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