Dorf Lord
Posts: 2
Joined: Wed May 29, 2013 6:22 pm

F3 Poly Fuse casuing my problems?

Wed May 29, 2013 6:35 pm

Computer Skill: Advanced
Electronics / Soldering skills: On par with a 5year old and / or a well fed chimp.
English Skill: Bearly remembers to use a spell checker.

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I've recently developed a problem with my pi (one of the very first batches sent out). After a short period of operation it would freeze, still displaying via HDMI but not accepting any input and sitting on a single red LED.

After some quick fault hunting with different PSU's / cables and so on I’ve come to the conclusion it may be an "F3 poly fuse" fault.

After a bit of Google-fu I found the power section of the troubleshooting wiki;

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

As the guide says I took readings using the TP2 test point and the F3, at power on it would be a stable 5v but after a few minutes of anything it would drop to 4.30v. The guide says anything more than a 0.3v drop could be an iffy poly fuse and I’ve got a drop of 0.7v.

But this raises more questions... if it is the F3, what do I do? My soldering skills are beyond poor and don't think I could re-solder a replacement. Would it just be better to buy a new Pi?

Thanks again.
DL.

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dobra-dobra
Posts: 32
Joined: Wed Dec 26, 2012 2:04 pm
Location: Poland

Re: F3 Poly Fuse casuing my problems?

Thu May 30, 2013 6:49 am

First of all you can try to contact distributor and ask for replacement.
If you can't do that (or doesn't want to) I think that you should try replacing that fuse. I did it myself and I'm far from being proficient in soldering. It's not really a big deal and if you won't try you will never improve your soldering skills. So that's not an excuse.
Good luck!

obcd
Posts: 917
Joined: Sun Jul 29, 2012 9:06 pm

Re: F3 Poly Fuse casuing my problems?

Thu May 30, 2013 8:06 am

Solder another fuse over the one already there.
It's more difficult to remove a component than to install one.
Have you checked the power consumption of your Pi?
Maybe it needs to much current causing the Polyfuse to trip?

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rpdom
Posts: 12953
Joined: Sun May 06, 2012 5:17 am
Location: Ankh-Morpork

Re: F3 Poly Fuse casuing my problems?

Thu May 30, 2013 8:46 am

obcd wrote:Solder another fuse over the one already there.
As the current (sorry!) fuse isn't broken, just not fully working, soldering another fuse in parallel with it would reduce the effectiveness of the fuse. The combined trip current would be nearly double what t should be (not full, because of the degraded state of the old fuse).
It's more difficult to remove a component than to install one.
I agree, but really you should only do that id the failed component has gone completely open-circuit.
Have you checked the power consumption of your Pi?
Maybe it needs to much current causing the Polyfuse to trip?
Very good point :)

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redhawk
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Location: ::1

Re: F3 Poly Fuse casuing my problems?

Thu May 30, 2013 9:05 am

There is no need to solder around the F3 polyfuse you can bypass this by providing 5v directly into the GPIO 5v rail. - http://elinux.org/RPi_Low-level_periphe ... .28GPIO.29
I would also monitor current usage as well if the Pi is taking more than it should and could explain why your F3 polyfuse is tripping out.

Richard S.

Dorf Lord
Posts: 2
Joined: Wed May 29, 2013 6:22 pm

Re: F3 Poly Fuse casuing my problems?

Thu May 30, 2013 10:38 am

Well, I decided to purchase a new pi (as good an excuse as any!).

And in a fit of madness I solderd a paper clip to bypass the F3 fuse. Its been working for 2 hours so far, lets see how long it lasts before I kill it.

Thanks again for all the advice here anyway!

obcd
Posts: 917
Joined: Sun Jul 29, 2012 9:06 pm

Re: F3 Poly Fuse casuing my problems?

Thu May 30, 2013 1:01 pm

@rpdom

Even under normal working conditions, the used polyfuse has a voltage drop of 0.15 - 0.2V.
This makes the TP1 - TP2 testvoltage close to the under limit considered safe for a stable operation.
We all know that silicium breaks faster than fuses, so basically all the polyfuse does is protect the system against bursting in flames when something goes wrong. If you connect some usb devices, the polyfuse has to handle that current as well. What's the point in using a 2 amp supply if you have a 1.2 amp polyfuse. So, if you solder one on top, the voltage drop will be lower and more usb devices will work. You will still keep your insurance company happy, as it's not that there is no protection anymore like in the case of using the gpio pins to power the Pi.
So, soldering an extra polyfuse is not something I would only recommend to people with a broken polyfuse, but even to people who intend to use a wifi adapter. A decent (more expensive) design would have a separate over current protection on the usb ports alone and leave them out of the Polyfuse current path. I know some design decisions where taken to keep the price low. If you only connect a keyboard and mouse or self powered hub to the usb ports, you want need a more robust polyfuse.
It's not like changing your 16Amp fuse in your house by a 60Amp one to keep your sauna heater
going. In worst case, pcb tracks can start to act like fuses.
You still can solder a wire afterwards to fix that.

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