Re: [CAD_CAM_EDM_DRO] Need help with O-scope

There is another technique that can get effective isolation
from ground. That is to use the A - B setting that some
dual channel scopes have. This subtracts the B input channel
from the A channel. Thus the trace produced is the voltage
difference between the two inputs.

To use it put the A and B channel probes on the two points
that you want to measure. Connect the B probe to where
you would normally connect the ground. Do not connect
the ground leads to anything.

You are still limited by the max voltage limits for the scope
inputs and probes however.

Pete.

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, April 14, 2007 2:58 PM
Subject: Re: [CAD_CAM_EDM_DRO] Need help with O-scope


> 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.
> 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.
>
> 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.
>
> Jon
>