherals plugged into the PI and the PI running. Checking with the PI off and card out is close to checking open circuit voltage, you could test voltage through a wet noodle and see very little loss