Again...strange measurements with transformers

[andia]

Hi all, it happened again. In my journey of trying to do transformers myself, I stumbled on another strange measurement I did today.
I wound 2 nearly identical transformers but I got strangely different measurement results.
So both transformers are wound on the same nickel50 UU core, 2 bobbins each containing 1/2 pri and 1/2 sec. 1:1 - 1000 windings per coil. both pri. and sec. coils connected in reverse.

When I measure inductance with my LCR meter (@100Hz), one transformers coils measure ~8,5H, the other ones coils measure ~0.85H. only a tenth of the other.

the only difference is a copperfoil shield layer between pri. and sec. on the transformer with the lower inductance measurement.
HOW? Can anyone explain me the measurement?
at first I thought there was an error in the coil or measurement, but all of them measure the same.

frequency response is nearly the same on both transformers.

thank you very much!

 

[Murdock]

What LCR meter do you use?

 

[andia]

GW Instek LCR-916

 

[lukas]

...maybe Cooper foil is to wide on the bobbin(need around 1mm space from both sides)

 

[andia]

yes, the copper is really over the whole width of the bobbin. why does it have to be narrower than bobbin width?

 

[Murdock]

GW Instek LCR-916

It’s a guess but maybe switch between series and parallel measurement on your LCR.

 

[andia]

the lcr is in automode and switches to serial when it senses continuity, so this should be ok.

 

[gyraf]

Copper foil being a shorted turn?

 

[CJ]

Yes, the ends of the Cu must not touch,

Another possibility is you swapped a Start and a Finish somewhere.