continued...(wouldn't fit in the 2000 character limit for a single post):
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From: R.G. (keen@******.com)
Date: 8/10/2000 3:51 AM
Subject: Re: wah inductor
How much variation is there with drive level? Especially *below* saturation? Are we talking about minus 20% or more?
Depends entirely on the core material's BH curve and what portion of the BH curve you transit. Minor loops are generally lower effective permeability than major loops.
Inductance is also smaller for some materials at low levels of excitation. It's that way for output transformer laminates.
BH curves are just that - curves. The incremental inductance varies at each point. Some materials have very high slopes, but saturate abruptly, so there's hardly any change until saturation hits with a bang. These "square loop" materials are the ones to have for things like magnetic amplifiers, because the "gain" you get by forcing the cores around the knee of the saturation loop is very large.
Linear ferrites and irons have a more gradual slope change, and it changes all the way up.
Is there an easy way to tell at what level the core goes into saturation?
For linear materials, no. You just have to pick a place where the BH slope has changed by an amount you decide indicates the onset of saturation. For square loop materials, it's obvious when you hit saturation, of course.
You can do some experimental tests for a given inductor and saturation by the pulse-inductance test. Feed the inductor a square pulse of voltage, and watch the slope of the current as it rises. Since V=Ldi/dt, The incremental inductance at each point is proportional to the slope of the current ramp. When the ramp departs from approximately linear and starts to bend upwards toward infinity, you're seeing the reduction in incremental inductance that indicates an increasing bend toward saturation. Using a good pulse generator and starting with a very short pulse and low duty cycle lets you control what happens to the inductor easily.
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From: Robert Strand (rstrand@********.au)
Date: 8/10/2000 4:59 PM
Subject: Re: wah inductor
As RG said, the saturation issue depends on a lot of factors but I'd just to like tie this issue in with what I mention before about gapped cores. When you put a gap in the core a good deal of the variations you get with closed cores goes away and the inductance is much more predictable and varies _much_ less with drive levels. That's why most inductors use gaps. However, if you want to use the saturation for distortion purposes you are face to face with all the variations again, a very small gap is sort of a middle ground but may be quite hard to saturate in circuits like wha's.
(Ha, I mistakenly said nano-henries per turn instead of turn square in my earlier post. The brain thinks one thing and the hands type another.....)
Regards
Rob
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From: R.G.
Date: 8/10/2000 6:23 PM
Subject: Re: wah inductor
It's nice to find someone else who understands inductors. Your comments speak of a fair amount of experience. Power supply design?
Near as I've been able to tell the "magic" in old Vox wahs is largely in the inductor taking a magnetic offset. I was floored when I measured that, but it's apparently real.
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From: Robert Strand (rstrand@********.au)
Date: 8/11/2000 5:11 AM
Subject: Re: wah inductor
It's mostly power supply stuff, but I have done small transformers, some RF stuff and loudspeaker crossovers. Facinating little beasties, they look so simple yet there are so many factors involved.
I read the comments about the Vox wahs on your site some time ago. What you said made a lot of sense and the offset would in fact be real. I'm still suspicious that the origin of the core magnetization is due the DC bias current through the inductor, after all it's a closed core pushed into saturation. One possible way to resolve the dilemma would be to find two units where the inductor was wired in reverse, if the bias is consistent with the current direction that would edge towards the DC current theory. It's quite a mystery. It's also a mystery whether the core material was chosen deliberately or fortuitously. I'm still guessing.......
Regards
Rob
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From: R.G. (keen@******.com)
Date: 8/11/2000 2:27 PM
Subject: Re: wah inductor
It's mostly power supply stuff, but I have done small transformers, some RF stuff and loudspeaker crossovers. Facinating little beasties, they look so simple yet there are so many factors involved.
Yep, that fits. Once you get over the hump of understanding, it's fun to try to produce something really tightly integrated where the side effects actually help you. We once did a transformer for a switching power supply on 3-leg 3-phase laminations that had drive and outputs on all legs but was driven from primaries on both the outside legs and had secondaries on all three legs. We could get both forward conversion and flyback conversion from the gapped center leg, so we had an output that had no gaps in the output - one or more secondary was always pumping the output. Closest thing to transforming DC I've seen yet!
I read the comments about the Vox wahs on your site some time ago. What you said made a lot of sense and the offset would in fact be real. I'm still suspicious that the origin of the core magnetization is due the DC bias current through the inductor, after all it's a closed core pushed into saturation.
That was a pure guess. The "saturation" was not anything you could see by watching the waveform. However, the even harmonics would dance right up on the spectrum analyzer for the older vox inductor and there'd be no sign of them on the newer inductor until long after the odd harmonics were very prominent.
The DC currents seem too small, but there wasn't any other good explanation except the cores coming from the maker with a magnetic offset.
One possible way to resolve the dilemma would be to find two units where the inductor was wired in reverse, if the bias is consistent with the current direction that would edge towards the DC current theory. It's quite a mystery. It's also a mystery whether the core material was chosen deliberately or fortuitously. I'm still guessing.......
Yep. I did get somewhat similar results with a second DC bias winding to induce an offset, but the effect was still not quite the same. I suspect that the original cores were just magnetically harder, so the externally induced offset couldn't fake the shape of the BH curve of the original material.
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From: Bart (sterling@*******.net)
Date: 8/12/2000 4:09 PM
Subject: Re: wah inductor
RG & Rob,
Any possibility the "brass" screw through the core could be the source of the offset? Purity of alloys in the hardware business occurs to me as another suspect.
Thanks for your work and thoughts in this area.
Bart
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From: R.G.
Date: 8/12/2000 4:37 PM
Subject: Re: wah inductor
Could be if it's somehow ferromagnetic, of course. I'll have to analyze that next time I get an original inductor to mess with.
. All that fiddly stuff gives me small cringes.
