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I've tried 5 different chargers, and none of them gets TP1>TP2 over 4.6 volts. I just plugged in a new charger (found here) which gives it 4.4 volts with only SD card attached, but 4.8 with SD card removed.

I tested the power before the polyfuse without SD card inserted: 4.94 volts
Voltage after polyfuse: 4.86 volts

So far, so good. Power off Pi, insert SD card, power up Pi and test again: 4.78 volts before polyfuse, 4.39 volts after polyfuse.

I tested the resistance of the F3 polyfuse, it seems to be a constant 1.2 Ohms. Is the fuse bad, or is the SD card somehow using that much power?

  • How many amps are the chargers rated for? Voltage may drop if current drawn is too high for the charger. – Gerben Nov 30 '13 at 15:00
  • Most of the chargers I've tested are rated @ 5v, 1000mA. Also, quote: "There is no upper limit for the current rating of a power supply. The Pi will draw what it needs. However, there is a point of diminishing returns in the 1A to 2A range, since the Pi has an internal fuse that will trip if it draws more than about 1A." - raspberrypi.org/phpBB3/viewtopic.php?f=24&t=22228 – coderMe Dec 1 '13 at 19:28
  • What if you measure the voltage before the fuse? elinux.org/R-Pi_Troubleshooting#cite_ref-17 – Gerben Dec 1 '13 at 20:51
  • @gerben edited. – coderMe Dec 2 '13 at 0:30
  • It almost looks like you have unregulated power supplies. Looks like the power adapters are not very good (for this purpose) as the voltage drops when current is drawn. Are the charges heavy (contain a transformer coil)? – Gerben Dec 2 '13 at 13:59
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Based on your post, I suspect your problem is the charger. The link you provided indicates you are using a Nokia charger. The specification for chargers allows up to 1.5A, but the voltage specification is much looser than a USB power specification, and is allowed to drop as low as 3.6v. This is perfectly acceptable for a charger, but not for powering the Pi.

Rather than fiddling with the Pi, and risking damage, you should check the charger. If you connect the charger to a load e.g. 5.6Ω (which should be rated at 5W). This should be 5V ± 0.25V

There are lots of good chargers around. The Apple chargers work, and most hub supplies should work.

  • Thanks, but this is the 5th charger I've tried. Other chargers have similar results, but none of them get more than 4.6 volts into the Pi. – coderMe Dec 1 '13 at 19:22
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I'm not sure what type of realistic faults the polyfuse can save you from. Most short circuit conditions will cause damage way before the polyfuse reacts.

After a fault, the polyfuse can take hours to full reset, so may have to be patient. I'm not sure what the usual voltage drop is, but yours seems higher than I'd expect is reasonable. (It's no so much about the drop as the ripple/spikes you would see on an oscilloscope)

I've had no problems just shorting the polyfuse out (or feeding the 5V into the GPIO header), and I probably poke around with GPIOs and so on more than most people do.

On the other hand getting the power in without the polyfuse in the way seems to reduce the number of unexplained crashes.

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