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26th January 2011 13:55

Alan Yates wrote...


Thanks John. I have not seen DC-magnet biased inductors myself, I do wonder about their long-term stability? Interesting idea, a way to use a smaller core at higher densities perhaps? But the coercivity of the bias magnet would need to be very good. In TV line-driver service this might be practical because of largely unipolar excitation, or its "swinging" properties might be important for the circuit in question?

On your speculation; it depends on where exactly it is biased along its curve. It might be biased to saturation in which case yes, it has a small inductance until the applied field starts to de-saturate the core and it can start influencing the magnetisation, at which point its inductance rises rapidly and remains at some high value in the linear region before it saturates in the reverse direction. Alternatively it might be biased just short of saturation in which case it looks strongly inductive for about twice the expected current in one direction, and rapidly saturates (becomes weakly inductive) in the other.

Controllable magnetics like saturable inductors and magnetic amplifiers are considered old-hat now days, everyone uses more straight-forward switching technology. Of course the magnetics in switching devices are still critically important and rather non-trivial to design properly. I don't claim to understand the magnetic properties of matter to the depth I would like, but this simple foray into designing switching supplies and measuring inductor core saturation practically helped me enormously in getting an intuitive feel for it.

I have a bit of a penchant for toying with "old-tech" like controllable magnetics. I've played with "hard ferrite" core memory, magnetic frequency multipliers and the like, purely a geeky exercise of course, but fun and educational none the less.



19th January 2011 22:14

John Chapman wrote...

Fascinating article, thanks. I have only a hobby interest in electronics, but while scavenging a 12 year old TV for bits was intrigued to find a 50 microH inductor in the line driver circuitry which was taller than usual, marked with "polarity" ie a line marked one terminal; and magnetic enough to stick to other inductors, fridge doors etc. It clearly has a magnet shrinkwrapped on the end of the inductor core. Having done a bit of stage lighting in the days of dimming lamps with saturable reactors, I started looking for internet items on saturation and inductors and came to your article. Can you comment on this piece of speculation. "This is a non linear inductor, which will display low inductance in both current directions, but in one direction there will be a sudden increase in inductance when the magnetic field due to the DC component of the current overcomes the applied fixed external magnetic field and the core desaturates."

Have I got this right, and for interest, would you like the part to test. Maybe these are as common as muck, and I have just never met one before, but they certainly aren't obvious on inductor manufacturer's websites.

Thank you. Dr John Chapman Plymouth UK

25th January 2009 23:21

Alan Yates wrote...


I'd be interested to see their circuit. What magazine was it in?

I just threw this one together to solve my immediate problem. It has proven quite useful, but I'd like to eventually do full B-H plots, which I understand it just a matter of integrating the signal from another winding on the same core.

Apparently you can do it with a single winding, which would be handy for commercial choke testing. But I have only heard this as a rumour in documentation of a commercial B-H test unit, and have no idea how it is actually accomplished.



25th January 2009 22:51

Mike wrote...

Hi Alan

I meant to have a talk to you about this project at the T+T today. I have a similar one from a magazine I was going to build, I'll let you know how it goes.

Mike vk2kmb