Re: [CAD_CAM_EDM_DRO] Need help with O-scope

At 02:55 PM 4/14/2007, you wrote:

>David G. LeVine wrote:
> > At 10:06 AM 4/14/2007, you wrote:
> >
> >>I am having a problem with a mill set up with Gecko drives. There seems
> >>to be a noise problem that has cropped up on the Z axis and I would like
> >>to look at it with a scope. I have the use of a scope but have not used
> >>one for several years and have forgotten a few things. The part I worry
> >>about is; do I have to use an isolation transformer to keep from popping
> >>an IC do to different ground potentials? The scope is a B K Precision
> >>model 1570A with API 1X and 10X probes. Any help appreciated. Ed.
> >
> >
> > Most 'scopes had isolation transformers built in. BK Precision
> > SHOULD, but may not. It couldn't hurt.
> >
>No, this is not true. No oscilloscope is powered directly off
>the mains, but that is not the same as an isolation transformer.

Technical issue, the transformer power inputs (of the Tektronix
scopes I am familiar with) have transformer isolation between the
power supply and chassis. The ground pin is another matter. In
fact, my 454 (purchased surplus) has a double Faraday shielded
transformer inside which is used to ensure line noise does not create
false signals. When I saw it, it surprised the h*ll out of me!

Yes, the chassis IS connected to the ground pin, as is mandated by a
number of regulating bodies. Often, but not always, the BNC barrels
are connected to the chassis.

>The frame of the scope, and the barrel of the BNC probe
>connectors will be connected to the power plug ground pin.
>Only on battery powered scopes will this not be the case.
>If in doubt, use an Ohmmeter to measure from the BNC barrels to
>the power plug ground pin, and you will have your answer.
>Making a mistake with this could turn the entire scope into
>scrap or get you seriously burned or shocked.

Take a look at how I suggested that the ground lead be tested BEFORE
applying it to the machine.

>There are cases where you need to isolate the scope from ground,
>such as to measure across the motor output terminals of a full
>bridge driver. There are usually much better ways to make such
>a measurement, for instance by making a differential measurement.

We are in full agreement about differential measurements, and I have
used Tektronix scopes with "cheaters" to isolate the ground pin, with
great caution and someone nearby and clear, just in case. A number
of isolation transformers with 3 wire connectors have the safety
grounds tied together to be UL and NEC (250.5x) compliant. Assuming
that the isolation transformer you have does or does not have those
connections tied together is a VERY dangerous assumption. ALWAYS
MEASURE WHEN NOT SURE!

When one sees big (10+ KVA) isolation transformers with equipment
grounds which tie to the safety grounds in the building, one gets
curious and cautious.

David G. LeVine
Nashua, NH 03060