CT phase lead problems ?

[Mathieu thm]

Hello everyone,

I was about to do a test of my 160mm DRSSTC outside, I wanted to check the phase lead but I discovered that it was doing something weird, see the video :

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The CT is a 900:1 with 10ohms burden resistor. (N30)
I may be missing something obvious, but does anyone know why and how to avoid phase lead in the CT?

[davekni]

Video is too close up to see overall setup.  However, I see a scope probe and ground clip near top of base.  Is that probing CT output?  How close is that probe to primary coil?  I'd guess you are getting induced voltage in probe tip/ground loop from primary coil.
Best to have CT outputs connected as twisted pair (or coax) until away from coil.  Burden resistor and scope probe are then away from coil and not picking up significant magnetic field.
Presuming CT cores are ferrite and not powdered iron.

[Mathieu thm]

Video is too close up to see overall setup.  However, I see a scope probe and ground clip near top of base.  Is that probing CT output?  How close is that probe to primary coil?  I'd guess you are getting induced voltage in probe tip/ground loop from primary coil.

Yes, the oscilloscope probe is connected to the CT with the ground clip, now that you mention it, it's certain that the probe picks up a voltage induced by the primary coil. I understand better now, thanks!

Best to have CT outputs connected as twisted pair (or coax) until away from coil.  Burden resistor and scope probe are then away from coil and not picking up significant magnetic field.

It's not a problem to have too long a cable between the CT and the burden resistor ?
Is there any way of knowing that the current signal no longer has a voltage induced by the primary coil?

[davekni]

It's not a problem to have too long a cable between the CT and the burden resistor ?

Generally not a problem, as long as wires are twisted.  I've ran ~10 meters of coax on CT output without issues.  In that case I used a burden resistor to match coax impedance.  Doubt that was critical.

Is there any way of knowing that the current signal no longer has a voltage induced by the primary coil?

Flip scope probe/ground loop over.  If no change in signal, you are far enough away.  If you wrap or tie probe ground lead adjacent probe, that will reduce loop area, so be less sensitive.  Less loop area allows closer placement.  Direct coax to scope input avoids even more loop area.

[Mathieu thm]

Thanks for the advice ! I hope to be able to try it out tomorrow.