In article ,
jim rozen wrote:
In article , Gary Coffman says...

A *floating* dual trace scope. Current can be measured with
a sampling resistor.

Or one that has isolated differential preamps, like one of mine does.
Ordinary scopes use unbalanced inputs, with one side tied to chassis.
Trying to measure phase voltages and currents with one made that
way will give you a *hot* chassis, which is a serious shock hazard.

The plug-ins for the 7000 series tek scops *look* like they're
isolated ground. They have the bnc shell isolated from the
front panel with a teflon insulator. But this is to minimize
internal ground loops. The low side is indeed tied to power
common. But the appearance has fooled many a grad student....

The teflon insulation is also to isolate a ring surrounding the
outer shell, to couple in information on what the attenuation of the
probe is -- to switch readout scale values. (Some of the probes have a
1X/10X switch on them.)

There are some which *are* truly differential -- but they use
two BNCs for a single channel of differential input, and have a
long-range offset adjustment to allow you to tune out DC offsets.

Unfortunately, one of the nicest has an interesting plastic
gear inside to couple a switch to the knob on the front panel, and the
plastic has gotten brittle over time. I've seen two of those with the
plastic disintergating. I own one of them. I would love to see a
drawing of the gear's dimensions, with the hopes that I could make a
replacement. The plug-in is too nice to consign to the trash bin.

It is a barrel (perhaps 3/4" long), with a hex hole through it,
and a set of gear teeth only about 1/8" long in the middle of it.

Enjoy,
DoN.
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