saturable inductor

There will be that classical shark fin, followed by a loud bang, molten solder splattering about, stuff flying around, smoke alarms blaring ...

Be careful and make sure there si something that will limit the current.

Spark, smoke, sirens... I failed to ask why it doesn't happen

Aww, Joerg, you take the excitement out of life.

I had the ongoing amusement one project of kibitzing a theoretically experienced analog guy (from MIT, no less!) on a switching amplifier. One of the things that I diagnosed was that mysterious (to him) shark fin, although it was only one of the reasons that things went BANG on that particular circuit.

The part that added the most humor (for me) was that my job title (and function) at the time was very firmly planted in "software engineer", although I was being allowed to design a (simple) board.

Well, if you have a current limit it won't ;-)

Some of the guys also overlook the fact that there is a rather large tolerance on ferrite core specs. Later, some folks in purchasing think that a ferrite is a ferrite is a ferrite. Phssst ... *BANG*.

Don't ya love it when that happens? I once snuck into a mechanical design review. I wasn't supposed to be there but urgently needed to pass a message on to one of the guys. Looked at the PowerPoint screen and just had to ask: "Looks like there is way too little airflow. And what happens if that fan up there fails?" ... "Uhm ..., well, ... s..t!" Major redesign.

OK, I kind of guessed (qualitatively) that at first dI/dt is "slow" then there is a step (?? how long does it take for mu to drop??). The current steps up by sqrt(mu) and then keeps ramping up mu times as fast. Does it make sense?

Depends on the core, it's in the ferrite datasheet. Usually the "grace period" ends at some point where the inductor basically acts like an air coil, as if the core wouldn't be there anymore. For most practical circuits such as switch mode supplies that comes pretty close to a dead short with all its consequences.

I am not worried about "dire consequences" - I am sandwiching two cores actually (ferrite and iron powder), what I need to know is HOW FAST ferrite "disappears". They should use something like this in series trigger transformers.

All I can tell you is "really fast". For example, it can take just a few nanoseconds to drive Fair-rite #43 material into complete saturation. BTDT, got broom and shovel and cleaned up the mess ;-)

The voltage applied to a transformer usually determines that. The higher it is the faster the primary current will rise and will core saturation be reached.

I'm kinda side-stepping Joerg's whole thread with this reply -- what he has to say makes oodles of sense in a switching-supply frame of reference, but I doubt that that's what you're thinking of (or you are trying to make a switched supply with your oddball stacked cores, which means you may be cracked -- go look in the mirror and ask yourself if you're sane as a check).

The total voltage on the coil is equal to the IR drop of the wire plus the back-emf on the coil. The back-emf of the coil is proportional to the the rate of change of the product of the current and the permeability of the core material -- and while the current (and magneto-motive force) is going up, the permeability of the core material is going down.

So initially your inductor will have high inductance and the current will ramp up at a more or less constant rate commensurate with the zero-current inductance. At some point the ferrite in the core will get saturated with flux and won't accept any more than air does and your current will start ramping up at a more or less constant rate commensurate with whatever else you have in your coil -- if it's just air, then it'll ramp up pretty darn fast, if that iron powder coil hasn't saturated yet it'll ramp up at a rate commensurate with the iron powder coil (until it saturates).

I _don't_ know how fast the transition will be between the two straight ramps, or how straight they'll really be (the coil resistance will affect that to some extent anyway).

You ought to be able to make an estimate of the transition if you can get your hands on a B-H curve of the material -- the sharper the transition between the sloped part around 0 H and the nearly level part around high H, the sharper the transition between the two 'modes' of the core.

Good explanation. "Pretty darn fast" means just that, it gets down to nanohenries and the current (hopefully) pegs against some limiter. Michael should keep in mind that B-H curves and such are just typical data. There can be huge tolerances in core behavior, I have seen in excess of 30%. Us analog/RF dudes usually stay a good 40% below the "pretty darn fast" range.

My second professional consultation job was for a company that LF transmitters that used magnetic amplification -- basically they used toroidal inductors that were varied by modulating the field in which they sat, to actively change their inductance.

I thought that was a fascinating turnaround -- take a problem (coil saturation), and turn it into an advantage.

Dunno how they controlled the huge tolerances in core behavior, though

-- I would assume they calibrated individually, though.

About that same time (grad school) I had a direct-conversion 40 meter radio whose receive frequency would shift a few hundred Hz in the presence of a strong magnet outside the case.

I've always wanted to build a radio that tunes the LO that way on purpose -- just an iron powder coil, a rare earth magnet, and a mechanism to bring them together in a controlled way. Of course I have No Clue how well it would work, but you could sure have fun with writing some marketing hype to go with the technique!

BTW, the magnetically controlled inductance is the old trick for electronic tuning of the LC tanks while keeping good Q and linearity. The parameters attainable are better then what can be done with varactors.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

It was also a nice method to achieve a mostly frequency-linear tuning range for oscillators. IIRC Drake and Collins used that. The downside was that the worm gear needed constant attention. So the usual setup was the receiver or tranceiver itself with a screwdriver (no, not the drink ...) and a bottle of rifle oil sitting next to it. In Germany we used the Ballistol brand which supposedly was also good to cure other ailments such as saddle sores after too much horseback riding. I don't know about that and would not do it but the stuff sure is quite universal in use:

Yes, although varactors sometimes win if you need to pass high power (say, >>0dBm) through your circuit.

Nah, real men use those:

Composite cores are available from a few mfrs, inluding some parts that are basically two toroids glued together. The most common application is in a swinging choke, used to reduce minimum continuous load current levels in power supply unregulated auxiliary outputs.

A part stressed over the full flux range (+/-) will show a change of permeability at fairly low flux levels - a monitored current waveform produced by a fixed alternating voltage will show 2 different current di/dt slopes, before saturation.

RL

I meant pure electronic solution by affecting the permeability of the core by the magnetic field of the external solenoid. This variant can have much better linearity and Q then attainable with varactors, which can be important for LO and input filters. AFAIK some early FM transmitters used this idea, too.

Speaking of the motor tuning:

IIRC this R-155 receiver has 8-section variable capacitor bank tuned by the electric motor. The input section is built using tubes (total of 6), and the rest of the schematics is made from the discrete BJTs and crystal filters. In the old times, they knew how to do the real stuff.

I put a notice on that. If there is PITA for whatever reason, the ballistol should help, is that right?

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

They sure did but they were blissfully ignorant when it came to power consumption. Power just came out the wall outlet and was free ;-)

According to the movie "My Big Fat Greek Wedding" Windex would be the answer. Very worthwhile to watch BTW, great fun.