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

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

Don't isolate the scope from AC power ground. The scope frame and ground
points will go to the potential of the probe ground which could be harmful
to
the health of you and your scope. Most 2 channel scopes have channel A+
channel B (A+B) input selection. If you select that and invert A, the scope
will display the difference between the 2 channels. If there is a large DC
component, select AC inputs to measure low level ripple. Be sure to connect
the grounds of both probes to the frame or AC ground of the device being
measured.

| Don't isolate the scope from AC power ground. The scope frame and
ground
| points will go to the potential of the probe ground which could be
harmful
| to the health of you and your scope.

RIGHT, when you use an isolation transformerthen to isolate your
DUT!!! never the scope!!!

| Most 2 channel scopes have channel A+
| channel B (A+B) input selection. If you select that and invert A,
the scope
| will display the difference between the 2 channels. If there is a
large DC
| component, select AC inputs to measure low level ripple. Be sure to
connect
| the grounds of both probes to the frame or AC ground of the device
being
| measured.

The CMRR of such constructs is usually so bad that you see nothing
worthful. The only propper way is a fully differential probe like
these ones from tek
(http://www.tek.com/site/ps/0,,60-15865-INTRO_EN,00.html) or a
isolated oszi like Tektronix 720 series.

MArtin

On Fri, 30 Jul 2004 20:02:10 GMT, "id" wrote:

Don't isolate the scope from AC power ground. The scope frame and ground
points will go to the potential of the probe ground which could be harmful
to
the health of you and your scope. Most 2 channel scopes have channel A+
channel B (A+B) input selection. If you select that and invert A, the scope
will display the difference between the 2 channels. If there is a large DC
component, select AC inputs to measure low level ripple.

I humbly offer my thanks!

The scope does indeed have an A+B channel option, plus the ability to
invert the b channel. I had no idea that that option would do what I
wanted, but in hindsite it makes sense. (A plus a negative B equals
the difference between the two, and here I was thinking Iwas 'smart n
stuff ....)



Off to go play, thanks 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

The CMRR of such constructs is usually so bad that you see nothing
worthful.

Google CMRR

A) Center for Magnetic Recording Research
Nope

B) Cabada's Most Referred Real estate agents
Don't think so..

C) Common Mode Rejection Ratio
Hmmm. Maybe...

Whatever CMRR may be, I just explored that little SMPS with the A+ -B
setup on the O-scope, and Worked like a champ!

It was really nice to see what was going on at the gate of the Mosfet
driver. Had I been able to look at that previosely, I'd of found the
bad Diode quicker

Thanks 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

"id" bravely wrote to "All" (30 Jul 04 20:02:10)
--- on the heady topic of " Oscilloscope, How TO?"

id From: "id"

id Don't isolate the scope from AC power ground. The scope frame and
id ground points will go to the potential of the probe ground which could
id be harmful to
id the health of you and your scope. Most 2 channel scopes have channel
id A+ channel B (A+B) input selection. If you select that and invert A,
id the scope will display the difference between the 2 channels. If there
id is a large DC component, select AC inputs to measure low level ripple.
id Be sure to connect the grounds of both probes to the frame or AC ground
id of the device being measured.

I have an old scope that has differential inputs on each channel.
Normally the inverting input is grounded with a strap and the
non-inverting connects to the probe. Would this work in your opinion?

BTW on the same old tube scope when set to display both channels the
trace becomes distorted, and the A channel seems to bleed into the B
channel. Each channel works perfectly on its own so I was thinking the
problem was in the chopper, perhaps a bad switch in the matrix?

The chopper seems to be housed in a small square box which feeds into
the vertical output tube driving the crt. The switching matrix seems
to be made up of what may be early fets or transistors, not sure
which, since it is very difficult to take a peek due to construction.
The scope is an Analab. (what an anal brand name, who thinks these up!)

A*s*i*m*o*v

.... High voltage can give a dangerously uncomfortable discharge.