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More power/heat related problems ...

jamodio

I think I found another problem I didn't notice before.

 

I have the R-pi hooked up in the prototyping gig I described on another thread (http://www.element14.com/community/thread/18981?tstart=0),

currently no keyboard, HDMI monitor, just network and access to console via the serial port.

 

I've been running a trend plot monitoring the +5V feed to the R-pi and the output of the 3.3V on board regulator. I noticed that gradually

the voltage out of the regulator has increased from 3.30V to almost 3.39V, it is actually not doing much, just responding to ping packtes

(flood mode) I'm sending from another machine and serving the serial console I use once in a while to run procinfo and network stats.

 

I guess the voltage increase is probably not noticeable when there is more load and current draw, the only thing I can think of is self

heating of the LDO. I just placed one of the little heatsinks I've got for the LAN9512 on the LDO and seems that output voltage is starting

to slowly drop. I'll keep this configuration to see if with the heatsink the voltage keeps droping.

 

image

 

-J

Parents

  • Still can't explain it but something is going on, over a period of 40 hours the output of RG2 has been increasing from 3.34V to 3.39V,

    temperature shows now 44.2C and trending up. This is without the heatsink.

     

    image

     

    I'm tempted to replace the part and see what happens.

     

    -J

  • in reply to jamodio

    You have looked for instability (HF oscillations) with your scope? These issues can be down to (input /) output capacitors. Maybe reflow the solder on these components as well as the regulator. Looking at the trace your 5V line seems to be trending upwards as well?

     

    I assume that you have double checked your readings against another measuring device? Or against another piece of kit with a similar regulator. Its just that the 3.34V start value is out of spec to start with.

     

    For the cost of these things if you are comfortable doing the job then what's to lose. Looks as though its easy to get at as well.

  • in reply to Former Member

    Still not a definitive explanation but I believe the issue is ambient temperature, I'm cranking up the air conditioner to reduce my office temperature from about 27C to 25C and now the output of RG2 shows 3.36V.

     

    I think that since there are several components generating heat and it gets conducted through the power planes, it may generate sort of a cascading event where the internal temperature of some parts start to trigger side effects.

     

    I'll hookup the rework station at some moment to replace the part and test with other regulators I've believe have in stock with the same package.

     

    While the part number on the schematics is shown as NCP1117, the part installed on at least the two boards I've is numbered SE8117T33, which is not from any well known major semiconductor company but a chinese cheap knock off, it could also be a counterfit part, not uncommon when sourcing some parts from China.

     

    This thermal image taken by "Remy" from Spain shows really where the heat is, if you follow the orange haze you can see how heat is being conducted by the board and not just staying around the hot components.

     

    image

     

    I wonder how many Raspberry Pi's will get "cooked" when people start to put them in cases.

     

    -J

  • in reply to jamodio

    Interesting image, where was that posted?  Any more info, like the temperature of the LAN9512 at the time?

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