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I'm making some custom low signal 1:50 step up transformer, and I noticed something interesting. My LC meter displays correct primary inductance when no secondary is present on a core. With the partially wound opened secondary its reading is few orders lower. My only guess is that it uses a relatively high frequency so the secondary becomes shorted by its own capacitance.

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    \$\begingroup\$ What kind of LC meter (frequencies used)? What kind of step-up transformer and core? \$\endgroup\$
    – Antonio51
    Jun 17 at 13:22
  • \$\begingroup\$ Some random multimeter. It's working frequency is not specified. I use a high permeability Magnetec M-070 core. The transformer is supposed to have 25:1250 turns. I have 400 turns of secondary wound right now \$\endgroup\$
    – e_asphyx
    Jun 17 at 15:46
  • \$\begingroup\$ I have seen some multimeters using 90, 100, and 900 Hz and others making "resonance". So, I guess it can't be because ... "shorted" by a "resonant" circuit. \$\endgroup\$
    – Antonio51
    Jun 17 at 18:48
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    \$\begingroup\$ What are your measures? \$\endgroup\$
    – Antonio51
    Jun 17 at 19:10

2 Answers 2

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As some pieces of information are not known, I guess only this.
You are measuring the inductor at a frequency (10 kHz or more, probably at a "high" resonant frequency) as pointed out in the picture (abs(impedance) measured).
In red, a pure inductor. In blue, with a coupled secondary.

enter image description here

To make sure, perhaps measuring input impedance with a scope would be more useful.

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It’s not the best to use a “random multimeter” when doing transformer prototyping. Get a proper LC meter. Doesn’t have to be expensive, just decent enough to do L/C/R/ESR measurements at various frequencies like 100, 120, 1k, 10k Hz. The DER EE DE-5000 would be a suitable meter I think, if you don’t have a big budget. I’ve used it to measure various transformers and the readings looked reasonable.

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