rownyr
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Joined: Wed Jul 11, 2012 1:25 am

Low voltage and hanging

Thu May 16, 2013 11:49 pm

Hi,

I'm currently setting up my new power supply for Pi. Previous was a 1A USB phone charger and everything worked as it should. However, with my new power solution I get delayed keystrokes, missing keystrokes and sometimes keyboard doesn't work at all.

I'm using AC/DC 19V 3.42A notebook PSU connected to the Pi through LM2596 step-down converter. Step-down converter lowers 19V down to 5V and it's rated 2A. With this setup, Pi is booting but there are some problems with keyboard. I measured TP1-TP2 voltage and it's about 4.3V with SD, keyboard, HDMI and ethernet cable plugged in. When booting it's about 4.9V at the beginning but after few secs it drops to 4.4V and then 4.29V and stays at this level. I measured the voltage also with SD card and HDMI only and it's about 4.41V.

Pi in its off state has 0.91V across TP1-TP2 with HDMI connected and 0.46V without HDMI. This is with keyboard and ethernet cable.
Without ethernet cable and keyboard numbers are respectively 1.54V(!) and 0.39V. I reconnected eth and keyboard and it still was 1.54V, but after power on/off cycle it did go back to 0.91V - I guess it's the capacitor(?).

The step-down converter I'm using is: http://www.aliexpress.com/store/product ... 38263.html . I set the voltage to 5.1V on it and measurement confirms that voltage - it's stable even at the boot time.

What could be wrong with this setup?

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mahjongg
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Re: Low voltage and hanging

Fri May 17, 2013 12:00 am

If the voltage between TP1 and TP2 is below 4.75V then specifically many USB devices may start behaving erratically!

If your PI isn't receiving the correct voltage even though you supposed it should, many things may be wrong:
  • Your PSU isn't up to the task
  • The cable between the the PSU and PI isn't up to the task
  • You managed to blow the polyfuse and it hasn't (completely) recovered (can take a long time, even days, sometimes never)
  • There is a short or power overload
Note that applying a voltage greater than six volt, even for a very small time (milliseconds) will trigger an overvoltage protection device on the PI, and the resulting short circuit current will "blow" the polyfuse.

rownyr
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Joined: Wed Jul 11, 2012 1:25 am

Re: Low voltage and hanging

Fri May 17, 2013 12:07 am

mahjongg wrote:If the voltage between TP1 and TP2 is below 4.75V then specifically many USB devices may start behaving erratically!

If your PI isn't receiving the correct voltage even though you supposed it should, many things may be wrong:
  • Your PSU isn't up to the task
  • The cable between the the PSU and PI isn't up to the task
  • You managed to blow the polyfuse and it hasn't (completely) recovered (can take a long time, even days, sometimes never)
  • There is a short or power overload
Note that applying a voltage greater than six volt, even for a very small time (milliseconds) will trigger an overvoltage protection device on the PI, and the resulting short circuit current will "blow" the polyfuse.
The TP voltage is 4.75-4.85V for the first two seconds after power on, then it drops to 4.3V. I can adjust output of the converter beyond 5.25V but will it help?

PS. I tested it with two USB->microUSB cables, including the one that worked with the phone USB charger (and there was no problems with it).

EDIT: measured resistance across polyfuse F3 and it's 0.04 Ohm.
EDIT2: TP voltage without anything connected (even without SD card) is 5.09V, the same on one end of F3 polyfuse, and 5.11V on the other end. After plugging the SD card back (only connected thing right now) TP voltage drops to 4.58V, on the other side of the F3 polyfuse it's 4.67V.

So, maybe it's something with the SD card? I'm using some microSD card in adapter. I tested the voltages with my 2nd card and the results were the same.
Last edited by rownyr on Fri May 17, 2013 12:31 am, edited 1 time in total.

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mahjongg
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Re: Low voltage and hanging

Fri May 17, 2013 12:25 am

Either somewhere there is a loss in voltage, after a few seconds, OR your power supply collapses after a few seconds.

it could still be the polyfuse that causes the loss in voltage, if it heats up, try measuring the voltage across it while its working, thats a better test than trying to measure its resistance which can change dynamically.

Another test is to try to measure current consumption to see if it suddenly increases dramatically. During booting the current consumption will rise from about 400mA to about 550mA when the GPU kicks in, that is normal.

rownyr
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Joined: Wed Jul 11, 2012 1:25 am

Re: Low voltage and hanging

Sun May 19, 2013 6:54 pm

I found the problem... I was using cheap alligator cables. Now I'm using three of them in parallel for +5V and three for GND (temporarily, for testing only). Voltage drop after three cables is still 0.2V...

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