The technology is pretty straight forward and simple, but I'm curious how much energy is actually required to magnetize a big woofer magnet?
Also, how much magnetic field strength is required to be produced by the magnetizer?
Or can one express it roughly with a thumb of rule, say the magnetic field generated by the magnetizer must be somewhere around "twice" (or whatever amount is needed) that of the magnetic field in the air gap of the speaker magnet in order to force the permanent magnets to become permanently fully magnetized, anyone informed enough on the subject able to tell about it?
Also, how much magnetic field strength is required to be produced by the magnetizer?
Or can one express it roughly with a thumb of rule, say the magnetic field generated by the magnetizer must be somewhere around "twice" (or whatever amount is needed) that of the magnetic field in the air gap of the speaker magnet in order to force the permanent magnets to become permanently fully magnetized, anyone informed enough on the subject able to tell about it?
For Ferrite/Ceramics the saturation field strength is 800000 Ampere-Turns per meter.
If you have a huge iron structure guiding magnetic lines, such as old style "C" shaped magnetizers, gap length is airspace between "C" ends.
One of mine is like that, there´s 15 centimeters between ends, which allows me to magnetize a finished 12" speaker inside its packing carton .
For a 15" one, I must magnetize it alone, frame up, sliding it in "around" the top pole.
For an 18" one (I make very very few of those) I must magnetize the magnetic circuit on its own, and later bolt it to a cast frame "outside" the magnetizer.
The fixture I showed partly envelops the coil inside an iron "cup" so you only put the magnetic circuit inside, no big deal about the frame, my "small" magnetizer is like that, and you are shortening magnetic field path by half.
sweetperfume´s is just the coil, so it has the longest effective path , the simplest but least efficient configuration.
In any case, if you have a beefy capacitor bank, no problem.
Turk means the coil is a BIG inductor running monster voltage and current.
Like on a relay coil, electric motor, etc. , you need a big flyback diode in parallel with it, or inductive kick can destroy the SCR or at least produce an uncontrollable peak.
In theory coil DC resistance can "critically damp" the pulse but thatv requires fine tuning.
The critical damping equation is simple, but we have unknown variables, specially coil inductance at real world conditions (measuring it in a bridge is not realistic) so a flyback diode provides peace of mind.
Diode must handle peak coil current with ease.
JMFahey
Can you share any details on your magnetizing fixture? What size wire is it wound with? How many turns? I am building a magnetizer, I have the capacitor bank almost done and need to start on making the magnetizing coil. Any info you can share will be very helpful.
Thanks
Very sorry oldaudioguy but it took me years to perfect it, burning many SCRs in the process, and as you must have found no practical data is available on the Net.
There´s only a few very crude tyro descriptions around, which seem to ignore Laws of Physics or give very poor results.
I can only go as far as reminding that "saturation flux is 800000 ampere turns per meter" , that distance to be covered is not only "visible air space" but also (what everybody dismisses) magnet itself is "air".
Ferrite has very low permeability, close to air, it´s not Alnico which is a sophisticated steel alloy.
You must also fully saturate the center polepiece, which otherwise is a low resistance path and shorts/bypasses your ferrite ring.
In short: calculate the real and full air path which is always longer than you think (hint, add 1/2 bobbin radius or 1/4 diameter to path length) , ferrite ring thickness and any wasted air space in between and increase desired Ampere Turn spec by at least 50% to be safe.
Otherwise you will "magnetize" bit not "saturate" , not even close.
This method works fine for brute force machines, for capacitive discharge it gets complicated (and needs some Calculus thrown in) because coil takes a time constant to increase current while at the same time capacitor bank discharges with its own time constant, which can be completely different, you must solve for the point where both curves cross, one exponentially rising, the other dropping.
It´s simpler if both have same time constant.
As I said, it took a lot calculating, correcting and experimenting plus burning stuff to reach good results.
Sorry again, and please notice Chinese sell Magnetizers and show the main fridge sized capacitor bank and an outside view of the magnetizing fixture but never ever show the actual coil or discuss its specs, not even basic DCR or Inductance.
