I'm not sure about the grounding, but I tend to believe that it would not make any difference - ungrounded transformers work fine.

But here's another thought:

I tend to doubt that the current sensing coil inside the meter is stepped down according to the ratio between it's scales and the tiny meter movement current rating.

Rather, I suspect there is some current stepping down, but only part way, and a calibrated shunt is placed across the meter movement.

My thought is, if there is a substantial meter shunt there, might that prevent my third winding, the external 6 turns, from sensing anything? In other words, is a possible meter movement shunt acting like a "shading ring?"

I'm not sure. It would take another iron core, not the meter's, to find out.

Eleanor White

Your meter has to have some kind of shading device, or the needle would oscillate at 60 cycles.

I would think damping would be handled inside the meter movement.

My thought is that a substantial shunt, external to the meter movement, might cause enough back-EMF to neutralize most of the flux in the ferrous ring. If that is happening, it might account for the lack of output from my externally-added winding.

I'm going to look for an old/burnt-out transformer from my local ham radio club to cut the windings off, and try that for this application.

Eleanor White

Could be that your meter is using a rectifier and a shunt and electronic circuit to develop current reading from its coil. I tried to explain earlier, but is there a possibility that the meter winding and your winding are canceling each other because of the direction the flux is going through your coil versus the meter coil. Just a thought. Just for curiosity have you tried reversing the direction of your windings on the core?

Interesting thought, that the direction of one winding on a multi-winding transformer might interfere with other windings. I know that changing the direction of one winding would change the polarity dots on a schematic, but I haven't heard that the physical winding direction can actually interfere with other windings.

I have an engineering career behind me, and I've been a ham radio operator for 57 years, and I don't remember seeing any prohibitions on winding directions, other than establishing polarity. But at the same time, I've never worked in transformer design, either, so I don't know for sure.

If I had to bet, at this point in the experiment I'd bet that a shorted winding, which a meter shunt is, is the most likely cause of the failure.

Once I can try a plain iron transformer core by itself - no shorted winding - I'll post the results here.

The tiny power extracted by the meter movement itself, and the likelihood that any rectification is full wave, suggests to me that the influence of the meter movement (by itself, excluding the influence of the shunt) would have negligible effect on the flux seen by other windings.

Eleanor White

Thanks for the update! Mutual induction can be an interesting topic. I may try to duplicate your experiment if I get a chance. CT's do have a polarity dot on them, and it does make a difference when connecting the CT's together, but that scenario is different than yours.

My last post went to some political thread.(oops) This site kills me sometimes. The picture indicates that there are two secondary windings on the core, one on each section of the jaw and connected in series and if you notice they are wired yellow in, blue out and then blue in and yellow out of the other winding. They are matched. I bet if you do a similar setup, using two coils instead of the one, you will see a difference.