2

Accidentally I short circuited 5v & gnd of R-pi, since then my R-pi is not working. How can I make it start again by means of replacing some IC or power regulator or anything ? Please help.... only act led is on...

  • 1
    Hello and welcome. Could you please describe how the power is connected to the Pi (and specify which model of Pi it is)? If all worked well you might just have blown the so called Poly fuse which will reset itself in time. – Ghanima Dec 30 '15 at 13:31
  • 4
    You are unlikely to be able to repair this, and now have a dead Pi. You can try leaving it disconnected for a day in the hope that it only blew the polyfuse and it will reset itself, but I suspect you allowed the magic blue smoke to escape. – Steve Robillard Dec 30 '15 at 13:33
  • 3
    Pretty sure the polyfuse can't protect you from that, but you can always cross your fingers. Someone with a better awareness of the schematics might be able to give you a detailed explanation of why you now have an ornamental SBC. – goldilocks Dec 30 '15 at 13:45
  • Moderator overflow! On second thought, if a LED is still on, it's less likely that it is just the fuse. It is the first thing between the powering USB port and the voltage regulator. If it blew it should be of high impedance and thus no LEDs active anymore. (So to be sure, please provide how you connected power, and follow Steve's advice.) – Ghanima Dec 30 '15 at 13:48
  • Rpi b+.I have powered it through 5v micro usb. – user77339 Jan 6 '16 at 11:49
4

I am trying to research "main polyfuse" tripping behaviour and so far I would be concerned about your RPi but not without hope.

If you have specifically shorted the 5 Volt connections (physical pins 2 and 4) on the 26 (or 40 on later Pis) to the Ground on Pin 6 this is a case that the polyfuse SHOULD protect against. At the voltages and currents involved, 5 Volts and say 2 to 3 Amps (or less) from a USB Wall-wart the device should recover (from the high-resistance value it goes to when it is heated by the excessive current flow under fault conditions) - most of the recovery happens within a minute or so according to Wikipedia but there remains a residue of "extra" resistance that can take much longer (hence the day that others will quote) to complete the recovery. Unfortunately that "extra" resistance is still enough to stop things working properly on the RPi - you will find other Q&A here about people experiencing issues that can be traced to lower than needed voltage levels from the Power Supply under various situations.

If you have a (Digital) Multi-meter you might - very carefully - find the TP1 and TP2 test points on the RPi's circuit board and measure the voltage across them when the Pi is powered up (even if it is not "working") - I can't recall the absolute figure but I think anything under 4.5 Volts on those two points (which is the +5V power rail AFTER the polyfuse [TP1] and Gnd [TP2]) is not good enough for the Pi to operate successfully.

If you can find the polyfuse (is labelled F3 on my RPi) and have that DMM you may wish to try and measure the resistance on the meter's lowest resistance range with the power supply disconnected. The resistance should be almost un-noticeably different compared to the resistance value you get from shorting the two meter probes together.

I have a totally unproven theory that packing the RPi back in it's antistatic bag that it came with (to protect it from static damage) and then wrapping it up in a cool place in another bag to seal it from as much condensation laden air as possible, and popping it in a freezer might improve the recovery from the remaining previous thermal effects that were caused by the overload. However there is no experimental evidence, that I have found that confirms or disproves this.

One other point is that a short on the power supply may have corrupted some of the data on the SD card if it was being written to at the time or the RPi had file-systems open and had data that had not been flushed out to the SD card - which is pretty much all the time it is running...

tl;dr: Leave it alone, un-powered for a day (or wrap it safely and pop it in the freezer for a lesser time if you want to experiment) and try again after you have checked that the SD card is not suffering from data corruption.

  • 1
    polyfuse SHOULD protect against - if the Pi was powered via it's USB "powering" port and not somehow backfeed via the GPIO connector. A fact that the OP should confirm. – Ghanima Dec 30 '15 at 16:16
  • Agreed - I wonder how many do NOT power via the USB connector - at least initially? In further consideration the fact that the OP is not seeing the Red Power LED is suggestive of a low 5V supply which would be consistent with a not(yet) reset Polyfuse that is allowing some power though (to operate the Green LED) but not enough to operate normally. Hopefully they will get back to us with an update... – SlySven Dec 30 '15 at 20:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.