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I have one raspberry PI and two PI-2, side by side, inside a little cupboard.

I'm constantly monitoring their temperature. I'm worried because one of the Pi-2 is quite hot.

The PI and the hot PI-2 are running raspbian jessie and mostly idling.

The cool PI-2 is inside an official case (without active or passive cooling) and running OpenElec (latest stable).

I made some measurements on the Raspi1 and the hot Raspi2 a couple of months ago and everything seemed normal.

  • Raspi1 prev temp: ~50C (it was cased then)
  • HOT Raspi2 prev temp: ~ 40C

Now ambient temperature is quite hotter, but I wouldn't expect so much a difference:

  • Raspi1 current temp: ~45-50C (it isn't inside a case anymore)
  • Cool Raspi2 (cased) current temp: ~57C, seems pretty stable
  • HOT Raspi2 (not cased) current temp: ~63-75C, the CPU-GPU bundle is scalding to the touch (rest of the board doesn't feel too hot)

I know the theoretical limit is at 85C but I'm worried that it has jumped from a nice temp to something near the limit, while the cased one is notably colder.

All of them are connected via HDMI to a mostly unpowered TV, and through wired ethernet to the same router. I have interchanged power sources just in case, but didn't have any effect. I also tried to disconnect HDMI and network from the hot one, but no noticeable effect.

They are not overclocked, and anyway (as already said) mostly idling.

Software: In order to compare apples to apples, I swapped the SD cards and removed the case from the cool raspi2. The cool one has gone about 15C down, now sits about 42-43 (as the other one did a couple months ago). The hot one was down by 5C, but after a few minutes got hotter than it was (to over 65). So now I know it's not a software issue or something weird caused by the case. Ambient temp is about 30C, perhaps a little more.

To test both in as equal conditions as possible, I used the same power source and even the same ethernet cable.

To test, first I let the device idling for a while and look at the measure. Then I tried to max out the CPU, opening three ssh sessions and executing one instance of "while (true) do find /; done" and two of "while (true) do ls / -R; done", besides another session with top and another with temp measurements. top indicated a sustained CPU (i.e. top us% + sys%) usage of around 90%

The hot device was consistently at about 2.0 W when idling. When maxed out it was set to 3.0 W.

The cool one was at 1.5 W when idling. When maxed out it jumped between 2 and 2.5 W.

Power consumption: I finally got a usb voltage/current tester. According to it, the cool raspi2 is consuming ~0.4 A while running an idle openelec, while the hot raspi2 (same USB power source, same cable, same SD card) is consuming around 0.75 A. Voltage is around 4.90 V in both cases. ** This seems to be the problem. ** When trying to locate the issue, sometimes after fiddling for a while, temperatures went down to normal or near normal, to be up again after a couple of days.

What should I do now that I've isolated the issue?

