Re: Problem with common leads in dual channel scopings?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>

Hi Harvey,

At 04:18 PM 8/29/2003 -0700, you wrote:

>Later I observed that one of the probe leads
>was smelling funny, and it was too hot to touch.

Most probes have a very large internal resistor in them say ~10Meg 
ohms.  If you put too much current(voltage) into them they overheat and 
often fail.  Damage to resistors inside the scope is also very possible.


>This brings me to a question of scope operation: can
>anyone simply say why grounding is necessary?

Scope often deal with very high frequencies where probes and wires start 
looking like antennas or inductors.  The "ground system" stabilizes 
everything so good reading can be taken.

>These
>scope leads were only monitoring voltages from an
>alternator stator being under 12 volts or so, so the
>voltage ranging precautions of the scope were
>available, BUT it might be true that since it is
>monitoring a circuit of higher amperage delivery,
>might any special precautions be necessary?

12volts will not hurt any scope and they are not affected by 
current.  However, do be very careful of spikes and inductive kicks hitting 
the probes with far higher voltage.

>Could it be possible that the internal
>impedance of the scope might act differently when
>observing a 480 hz signal? Doesnt sound too likely,
>more like a problem involving the probe connections I
>thought...

The probe will easily do say DC to 50MHz.  High voltage is getting into 
something and tearing the probes and scopes up.


>In any case a solution to these monitorings was found,
>by making the dual channel leads connected to the
>voltage rise of the circuit, WITHOUT any of those
>probe leads having a common connection point.

Be sure your ground really IS at ground!  High voltage can sneak back onto 
the ground side too if the grounding has some big problem.

Sounds like you are really having a lot of fun there!  I am not sure I 
understand it all though.

Cheers,

         Terry



>HDN