And most DIY or surplus coils pictures show the same (i.e. "no details")
FWIW I almost travelled to USA to the Peavey Auction of 2018 specifically to buy the one they used for Black Widow speakers, a beast capable of 250mm diameter magnets ... but I knew about that auction 15 days late (when a customer showed it to me) and it had already been sold
There´s only a few very crude tyro descriptions around, which seem to ignore Laws of Physics or give very poor results.
I can only go as far as reminding that "saturation flux is 800000 ampere turns per meter" , that distance to be covered is not only "visible air space" but also (what everybody dismisses) magnet itself is "air".
Ferrite has very low permeability, close to air, it´s not Alnico which is a sophisticated steel alloy.
You must also fully saturate the center polepiece, which otherwise is a low resistance path and shorts/bypasses your ferrite ring.
In short: calculate the real and full air path which is always longer than you think (hint, add 1/2 bobbin radius or 1/4 diameter to path length) , ferrite ring thickness and any wasted air space in between and increase desired Ampere Turn spec by at least 50% to be safe.
Otherwise you will "magnetize" bit not "saturate" , not even close.
This method works fine for brute force machines, for capacitive discharge it gets complicated (and needs some Calculus thrown in) because coil takes a time constant to increase current while at the same time capacitor bank discharges with its own time constant, which can be completely different, you must solve for the point where both curves cross, one exponentially rising, the other dropping.
It´s simpler if both have same time constant.
As I said, it took a lot calculating, correcting and experimenting plus burning stuff to reach good results.
Sorry again, and please notice Chinese sell Magnetizers and show the main fridge sized capacitor bank and an outside view of the magnetizing fixture but never ever show the actual coil or discuss its specs, not even basic DCR or Inductance.
And most DIY or surplus coils pictures show the same (i.e. "no details")
FWIW I almost travelled to USA to the Peavey Auction of 2018 specifically to buy the one they used for Black Widow speakers, a beast capable of 250mm diameter magnets ... but I knew about that auction 15 days late (when a customer showed it to me) and it had already been sold
Here's a magnetizer from Rod Elliott's audio site, though it's for an "anlernative" purpose and doesn't mention speaker magnets:
https://sound-au.com/clocks/magnet-charger.html
https://sound-au.com/clocks/magnet-charger.html
Sorry for this, but no one should go asking for the design elements to it’s core for building anything. What here should be discussed should be about how things work. Not actual things.
For magnetizers there is little to no info available anywhere.
please keep in mind the people here have invested a lot of efforts and money for getting results and this info will not given out no matter what.
Just ask for something that have you kept stuck at it with just principles and not actual implementation information.
For magnetizers there is little to no info available anywhere.
please keep in mind the people here have invested a lot of efforts and money for getting results and this info will not given out no matter what.
Just ask for something that have you kept stuck at it with just principles and not actual implementation information.
No problem. I will work it out. I find it interesting that you assumed I would be magnetizing ferrite speaker magnets. Not my plan at all. Too bad about Peavey--I did lots of work for them over a few decades.Very sorry oldaudioguy but it took me years to perfect it, burning many SCRs in the process, and as you must have found no practical data is available on the Net.
There´s only a few very crude tyro descriptions around, which seem to ignore Laws of Physics or give very poor results.
I can only go as far as reminding that "saturation flux is 800000 ampere turns per meter" , that distance to be covered is not only "visible air space" but also (what everybody dismisses) magnet itself is "air".
Ferrite has very low permeability, close to air, it´s not Alnico which is a sophisticated steel alloy.
You must also fully saturate the center polepiece, which otherwise is a low resistance path and shorts/bypasses your ferrite ring.
In short: calculate the real and full air path which is always longer than you think (hint, add 1/2 bobbin radius or 1/4 diameter to path length) , ferrite ring thickness and any wasted air space in between and increase desired Ampere Turn spec by at least 50% to be safe.
Otherwise you will "magnetize" bit not "saturate" , not even close.
