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Hi All,
Last weekend I tested high-permeability ferrites as dielectrics, and I just had two feverish days because of all the weird things that were happening.
I connected 2 electrodes to these ferrites, and observed what happens when both AC and DC current are injected. A lot of things happened.
As this test followed my idea of an electromagnetic version of the Archimedes screw (see other thread), I was looking for signal at a double frequency of the ac input, which would be a sign of a double half-wave rectification due to an effect of Lorentz force on electrons bathing in the magnetic field created by their own direct current.
And I did have one, which depended a lot on DC polarization through ferrite!
In the setup, I placed an LC notch circuit to attenuate the fundamental frequency, and used an SDR to visualize the spectrum (it is clearer than the scope). See the measurement schematic and copies of spectra.
While we can barely see the harmonic 2, polarizing the ferrite in DC raised its level up to 40dB more! The polarization voltage must not exceed the AC voltage, otherwise the gain will no longer increase and even decrease.
I have done dozens of tests before I understand. This is not Lorentz's force at work but non-linear junction effect between the copper contact and the ferrite.
It should be noted that good contact is very difficult to obtain. I had first glued copper strips on the ferrites, with a lot of pressure. Despite this, the DC current fluctuates enormously, the resistance is not stable, but the frequency doubling effect is extremely clear.
I finished by tightening my contacts with screws, and there the resistance is stable, but the effect is not more pronounced.
The resistance must be of the order of KΩ to a few KΩ.
My first test was done with a large ferrite core, the next ones with smaller ones (see photo, for the last one with screws, the transformer coils were kept). The frequency doubling has been obtained for all.
If you are trying to repeat the test, choose ferrites whose resistance can be detected directly with an ohmmeter. They seem less frequent than good insulating ferrites. I only found three kinds in my collection.
When I realized that it was a junction effect, I had fun replacing the DC polarization with a 1 KHz signal, so I got an amplitude modulation. I also tried a frequency mixing of same principle as in superheterodyne radio receivers.
I finally replaced a ferrite by two inverted diodes connected in series, each in parallel with a resistor, and this setup gave similar results confirming that we have a diode effect with a copper/ferrite contacts. If the result is similar, on the other hand the effect is considerably less pronounced than with diodes.
I don't know if the principle can be used, it's not what I was looking for, but I think it's always good to know and perhaps to investigate further!
« Last Edit: 2019-01-21, 18:00:27 by F6FLT »
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Topic: Ferrite-copper diode and non-linear effects in ferrite dielectrics (Read 2877 times)