raven
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    You should not be needing heatsinks without overclocking, especially on a mostly idle pi. Have you checked with (e.g.) top that it really is mostly idling, and something weird isn't going on? Switching the SD cards is not really confirmation that they are doing the same thing, it just implies they probably should be. – goldilocks Jun 27 '15 at 12:45
  • Yes, I already checked. CPU is around 1% (that's 1% of one core IIRC). – raven Jun 27 '15 at 12:54
  • Have you checked the power consumption (amperes) of both? –  Jun 27 '15 at 13:21
  • I have now, see edited question. I don't have an easy way to directly check output current from the USB charger, only power (watts) from mains. I hope it's enough. – raven Jun 27 '15 at 15:46
  • You can get a cheap USB voltmeter/ammeter on ebay, it would be most useful in measuring the USB current draw. – Greenonline Jun 27 '15 at 18:19
  • @Nasha it will arrive around the end of july. Meanwhile, what can be done if the cause is an excessive current? – raven Jun 30 '15 at 07:20
  • An excessive current might imply a short circuit between two pins, not necessarily on the Pi itself. Also a parasitic capacitor between two lines that makes high frequency signals draw excessive power. But I'd say unlikely. Never too careful though. Also check the waveform and measure the voltage of your power supply; it must be exactly 5 volts, steady, no noise or very low ripple. Some USB chargers really are crappy, first cause of troubles in most cases. –  Jun 30 '15 at 07:32
  • @Nasha the voltage was low. Can this be the problem? I would expect it to be unstable, if anything; not to get red hot. – raven Jun 30 '15 at 21:02
  • A voltage too low might also induce a current increase due to switching circuits. Logical levels are expected to be of a certain voltage. Below that, a switching circuit might operate in an area that is too close to the electrical switching threshold. To rule it out, just use a power supply that [you checked] delivers a steady 5V. –  Jul 01 '15 at 07:40
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    @Nasha thank you very much. I don't have a good way to check voltage stability, but I switched to a supply that at least delivers in the specified range (4.75-5.25), and temps have gone down. Right now all three devices are at 50 (+/-2) C, which seems right to me. I'll get some better quality power supplies. – raven Jul 01 '15 at 13:27
  • @Nasha I finally got a USB amperimeter. Consumption is too high in the faulty one. What to do now? – raven Jul 25 '15 at 10:05
  • What do you mean "too high"? Now if you're sure one has been damaged (by excessive temperature or inappropriate voltage), there's not much you can do, IMHO but just replace it. But do not either assume the voltages are stable, monitor them continuously too. If they changed, the same cause will lead to the same effects. Just be sure. –  Jul 25 '15 at 16:42
  • "Too high" here means "double than another board doing exactly the same". It shouldn't have been damaged by temperature, AFAIK it hasn't been over ~86 C (theoretical limit is 85) and not for too long (an hour at most between 84-86). But that was after founding it wasn't right. My guess is a bad connection in some place, that would explain the transient nature of the problem. (It has started working fine for one day or two a couple times) – raven Jul 25 '15 at 21:35
  • If you make an answer with all the info you provided, I will gladly accept it. – raven Jul 25 '15 at 21:39
  • @goldilocks Yes you need heatsink even without overclocking if you want to run the thing at full load (all 4cores at 100%). Apparently there is no need to worry because if the SoC gets too hot, it will throttle down significantly (your computing performance will decrease as well) to keep cool. But if you notice sudden decrease in performance, you should always check 0) the frequency under load and then 1) the temperature and 2) the voltage, both of them can be checked from the command line, because – xmp125a Jan 10 '17 at 09:27

1 Answers1

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Based on the comment thread, I will put forward the following advice that has done well for me on both original RPi and RPi2: The quality of your power supply matters.

If the power supply you're using isn't adequately regulated or is trimmed too high, you may find your Pi is operating outside of its optimal electrical specifications. Running a power supply above 5.25 VDC can lead to current leakage inside the chip (even at idle) that can be measured as increased temperatures.

Not that I think anyone wants to read this, but here's a white paper on transistor current leakage and how ARM is designed to address that problem. It doesn't prove that this is the problem in this circumstance, but based on the comments, it certainly seems to point in that direction.

So, in short, quit buying cheap power supplies for your Pi. Not that good quality ones will be that expensive, but if you buy from eBay or (sometimes) AliExpress, you absolutely get what you pay for. Buying from a supplier that everyone would agree is reputable and concerned about customer satisfaction (Sparkfun and Adafruit being two easy examples, there are many others), you shouldn't have a problem.

WineSoaked
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  • Well, if you really followed the conversation until the end, this answer is not right for my particular case (current was within specs). Anyway, maybe I should accept this answer, as it's probably quite useful for most people with a similar problem to mine. I'll let it a few days and if nobody comes with a better one, I will accept it. Thank you very much for your effort. – raven Aug 09 '15 at 19:34
  • So, reading the conversation, it looks like what you might be experiencing is indeed some manner of thermal damage that has been sustained by the ARM core. It's difficult to know for sure without a full forensic workup on what temperatures that the chip experienced and for how long.

    Thermal damage to silicon can cause transistors to be stuck on or off, along with package issues (weakening of the solder balls that attach the chip to the circuit board, and so on). A lot of it goes into details for which I'm neither qualified nor experienced. But my "answer" was to summarize, that's all.

     – WineSoaked Aug 09 '15 at 20:54