Re: AC spike go pass XFRM and regulator to reset CPU [Part 2]: It is irradiated?

Rodo wrote:

Hi all,

A while back I posted a problem I was having with a spike (see original
text below). Someone suggested that the spike would be in the ground
line as well. So I connected the scope probe and ground to the ground
of the circuit and yeap, there it was. I felt rather silly connecting
the scope probe and ground together but oh well ...
This week I disconnected the probe but left it next to the circuit and
I noticed that I also got the spike. So I thought it must be getting in
the scope from the AC ground. So ... for some reason I move the probe
on top of the scope which sits in a higher shelf and repeated the test.
I got no spike. Several tries show the spike if the scope probe is
close (1 or 2 inches) to the circuit but it goes away if I place the
probe further from it. So it looks like the spike is being generated
and transmitted (for lack
of a better word) everywhere to the circuit? Is this something that
would require more ground to shield the PIC?

Comment/ suggestions welcome .... this is driving me nuts :-(.

Thanks in advance.

Scope probes are usually very high impedance and it is not unusual for an
open probe to pick up noise from everywhere. You need a sound approach
to exploring this phenomena.

Are you trying to do this exploratory work in a lab or is the environment
you are in the shop floor? This may have a bearing on the noise you see.

If you are working in the lab is the ground in the lab a very low
impedance. I have had substantial copper busbar fitted around all the
sockets in a lab to ensure a very low impedance ground network. Check
your scope's grounding is secure (for safety reasons) and as low
impedance as possible. You should also probably use a LISN (see
http://en.wikipedia.org/wiki/LISN) to help with performing measurements.

If you connect the probe to a small coil (just a few turns of stiff wire
wrapped around a 4mm diameter former) you could hunt for the localised
source of the spikes (in a non-contact way).


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