The squeal of a newborn inductor - Page 1
Forum section: electronics / general

author: Artlav
author_rank: Frequent Contributor
timestamp: December 31, 2014, 01:22:05 pm
content: Consider the humble hand-wound inductor, precariously situated on top of a buck converter.32x16x8 ring of soft ferrite, 40 turns of 1mm diameter wire for 100uH and 6A of current.You might be wondering why there is a bit of tape on it.Some might know that newborn scream bloody murder on the first moments in the living land.Hearing the same from a newborn inductor was not something i expected.But scream it did.Squeal, whine and sputter.I even thought i screwed something up, and the circuit was about to oscillate the smoke out of itself and take the PSU with it.But nothing seemed amiss, so i tried to comfort the newborn beast by giving it a hug with my fingers.So soothed it went completely silent, and the power consumption reduced somewhat.Further examination showed, that not only the thing was making sounds, it also introduced noise into the current flowing through it.There was a sine wave superimposed.Some sort of a mechanical resonance, or maybe just vibrating magnetic fields.A bit of tape was added to secure the windings at a random spot, and all was suddenly still and quiet in both planes of being.I wonder what it was.Did a manage to hit some sort of a resonance mode between the vibration of unsecured coils and the current's frequency?What is the exact mechanism that forms the superimposed sine wave?I gather it can be the changes in inductance, but there does not seem to be enough of a play to change it that much.And if it's electrical, then where is it's C counterpart? It would take 60nF to hit the right frequency.Most importantly, is wrapping the whole thing in tape a valid solution?

author: robrenz
author_rank: Super Contributor
timestamp: December 31, 2014, 01:59:49 pm
content: Just guessing,(machinist here) the mechanical vibration is causing a secondary induced voltage by physical motion of the coil in the electrically varying flux field.Edit: The way that inductor is supported on the thin neck of twisted wire is lowering the mechanical resonant frequency of the inductor. The electrical frequency must have been close enough to a multiple or sub multiple of the mechanical resonant frequency to cause the "scream". Touching the inductor or adding any mass is going to change the mechanical resonant frequency. Once the mechanical resonant frequency is far enough away from the electrical excitation frequency the "scream will stop."Sounds plausible, I hope its correct

author: colecaz
author_rank: Newbie
timestamp: December 31, 2014, 09:38:56 pm
content: And now you know why transformers are dipped in lacquer or varnish. It's the magnet fields causing the wires to vibrate in time with the current through them and make a mechanical noise from hitting things around them. When you put your fingers on the windings you dampened the vibrations.

author: tautech
author_rank: Super Contributor
timestamp: December 31, 2014, 09:48:13 pm
content: It would be interesting to see the inductor input current waveform also for additional understanding.V & I on the same screen shot.Guess you where just unlucky with turns #, wire gauge and ferrite choice.Build another to attempt to replicate the squeal.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 01, 2015, 12:26:50 am
content: Two lessons learned:1. Ungapped ferrite is terrible for inductors!2. Ferrite ismagnetostrictive, some varieties moreso than others.Magnetostriction means a change in the dimensions (a strain) due to a magnetic field. The response goes as B^2, so it is unidirectional (it only shrinks (or expands), for either polarity). This is analogous to electrostriction (take a guess), but is different from piezoelectricity (which is polarized and the strain has the same sign as the electric field).Since it's a B^2 thing, it has a frequency doubling effect. Hence if you record the hum from 50/60Hz power transformers, you will find that it's 100/120Hz dominant.The effect seen here is, magnetostriction causes the core to expand. It expands uniformly, which can be seen as a superposition of many acoustic resonances propagating in both directions (clockwise and anti- around the ring). Intuitively, I don't see that this should cause a problem -- but, if there happens to be just enough nonuniformity to prefer a standing wave, it will develop as the difference between the others. Possibly there could be a feedback mechanism where the motion of resonance affects the strain of the effect, I don't know.Is this dangerous -- more than just a nuisance? Absolutely! Some manufacturers warn against operating cores at their resonant frequency (I think EPCOS/TDK sometimes give this warning??). On one occasion, I was testing an ungapped ferrite core in a similar (pulsed to saturation) fashion, and as I adjusted the frequency, *tink*... the thing suddenly popped into about a dozen small chunks of ferrite! Broken evenly in pieces, due to the standing wave!Also as you can see, magnetostriction is of course more than a one-way street; it couples magnetic and mechanical forces. Thus, ferrites can be used as transducers, just as piezo materials can. Special mixes have been formulated to exaggerate the effect, and can occasionally be found in shapes suitable for ultrasonic drivers, sonar, etc.Tim

