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I built this transformer core out of 30 mm mild steel. It is 13 cm by 8 cm. The primary winding is 30 meters of #29 wire, and the secondary is 60 meters of #29 wire. The windings measure 5 ohms and 10 ohms. A 100-ohm resistor is across each winding to absorb voltage spikes.

When I apply a 15 volt pulse to the primary, the secondary only produces 5 volts. The core will have eddy currents, and is not the most efficient design, but 17% efficiency seems overly low. I tried resetting the core magnetic field by applying reverse voltage, but it has no effect on the output.

The 15 volts on the primary seems to produce a very strong magnetic field, the pieces of steel are locked together when it is energized. But this field does not seem to induce any significant voltage in the secondary.

Am I having massive hysteresis loss?

What modifications do you suggest to get the output at least above 80%? Transformer

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    \$\begingroup\$ Without insulated laminations, the core is just a huge shorted turn. \$\endgroup\$
    – bobflux
    Dec 18, 2020 at 10:58
  • \$\begingroup\$ What are the characteristics of your pulse, pulse width, rep-rate? \$\endgroup\$
    – SteveSh
    Dec 18, 2020 at 11:48
  • \$\begingroup\$ What is the output impedance of your pulse generator? \$\endgroup\$
    – Sredni Vashtar
    Dec 18, 2020 at 11:57
  • \$\begingroup\$ I don't have any experience with power pulse transformers, yet. (I do have an interest in one used to impedance-match rocket squibs to a triggered xenon tube output, though.) For signal types, though, I know the main ideas are to try and minimize parasitic elements: leakage inductance, winding capacitance, etc. And a ferrite core is almost always used as the higher-frequency components in the signal also have to cross over well. What in the heck are you trying to achieve with this? Is this a power pulse application for heating or something? \$\endgroup\$
    – jonk
    Dec 18, 2020 at 21:05
  • \$\begingroup\$ It is for a switching power supply using multiple inputs. I have ordered some laminated block cores, and will post the results. images.app.goo.gl/DLCngowY37YubG6y5 \$\endgroup\$
    – Doug Green
    Dec 20, 2020 at 10:03

1 Answer 1

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The core will have eddy currents, and is not the most efficient design, but 17% efficiency seems overly low.

Not at all. I believe that most of the coupling losses you see are due to not having laminations. This is why people use laminations or ferrite cores.

The 15 volts on the primary seems to produce a very strong magnetic field, the pieces of steel are locked together when it is energized. But this field does not seem to induce any significant voltage in the secondary.

If the applied voltage is constant then there won't be an induced voltage once the primary current has settled to a constant value. It will settle to a constant value due to the coil resistance.

Am I having massive hysteresis loss?

No, you are having a massive eddy current loss.

What modifications do you suggest to get the output at least above 80%?

Try some ferrite core material or use laminated silicon steel.

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