I may or may not understand your problem. You may have your scope grounded.
Does the scope have a 3-prong plug? Yes? Then most likely the ground lead
of your scope probe is connected to earth ground, so when you attempt to
probe various points in the SMPS, you are effectively creating a short to
mother Earth. Go to Home Depot and buy one of those 3-prong to 2-prong
adapters. Plug your scope into the 3-prong side, then plug the 2-prong side
into the outlet. Your scope is now isolated from mother Earth. Be careful,
use an isolation transformer, and remove all jewelry before measuring live
circuits.


"James Lerch" wrote in message
...
Greetings All,

I have a two channel Vu-Data corp Model Ps950 Oscilloscope. I'm
trying to measure a signal relative to a point in a circuit not at
ground potential, but I don't know how?

For instance, I was working on a small SMPS yesterday. The first
stage of the power supply simply recitified the AC input. I was
attempting to measure the DC output and any AC ripple.

When I looked at the output of the rectifier with the O-scope I saw
large AC component on both the positive and negative sides. When I
used my DMM across the rectified output, I measured very little AC.

Before I describe my momentary lapse of common sense, here's how the
first stage rectifier is composed:

Each leg of the AC input goes thru a large Inductor. Out of the
inductors into a full wave rectifier with a large Cap on the output.

Since the DMM didn't measure much AC across the output cap, I decided
to put the O-Scope probe ground on the negative side of the cap. When
I did, I promptly let the magic smoke out of one of the inductors, and
fried two of the four diodes in the bridge rectifier.

After repairing the damage I did, and replacing all the diodes in the
circuit, I got the power supply working again.


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and
Coating site)

Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge