[SI-LIST] Re: A short story

  • From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Sat, 9 Apr 2005 21:27:21 -0400

Most likely, there were and are no shorts on your boards.  But there are
lots of capacitors.

An ohmmeter measures the current.  Depending on the charge of those
capacitors at the time you measure it, the current may be small, big,
positive, or negative.  After powering off the board, they probabbly had
some charge they didn't have before, and so the current and the reading
differed.

If you left the ohmmeter connected overnight, it might approach the same
reading regardless of whether the board was ever powered or not.  (It might
take longer than overnight.)

Try measuring the resistance of just an electrolytic capacitor.  Reverse the
leads a few times and see what happens.

Even without capacitors, 0.5 ohms isn't much of a short, if you are
measuring at the power planes.  Your 14A regulator wouldn't blink.  Expect a
real short to measure a lot less resistance.

Regards,
Andy

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