It does make sense to a certain degree.  All of my "bench" power supplies
are Power One and similar linear units liberated from other equipment.
None have separate ground connections on teh output; all have red & black,
and the black is tied to the input AC ground AFAIR.  The only place I have
a green wire is on the input side, where there' also a black AC hot wire.

Even if the PIC's DC gound was isolated from the AC ground, thogh,
attaching the scope ground clip should have tied PIC and scope grounds
together, eliminating th hum - no?  It always does for me.

As for what you were suggesting, if it was "tie the DC ground to the AC
ground", I'd agree.  That's not what I understood you to be saying fo just
reading the post though (but hey, I'm on my first cup of coffee, so I may
just be dense this morning).

Dale
--
"Curiosity is the very basis of education and if you tell me that
curiosity killed the cat, I say only the cat died nobly."
          - Arnold Edinborough


On Thu, 14 Feb 2002, Alan B. Pearce wrote:

> >Along with all power on that circuit as the breaker pops with extreme
> >prejudice, if you're in the US where black is AC hot and green is ground.
>
> hang on, he started off talking about running his PIC circuit from a power
> supply, and all he could see on the scope was a massive hum waveform. I
> don't recall seeing any PIC chips that run off mains voltage directly, he
> definitely talked about a power supply.
>
> I think you will find the power supply has red, black and green terminals on
> the front, for positive, negative and ground, with the power supply being
> totally floating from ground. Most bench power supplies I have seen are
> wired this way.
>
> The reference to mains seems to be a red herring (sic) that has developed
> because he talked about trying to measure the mains with the probe. He also
> says he has zero experience with scopes, so it would seem this was a
> dangerous blunder of the inexperienced. I have not seen anyone else try and
> get him back on the right track of debugging his pic circuit with a scope.
>
> Original mail below with emphasis by me.
>
> >After reading the benefit of an oscilloscope, i took one that is not used
> in
> >my work to debug (is this used in hardware too ?) a pic circuit. I have
> ZERO
>                                                     ^^^^^^
> ^^^^^^^
> >experience with scopes, so, and i attach the probe to Ch1, the probe has a
>   ^^^^^^^^^^^^^^^^^^^^^^^^^
>
> >clip, i grip it to gnd and the pin of the probe to what was i really want
> to
> >measure... BUT, i have a 50Hz superimposed over my signal, and showme not
>             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>
> >really what i expect, then measuring the line ac make some spark in the
>
> Now looking at the bits I have underlined, does this not make sense for an
> ungrounded circuit behaving as a hum aerial ???? Lets get back to the
> original problem.

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