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I am using a step up transformer and I am getting a weird discontinuity in the secondary output. Has anyone seen anything like this before? I cant figure out what might cause this. In a one test setup I can get a nice sinusoidal waveform with the same input voltage when I have a large resistive load (500ohms) on the secondary. When I hook up to this inductive load (5ohms esr) I am getting the weird waveform. Is it possible the output impedance of the transformer is too high for the lower impedance load? Thank you for any insights.

enter image description here

Dark Blue: Primary Current, Light Blue: Secondary Current, Pink: Secondary Voltage, Green: Primary Voltage

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  • \$\begingroup\$ Looks like saturation. Do you have a spec sheet for the transformer? \$\endgroup\$ May 14, 2020 at 22:41
  • \$\begingroup\$ I wound it myself. Based on my calculations I should have plenty of headroom before saturation with the volt seconds I am applying. Furthermore I am observing the same issue across a range of input voltages on the primary. The waveform stays pretty much the same although the amplitude changes. At the lower input voltages I would not be anywhere close to saturation. \$\endgroup\$
    – Marlowe
    May 14, 2020 at 22:55
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    \$\begingroup\$ Is your load inductor air core or iron core? Gapped or not gapped? \$\endgroup\$
    – Marla
    May 14, 2020 at 23:28
  • \$\begingroup\$ The load inductor is iron core, not sure about the gap. I believe that I am not saturating the load inductor because I can apply more volt seconds to the load inductor with a power supply without saturating it and I am getting nice wave forms with just the power supply connected to the load inductor. \$\endgroup\$
    – Marlowe
    May 14, 2020 at 23:55
  • \$\begingroup\$ I was thinking more along the lines of the inductor Not saturating, rather, the load seems to go to a higher impedance state at zero crossing. Perhaps indicative of a gap closing on each cycle (resulting in a higher impedance). I could imagine the core vibrating at frequency, alternating a very small gap and closed gap. Note the secondary volts increasing , indicating higher impedance load. \$\endgroup\$
    – Marla
    May 15, 2020 at 0:16

2 Answers 2

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I do not suspect transformer core saturation. At a fixed input voltage to a transformer, you can short circuit the secondary, and the transformer core will not saturate. So, if your core does not saturate with open circuit secondary, it will not saturate with a load. (Assuming fixed frequency).

I was thinking more along the lines of the inductor load Not saturating, rather, the load seems to go to a higher impedance state at zero crossing. Perhaps indicative of a gap closing on each cycle (resulting in a higher impedance). I could imagine the core vibrating at frequency, alternating a very small gap and closed gap. Note the secondary volts increasing , indicating higher impedance load.

Edit 1: Regardless of my mentioning inductor gap, I suspect the anomaly is in the load.

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  • \$\begingroup\$ "if your core does not saturate with open circuit secondary, it will not saturate with a load. " did you mean "with short circuit secondary" ? \$\endgroup\$
    – muyustan
    May 15, 2020 at 2:48
  • \$\begingroup\$ @muyustan . . yes, with a load including zero ohms short. \$\endgroup\$
    – Marla
    May 15, 2020 at 2:59
  • \$\begingroup\$ you did not edit and fix it?? I am not sure if you did a mistake but I asked you and you approved me?? confused \$\endgroup\$
    – muyustan
    May 15, 2020 at 3:46
  • \$\begingroup\$ @muyustan is it this you are having problems with: if your core does not saturate with open circuit secondary, it will not saturate with a load.? \$\endgroup\$
    – Andy aka
    May 15, 2020 at 9:10
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    \$\begingroup\$ Core saturation has nothing to do with secondary load current and EVERYTHING to do with magnetization current - read this answer. \$\endgroup\$
    – Andy aka
    May 15, 2020 at 9:28
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Look closely: -

enter image description here

I see the positive peak of the primary current at 2.1 graduations and the negative peak at 1.9 graduations. There is DC content and this is likely because when you applied the excitation voltage, you applied it at the zero-crossing point of the waveform and this will produce a standing DC current in the primary. It isn't helped by the inductive load because it will also be subject to an initial voltage waveform that is passing through zero volts and, in turn it will be biased at a DC level.

It is a saturation anomaly as far as I can tell.

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