Re: Scoping I-V phase shift through L

Hi David,

	This is actually a bit complex...

	The Tesla coil measurements are done (in my case) with electrically
isolated fiber-optic current probes.  Thus they can be used at the top
terminal where the voltage are like 500000 volts.  The scope is
electrically isolated by 30 feet of glass fiber so the scope is very safe.
Two probes are used, one at the base of the secondary and on between the
top of the secondary and the toroid.  They are high bandwidth and phase
correct so measuring current and phase is easy (one you have the probes
;-).  Details of this are at:


	However, if you want to see current and phase of a typical line powered AC
transformer, there is one BIG caution!  One side of the AC line is hot and
will deliver thousands of amps to ground.  Like the ground clip of a scope
probe!  You can hook the probe tip to 120VAC with little problem but if you
connect the ground clip to the hot wire of the AC it will vaporize in a big
blue flash!  This experiment has been performed thousands of times by
people trying to see AC on a scope without thinking ;-))  Perhaps you can
make your measurements from the secondary of the transformer where the
transformer itself will provide the isolation needed (and the voltage is
pleasantly lower).  Otherwise, you will have to hook one probe tip to the
hot wire and NOT use the ground clip.  The other probe would have to
connect to a resistor (that is properly rated) between the neutral wire and
the transformer (again no ground clip).  You really "could" use the clip to
neutral but one little mistake and BLAM!!  Fully isolated current
transformers are an option too but I bet you don't have one or you would be

	Of course, be very very careful!!  I would not recommend trying this with
the AC line unless you know EXACTLY what you are doing.  Thousands (if not
tens of thousands) of scopes have been destroyed and as many fingers have
been burnt by people trying exactly what you describe.  If you are really
having a bad day, you could get killed!

	However, if you are working on a low energy battery powered circuit.  Such
danger is probably almost zero...




At 10:07 AM 11/10/1999 -0800, you wrote:
>I have heard many folks on this list talk about using a 0-scope to measure
>current and voltage phase shifting through a inductor (at least I think
>that is what was being done :-)). How do you do it? I have no idea how I
>would measure current on a scope. I have a older analog dual trace scope,
>and am interested in (if possible) trying to measure how much a inductor (a
>transformer) causes the voltage and current to go out of phase. If this can
>be done with a scope and other commonly available equipment I would be very
>happy if someone could explain it to me. 
>David Trimmell