author: Refrigerator
author_rank: Super Contributor
timestamp: January 01, 2015, 12:37:37 am
content: ^Nice !I've had a couple joule thief circuits squeal, i've always thought that it was due to the windings being too loose and making them vibrate.

author: Artlav
author_rank: Frequent Contributor
timestamp: January 01, 2015, 11:57:01 pm
content: Quote from: robrenz on December 31, 2014, 01:59:49 pmActually, that point seems to be the main source of the sound.It is also the location where the winding is the most loose, so it can be just that.Quote from: tautech on December 31, 2014, 09:48:13 pmJust PWM - when it's rising it got 24 volts across it, when falling - nothing.2 Ohm load, no capacitors.Quote from: tautech on December 31, 2014, 09:48:13 pmDid that.Same thing.Wrapped in tape - same no thing.Not a fluke.Quote from: T3sl4co1l on January 01, 2015, 12:26:50 amHm, what exactly is gapped ferrite in case of a ring?Would one with permeability of 50 (instead of 2200 for N87) be "gapped"?And yes, at first i tried a regular N87 ring of the same size and 10 turns. 1000uH of inductance, but it saturated pretty much at once.I'm still trying to wrap my head around how to pick the right magnetic material and it's size.Permeability is the only parameter specified, and yet 3 turns on 2200 material is not the same as 40 turns on 50 material of the same size, even if the inductance is the same.Just doing the math gives me permeability of no more than 70 for the given ring size, if i want 6A, and thus 41 turn on a material of 50.But what does the mathmeanstill eludes me.Quote from: T3sl4co1l on January 01, 2015, 12:26:50 amI gotta try this.Might also make a nice video...Quote from: T3sl4co1l on January 01, 2015, 12:26:50 amMakes me wonder how big a grain of truth is behind audiophoolery's claims for their 5-digit-price coupling transformers and filters.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 02, 2015, 01:53:14 am
content: Quote from: Artlav on January 01, 2015, 11:57:01 pmYes... sort of. But ferrites aren't common in that range -- you'd be looking at #61 or #67 NiZn ferrites, which are expensive, have low Bmax and very low hysteresis/eddy current losses: they're intended for very high frequencies.The fundamental problem is, energy density goes as 1/mu. So hi-mu cores are great for transformers, which ideally store no energy, and terrible for inductors, which must (note the distinction between transformers and coupled inductors, which also have multiple windings).Air having the lowest mu, also has the highest energy density; but it's hard to couple directly into air when we are limited to the use of copper wire, so it is most efficient (by size, weight, cost and efficiency) to use a permeable core with an average mu of 10-100. (Average meaning, if the core is gapped, you can find the effective permeability assuming the length is the same: mu_eff = l_e / (l_e / mu_r + l_g), so that a core with l_e = 100mm and mu_r = 2000 with l_g = 0.5mm air gap (the gap being mu_r = 1), has mu_eff = 181.)QuoteIn fact, it will saturate in:t = Bmax*N*A_e / Vso if, starting from zero current, you apply a square wave pulse of 10V, and it has Ae = 100mm^2 and 10 turns, a ferrite with Bmax = 0.3-0.4T will saturate in 30-40us. (Note that 100 mm^2 = 100u (m^2), so SI units work out very handily when using both us and mm prefixes.)In that time, the current won't rise very far. If your application requires pushing a current through that poor inductor (like a current mode buck converter), you'll have some problems...QuoteIf nothing else, the Maxwell stress Pm (literally, the pressure due to a magnetic field -- the attraction or pressure you feel between two magnets) is also the energy density (because Pa == N / m^2 == J / m^3!), and since Pm = B^2 / (2 * mu_r * mu_0), you want as low an  permeability as practical.Another way to put it: if you're eyeing a particular ferrite core (E-E or whatever, say), you know you need at least this amount of air gap (setting mu_r = 1 and finding v_g = E/Pm, and then l_g = v_g / A_e) to store the energy you want to put into the inductor. Of course, the energy is given by E = 0.5 * L * I^2. You would finally set number of turns based on inductivity (A_L) for that core, at that gap, and the desired inductance.Or, you can do it by flux, because inductance is the conversion factor between flux (the volts-for-how-long you applied to the winding) and current (how much the current changed during that time). If you need 2A at 10uH, that's 20uWb == 20uVs. This exactly fixes how many turns are required for a given core, regardless of gap: Bmax, V and (t or 1/F) are connected, absolutely independent of gap or mu. Finally, once you know the core and number of turns, you set the gap based on the inductance required.For cores of fixed permeability, you don't have the added degree of freedom (adjustable gap), so you really just run through a parts listing and find one with enough volume to store the energy, that's also big enough to put on the turns required for your inductance. Material choice is based on frequency and losses (#26 or #52 powdered iron is... just about useless for anything really.. #8 and #35 are pretty good for most purposes, #2 for high frequency and Q.. and other materials like MPP, Kool-Mu, etc. that have varying properties).QuoteThe transformers are usually made with high-mu materials like 50/50 NiFe, permalloy or supermalloy, which are of course fairly expensive, in and of themselves. That would almost justify the $20 price tag. Special winding arrangements (interleaving, bank winding) to reduce leakage inductance and stray capacitance might even justify a $40 price tag.Beyond that, it's allllllllll marketing.As many buzzwords as you can throw at it: silver wires, gold wires, sonic clarity, soundstage, presence, etc.And for filters, you can make reasonable low distortion coils from gapped iron, gapped ferrite, or just go naked (air core), which will similarly increase the price in a relatively marginal way, but anything beyond that... you know the Dave Jones buzzword.Tim

