hand making inductors

Several incarnations ago I made a few inductors for production test jigs. At that time, "Reference Data for Radio Engineers" and the ARRL "Radio Amateurs Handbook" included graphs, nomograms and formulas for air-core inductors. The folks who sold magnetic cores (e.g., Amidon, Fair-Rite, Micrometals, et al) were pretty good at publishing design aids for their particular products. Typically you had to add or remove a few turns from the calculated value to get the inductance you wanted, but once this was done the unit-to-unit repeatability was good if you took care to construct them identically. I suspect that design information is squirreled away on the 'net someplace, and I wouldn't be surprised if it has been implemented in online calculators.

If you don't have access to an LCR meter - or your LCR meter doesn't cover the range you need, or measures at a frequency far removed from the actual application - you can make reasonable inductance measurements by resonating the device with a known-value film capacitor, using your 'scope in X-Y mode to detect resonance.

Dale
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
At the accuracy required for output chokes, messing around adjusting inductors for precision better than 10% is really only gilding the lily. Other uses for inductors in solid state audio may be in small signal LR, LC or RLC filters but these became redundant with the development of opamps.

Considering that audio DIYs often modify output filters with choke increments of x2, x3 or a half or quarter value, it seems a waste of time and money precisely measuring a damped output choke any more than you need to accurately specify main rail smoothing caps. For typical output use, general coil winding guidelines from a trusted source will give more than adequate accuracy. From my perspective as a making one for each amplifier completed, and that's well over a hundred, I've never had a need to measure, even with repair work.

Curiosity and collecting a pile of instruments as an expression of our serious interest in electronics is another matter. Some of us like to do this and others don't understand circuit requirements and fret about things that aren't professionally made, labelled with certain values and with overkill levels of precision to aid our sense of security, too. Those are personal issues.

There are occasional designs like Quad power amplifiers, that use an RLC bridge for feed-forward error correction and this does require better precision than the simple, first order filters we usually refer to. Those, I would probably purchase with a kit or individually with the specified accuracy in any case, considering the likely one-off nature of the purchase and also the mounting requirements.

This little calculator is so easy and there is a multi-layer version on the site too: Pronine Electronics Design - Single-Layer Air Coil Calculator
 
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wintermute

Administrator
Paid Member
2003-08-03 11:43 am
Sydney
I like this site for working out air cored coils... --> Mark 2 - Inductor Sim

I was winding coils for crossovers, but should be still applicable. I found I needed to up the inductance 10% on what the calulator stated to get the right value, but I was using 14Gauge wire (which is hard not to get kinks in) so using thinner wire it may work out more accurate.

Tony.
 

Mark Johnson

Member
Paid Member
2011-05-27 3:27 pm
Silicon Valley
I agree that most audio applications of inductors don't require extremely precise or accurate values, at least in my personal experience. I also agree that textbook and web page formulas, tables, calculators, and nomograms usually give quite reasonable results that match real world measurements, at least in my personal experience.

Nevertheless I have given myself permission to purchase a couple of inductance measurement instruments. It's part of my DIY hobby enjoyment, collecting equipment. It's also deeply satisfying to wind a coil according to a formula, then measure it and find the value is spot on. (Here is an example) . And yes, it is a reassuring relief when practice and theory agree. Even the 100th time.

The final argument that won me over, and convinced me to buy an Agilent LCR meter right now, is that unmarked surface mount components are the wave of the future. Like it or not, thru-hole devices are slowly dying out. In some cases, quickly dying out.

So I bought an LCR meter with SMT tweezer probes; it immediately tells me whether I've picked up an inductor, capacitor, or resistor. It also tells me the component's value, either in henrys, farads, or ohms. Voila, this instrument is not a luxury, it's a necessity. BTW it cost less than a pair of Jimmy Choo's or Manolo Blahnik's.
 
Most amplifier output inductors are sized smaller than I would prefer... so it might advise to size the gauge of the wire up substantially.

The best way to size the inductor in an output zobel might be to look at the ringing when driving a capacitive load of appropriate value, and consider the impedance that the inductor creates vs. frequency since its purpose is to prevent the output section looking at a very low impedance at some rather high frequency...

_-_-bear
 

matze

Member
2012-09-12 5:45 pm
If you don't have access to an LCR meter - or your LCR meter doesn't cover the range you need, or measures at a frequency far removed from the actual application - you can make reasonable inductance measurements by resonating the device with a known-value film capacitor, using your 'scope in X-Y mode to detect resonance.

Dale
If you have a generator and a 'scope or AC meter available, measuring is the best. You can also form an L-C series resonance and feed it from the generator via a resistor. It is then easy to find the frequency with voltage minimum over L-C.

Matthias
 
Looking for voltage minimum or maximum are rather uncertain ways to detect resonance. I use a 'scope in X-Y mode to detect the frequency where voltage and current are in-phase. It's a much sharper measurement and, even with an inexpensive frequency counter, the overall inductance measurement has an uncertainty essentially the same as the uncertainty of the capacitor value.

Dale