Power Supply: The Mystery Continues

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I continued testing the power supply. It would work most of the time it was powered on and give the correct voltages, but then sometimes when it was powered it the +5V circuit breaker would trip. I managed to capture this on the oscilloscope The peak voltage was 5.16V and then the breaker trips and it goes down to around 1V. 

Time to keep investigating the G824 module to see if there is something wrong with it. (Michael from RICM has been very helpful with troubleshooting so far). 

I borrowed a nice mixed signal scope from the EE shop. It blows my HP 54200A out of the water.

The +5V regulator is supplied by the +8V line. So Michael suggested looking at both of the lines as they power on to see what they look like.
Not Tripped

Tripped
Above are the results of this. I measured the peak voltages for both lines when they tripped and didn't trip. The +5V line averaged around 6.6V when it didn't trip and averaged around 12V when it did trip. The +8V line averaged around 9.3V when the +5V didn't trip and ranged from 11.5V to almost 13V when the +5V did trip.

It's time to investigate the +8V line.

Comments

  1. lastchancenameJune 6, 2017 at 8:50 PM

    If I may make a first guess, I'd say the +8V (yellow line?) is probably OK, but showing the consequences of being loaded by a dead +5V (blue) rail. (Yellow keeps rising after the failure event)

    NOT TRIPPED shows a nice clean 5V supply - so something downstream seems to be happening.
    That flat period after the 5V falters initially then goes south to 2V5, means something is taking 25mS after power-on to die, either under system wake-up sequencing, or as the faulty component implodes internally (heat, breakdown or whatever)

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    Replies
    1. lastchancenameJune 6, 2017 at 8:52 PM

      P.S. 2V5 at substantial current should show as heat in one way or another.
      Return the MSO, and borrow a thermal camera(!)

      Delete
    2. DawsonJune 7, 2017 at 6:36 AM

      Very good guess. The yellow/8V line is indeed OK, but suffered from overshoot. Whether from the breaker tripping or something else I don't know. The +5V regulator board is powered by the +8V line. The breaker only cuts off the +5V output from the board, not anything entering the board, so the +8V would still rise to its normal value.

      One thing I forgot to mention in the blog, but will go back and edit, was that I reformed the capacitor C5 for a couple more days. (I will post a schematic). C5 is the filter cap for the +8V line, so it was possible it wasn't filtering properly.

      All in all I think the ultimate reason it was tripping was because there was no load on the supply. I got some heavy duty power resistors and load tested at 5A, shown in the next post.

      P.S. Thanks for reading and commenting! I appreciate the support.

      Delete

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