author: GonzoTheGreat
author_rank: Regular Contributor
timestamp: November 21, 2017, 05:59:06 pm
content: Quote from: T3sl4co1l on January 02, 2015, 01:53:14 amI though that the energy stored by an inductor is proportional to its inductance because E=0.5*L*i^2...and the inductance is higher when mu is higher, because e.g. 10 turns of wire wound around a high mu core has a higher inductance than 10 turns of wire wound around a low mu core (such as air).So it seems to me that for the same current and the same number of turns, the inductor with a higher mu core will have more energy stored in it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 21, 2017, 06:38:41 pm
content: Quote from: GonzoTheGreat on November 21, 2017, 05:59:06 pmIndeed it is, but you can't get the same current (at the same inductance) in an inductor with less air gap.Note that the energy equation is true when L is constant. When L is variable (dependent on I), you can't use that anymore -- instead, you must integrate V dI as the inductor is charged. The intuitive effect is this: while the inductance is large, current rises slowly, and not much energy is stored; then the current rate suddenly increases, as the inductor saturates, and it is no longer its nominal value, but much less, and this saturated value therefore stores much less energy for a given current.Example: suppose a 1mH inductor saturates at 1A, and has 10uH above that current. Its energy storage at 1A is 0.5*(1mH)*(1A)^2 = 0.5mJ. It's not usable in the 10uH range, because of nonlinearity.Suppose we air gap the inductor, so that it has 100uH up to 10A, and 10uH above there. Now it stores 5mJ at saturation, and is usable at higher currents.Tim

author: TerraHertz
author_rank: Super Contributor
timestamp: November 22, 2017, 09:13:23 am
content: Very interesting and informative thread. Much inductor-foo teaching. For one thing I had no idea ferrite was magneto-strictive, let alone that you can make it shatter like that. Now I want to try doing that.Artlav, you twisted the inductor end wires together tightly? Youreallytrust the enamel that much?Also... not sure I should admit this. Am I (probably) the only one who didn't realize (till just now) that T3sl4co1l is Tesla Coil?Ever since I first saw that user name I've been wondering what it meant. Doh.Someday I'll work out what the '7' is about. Don't rush me.Edit to add: Annnd, I just asked a question about a nearly 3 year old post. Oh boy.