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RG1 1.8v regulator

Former Member

Ok, so in a different thread I threatened to remove RG1 and do some current measurements on it's output after seeing those thermal images that show it's not generating any heat...

 

Well, I did it tonight. Some photos here: https://picasaweb.google.com/selsinork/RPi18v

 

The jumper pins in the output let me either just put a jumper on and verify the Pi boots ok, or wire a multimeter in series to get some current readings.

 

The results were interesting to say the least. I had to go back and check I was reading the multimeter correctly, that it wasn't broken etc.

 

On initial power up I see a negative current for a second or so which then reverses to about 0.5mA (yes half a milliamp, that's not a typo) for a few seconds while we get the first sd-card accesses. Once we're booted and sitting at the login prompt the current reading fluctuates from around 0.001mA to maybe 0.04mA. 

 

I'm using the 40mA range on a decent Fluke multimeter, so I've no reason to doubt the results. There's obviously going to be some inaccuracy down at that level due to length of meter leads etc, but the result is fairly clear.  You'll understand why I was checking the meter was working and I was reading it correctly though image

 

 

So from there onto the next test, lets try completely disconnecting RG1 and see if the Pi boots while using the LAN9512 1.8v 'output'.  Yes it does! 

 

I think that's reasonably good indication that jamodio got it spot on, the lan9512 shouldn't be connected to the 1.8v plane and it's heat problems are going to be largely due to supplying current on it's 1.8v filter pin that it was never designed to do.

 

So anyone willing to pull RG1 off a Pi and verify my results ?

Parents

  • I couldn't resist a little hardware hacking on a Saturday morning.

     

    My LAN9512 isn't running hot, only luke warm I would say, but I can guarantee a system hang by copying a file from an nfs mount to a usb flash drive in 10-20 seconds. USB keyboard is a little glitchy too. Tried 10W iPad charger and Dell monitor with integrated USB hub for power also without keyboard over ssh. That's motivation enough for me to tinker.

     

    Carefully, I cut the circuit traces from pins 15 and 38 on LAN9512 and the 1.8V side of C29 to the vias. I then very carefully soldered some wire between these three points. This more closely resembles how the circuit should be (minus the 100nF caps). (The 9512 reference design says 4u7 should be by pin 38 btw) See photo:

     

    image

     

    Measurements taken before show 1804mV on both RG1 and C29. After modification I have 1797mV on RG1 and 1809mV on C29, so I am confident the regulators are now isolated. The effectiveness of my filter circuit is a little more dubious.

     

    Didn't fix the system hang copying over LAN to USB, but I can copy from LAN to the SD card (didn't think to test that previously). At least my Pi still works.

     

    Now that VDD18CORE is isolated from RG1 I can measure the current drain on RG1. More results to follow.

     

    [edit] After lifting RG1 and placing my exceedingly cheap multimeter in series with the output I measure about 20mA briefly, then 80mA briefly then 60-70mA for a while while booting then finally settling down to 50.5mA at the login prompt (presumably, no video attached, only power). Would have been nice to take a measurement before, I know, not very scientific of me.[/edit]

Reply

  • I couldn't resist a little hardware hacking on a Saturday morning.

     

    My LAN9512 isn't running hot, only luke warm I would say, but I can guarantee a system hang by copying a file from an nfs mount to a usb flash drive in 10-20 seconds. USB keyboard is a little glitchy too. Tried 10W iPad charger and Dell monitor with integrated USB hub for power also without keyboard over ssh. That's motivation enough for me to tinker.

     

    Carefully, I cut the circuit traces from pins 15 and 38 on LAN9512 and the 1.8V side of C29 to the vias. I then very carefully soldered some wire between these three points. This more closely resembles how the circuit should be (minus the 100nF caps). (The 9512 reference design says 4u7 should be by pin 38 btw) See photo:

     

    image

     

    Measurements taken before show 1804mV on both RG1 and C29. After modification I have 1797mV on RG1 and 1809mV on C29, so I am confident the regulators are now isolated. The effectiveness of my filter circuit is a little more dubious.

     

    Didn't fix the system hang copying over LAN to USB, but I can copy from LAN to the SD card (didn't think to test that previously). At least my Pi still works.

     

    Now that VDD18CORE is isolated from RG1 I can measure the current drain on RG1. More results to follow.

     

    [edit] After lifting RG1 and placing my exceedingly cheap multimeter in series with the output I measure about 20mA briefly, then 80mA briefly then 60-70mA for a while while booting then finally settling down to 50.5mA at the login prompt (presumably, no video attached, only power). Would have been nice to take a measurement before, I know, not very scientific of me.[/edit]

Children
  • in reply to Former Member

    are you able to measure the temperature of the lan9512 with this mod in place?   I'm only getting arount 48C on any of thr Pi I have, which I don't consider too bad, certainly not the 'blisteringly hot' others have reported..  So either I have asbestos fingertips from too many years of burning them with soldering irons, or there's some other factor involved.

     

    Ideally we need to find one that runs hot, or how to duplicate that condition, and try this mod on it to see what difference it can make.

  • in reply to Former Member

    selsinork wrote:

     

    are you able to measure the temperature of the lan9512 with this mod in place?

    I have an IR thermometer on its way from China which might take a few weeks. I might also be able to dig a thermocouple out of my junk box somewhere. The plan is to take temperature readings in various modes of operation when running the 1.8V plane off each regulator (RG1 and LAN9512) individually. Otherwise there is the finger test. Results to follow.

     

    Quick update before I go to work:

    I found a thermistor from the Silicon Chip Multi-Purpose Fast Battery Charger kit from the Feb 1998 issue. Not sure what its specs are if anyone has this issue handy, otherwise I'll have to go digging for the plans or do a calibration. Here is a pic of the test setup:

     

    image

     

    The pinout for the 4-pin header is: GND, RG1OUT, Pi 1.8V plane, VDD18CORE

    This will also allow for current measurements, particularly of interest is the direction of current from VDD18CORE to RG1OUT. Other current measurements in series with the regulator outputs may not be accurate when both regulators are connected due to additional resistance of the multimeter. This may tell us if the LAN9512 is sourcing or sinking current during normal operation. I suspect sourcing due to thermal images showing a cold RG1. The GND pin can be used to connect an external adjustable 1.8V source for further testing behaviour of LAN9512 in this unsupported mode of operation.

     

    Update:

    Initial results were not definitive so I took a few more current measurements to maximize the drain on the 1.8V supply. I could get consistently 100-110mA with peaks of 130mA by playing back a 1080p video with omxplayer. I pushed some network traffic through for good measure which did have an observable effect on the equilibrium temperature.

     

    Thermistor resistance at room temperature (about 20 degrees C) was approx 110KOhm, so using a temperature chart for a 100K thermistor we have (roughly, at equilibrium):

    Running 1.8V plane from RG1 57.5KOhm (approx 37 degrees C)

    Running 1.8V plane from LAN9512 53.7KOhm (approx 39 degrees C)

    Holding thermocouple in hands 63.5KOhm (approx 35 degrees C)

     

    These temperatures are fairly rough of course, but give a relative indication of temperature. Not the dramatic effect I was expecting, but it does run hotter as expected with that extra drain through the internal regulator, not blisteringly hot though.

     

    Perhaps in the worst case if VDD18CORE is slightly higher than that of the output of RG1 then not only does it power the 1.8V plane, but leaks back through RG1 to ground. In which case you may be better off removing RG1 so at least the drain on the LAN chip is capped at around 100mA.