squeezing a field

Not a whole lot. In the near-field region, B obeys Laplace’s equation, which means among other things that the field falls off on the length scale of the gap, not of the whole inductor.

Cheers

Phil Hobbs

You are right. A similar part is 47 uH in free air, 44.6 mounted on a multilayer board, and 42.1 squeezed between the board and a big chunk of aluminum.

So it will work.

Somebody who knew what they were doing could model it in LTSpice. The adjacent metal-work is a poorly coupled shorted turn. Model your inductor as 47uH coil with 0.135 series resistance and 4pF of parallel capacitance, and model the metal as a coupled - perhaps 1nH single turn

- with perhaps 1% coupling and maybe a milliohm of resistance. The difference between your two test situations is going to be mainly the coupling you get - though the loop resistance of the shorted turn is going to be lower in the second case.

Assuming that it will work might be a touch optimistic.

Depending on how closely packed the parts were, you might gat adjacent parts acting as pole pieces. So space >> than body hight.

Assuming also that your metalwork is Al and not Fe.

RL

RL

plugging numbers pulled out of thin air into LTSpice is better that doing the actual measurement?

It is for people who don't actually work with real parts.

I wouldn't know how to use Spice to model mag field coupling into PCB copper planes, or into a big hunk of aluminum. And it's good to get away from a screen and do something real now and then. Drill holes, dremel, solder, measure, things like that.

These products will usually be a 6-layer PCB in an extruded aluminum box with a fat bottom flange. My perferred inductor just fits on the bottom of the board, assuming the solder spaces it off the board maybe

Peter Baxandall (of tone control and QUAD amplifier fame) claimed to use analogue computing to work out his designs i.e. He built prototypes and measured them.

You youngsters probably don't remember a time when there wasn't Spice.

I did some simulation in Basic-Plus, and it was a nuisance. My first PC sim program was Tatum labs ECA, which required a typed netlist. But it was pretty cool.

Obviously not. My point was that John Larkin could use LTSpice to see what his numbers actually meant.

Putting a chunk of metal next to a drum core inductor doesn't change the drum core inductor in any way - it just adds a shorted turn to the assembly, and messes up what a dumb inductance measuring device sees.

In this case, that dumb measuring device is John Larkin, who doesn't want to think about what is actually going on, as opposed to his inductance meter, which can't.

Actually, it is extremely useful for people who work with real parts and who want to know exactly what is going on. You can see stuff that is very hard to measure on real parts.

But he understood what he was measuring - a least most of the time. His revolutionary ideas about capacitor microphones and the patent application he made fell down when he found out about the Philips capacitative pressure gauges which had been exploiting the same principle for about a decade before.

This played out in the pages of Wireless World, between the first and second parts of a two part article. I don't think he mentioned the Philips pressure gauges, but I've got a 1954 reference to them in my

He was remarkably good, just not totally perfect.

His footnote reference to "squegging" in the 1959 class-D oscillator paper is another minor drop-off. He can't be blamed for it, but a super-hero might have done better.

Hey!!! Who are you calling a youngster?!!! :-)

I have a spreadsheet I wrote for calculating the relationships between resistance, capacitance, frequency and time constant (put in two and the others appear, put in three and the error% appears). It also gives dB loss below or above the 'cutoff' frequency. Some years ago I also made some lookup tables for combinations of 5% tolerance resistors in series and parallel.

Those and a pocket calculator are still the only 'computing' I use for design work.

If anything, that enhances his reputation rather than diminishing it, A single inventor working on a project in his spare time compared with a laboratory-full of specialists working full-time on commercial projects (if I remember rightly) one of which was almost certainly partly financed by the Ford Motor Company.

The articles were in WW Nov/Dec 1963. At the end he refers to two Dutch papers:

Philips Technical Review Vol9 Nr12 1947/48 pp357-363

Omroep-technische Mededelingen Feb15 1961

These are both describing to low-noise condenser microphones but he points out that they don't have some of the desirable features of his design.

That may have been the article on capacitive pressure guages for car engines in the Philips Technical Review. Their main problem was that the temperatures and pressures they were trying to measure gave a short diaphragm life if the diaphragm was thin enough to respond to the required frequency range with sufficient sensitivity.

The Philip's capacitative pressure gauges were scientific instruments, not mass market products, and if I could find them as graduate student, somebody whose day job was at the Royal Radar Establishment in Malvern should have been able to do that too. My direct boss at George Kent in Luton - Colin Hunter - had done his electronic appenticeship at the RRE under Peter Baxandall. He wasn't any kind of single inventor working on stuff in his spare time, but rather a full time expert exploiting the resources his job gave him to follow his interests outside of his day job.

Very likely. 1947/48 was pre-transistor. The planar process was invented around 1955, and Fairchild started selling cheap planar transistors around 1959. They revolutionised circuit design,and I latched onto that in 1965 as a graduate student in chemistry. By 1963 you could suddenly do a lot with planar transistors which hadn't been practical with earlier parts.

It wasn't. The references in my Ph.D. thesis are to J.J.Opstelten and N. Warmholtz, App.Sci.Res.Hague B4 page 329 (1955) and J.J.Opstelten, N. Warmholtz and J.J. Zaalberg van Zelst ibid B6 page

There may have been work on a product for the car market, but they weren't cheap enough for that.

Etched silicon diaphragms with strain gauge sensors in the diaphragm was the mass market product, and they came a lot later.

No surprise there, though I am a bit surprised that you would admit it in public.

Some of the resources there were quite impressive. When I visited a few times in the early 1980s the acoustic anechoic chamber was remarkably quiet. I think the background noise was around -10dB(A).

John

I wrote a PowerBasic program to find resistors in stock that can hit a target divider ratio and Thevenin impedance. That's in PowerBasic.

It saves a lot of time and helps minimize the number of BOM items.

I love Basic, but the code snobs hate it.

I wrote some other apps, like resonance calculators, filter design, unit conversions. I could post a link.

I also have scribbles about what can be done with one quad resistor pack.

I don't understand why you use the word 'admit'. I make my own tools to meet my particular requirements, there is nothing shameful about that.

Yes, some of those were developed for research into car engines.

As far as I know the were never intended to be consumer products for car users.

Not always as easy in those days. I found great difficulty getting any electronics information because the librarian 'filtered' research and inter-library loan requests and blocked any she decided weren't for my core research.

I was aware of that, but he wasn't working full-time on those projects and he wasn't backed ny a team of researchers. He was doing it single-handed in his spare time, even though a blind eye might have been turned when he was using 'the firms' equipment.

Quote: "This circuit was first successfully demonstrated in July 1959..."

He used SB240 Surface Barrier transistors in the RF part of the final (1963) version but 0C44s in the prototypes. Both versions used 0C45s in the audio stages.