This method works fine for brute force machines, for capacitive discharge it gets complicated (and needs some Calculus thrown in) because coil takes a time constant to increase current while at the same time capacitor bank discharges with its own time constant, which can be completely different, you must solve for the point where both curves cross, one exponentially rising, the other dropping.
It´s simpler if both have same time constant.
As I said, it took a lot calculating, correcting and experimenting plus burning stuff to reach good results.
Sorry again, and please notice Chinese sell Magnetizers and show the main fridge sized capacitor bank and an outside view of the magnetizing fixture but never ever show the actual coil or discuss its specs, not even basic DCR or Inductance.
And most DIY or surplus coils pictures show the same (i.e. "no details")
FWIW I almost travelled to USA to the Peavey Auction of 2018 specifically to buy the one they used for Black Widow speakers, a beast capable of 250mm diameter magnets ... but I knew about that auction 15 days late (when a customer showed it to me) and it had already been sold
Sorry but 150-300 Amp thyristor is too small.
Look at Chinese magnetizers data I link below ad they mention 15000 to 40000 A output capability.
Admittedly those are millisecond wide pulses, but even so....
http://www.vatmag.com/products_28/364.html
Look at datasheets, "official" spec is continuous duty, such as handling electric motors, etc. , all have a much higher single pulse rating which applies here, as long as you allow for, say, 30 seconds or more per speaker magnetization, in any case derate at least to half that peak, basic "good Engineering practice".
That is a very good and complete manual for an excellent magnetizer.
But it´s a Lab/University type machine, capable of small magnet charging, designed for testing samples , say a thumb sized rod, a small Alnico or Ferrite cube , "up to" a Magnetron magnet.
Power is 440 Joules and it fits on a bench top.
Commercial loudspeaker magnetizers I linked above go from 4500 Joule (10 times larger than the Lab/Bench type) to mind boggling 90kJ
FWIW I am using around 6kJ and it works fine for up to 156mm*20mm Guitar speaker magnets, think Celestion Vintage 30 or G12H
That said, a "large" machine is basically the same as the RFL440 , only scaled up 10-15X or more.
But exact same Physics behind it.
Functional and simple design, like all of Elliott´s
Does work, of course, almost same as RFL440, and somewhat higher energy storage:560 Joule.
Still in the experimenter´s area size, not industrial.
Again, same Physics apply.
That, a modern update covering Rare Earth magnets.
Which are hard to magnetize: Ferrite is roughly 3X as hard as Alnico ; rare Earth ones 3 to 4 times as hard as Ferrite.
FWIW I can NOT magnetize them, should build a new one when/if required.
My rule of thumb (staright from Philips manuals) is 800kAT/m ; they cover newer ferrites so suggest:
| Ceramic (Hard Ferrite) | 10,000 – 12,000 | 796 – 955 |
Science is freely available in the Net and at University Libraries, all kinds of Electro Magnetism books are available, plus a few dedicated ones, I mentioned Scientific and Engineering values many times, now fine details on a Commercial product ... different thing.
My own "Bible" is late 60´s early 70´s Philips Eindhoven books about magnetic materials they manufactured: one is dedicated to magnetic and Piezoelectric materials, go figure; the other is "Permanent Magnets and their applications" , covering their trademarked Alnico: "Alcomax" and Ferrites :"Ferroxdure"
There is a third relevant book, about "Loudspeakers", not covering magnet design but showing commercial examples, which to me are practical "reality checks".
There is a widespread "snobbish" attitude in DIY circles: "Commercial=cheap-bad ; DIY/esoteric= GOOD!"
I follow quite the contrary: Industrial-Commercial must be quite good (at its price range of course) or it simply will not sell. period.
Or it will temporarily sell, based on false promises, and then go the way of the Dodo.
Open market is literally a cutthroat jungle.
IF Science and Engineering serve to get a competitive edge over competition, it is suicidal to freely spread it around in exchange of nothing.
Notice the Chinese will sell you anything and for a (relatively) good price, but they NEVER EVER spread design details, except in vague words used in marketing.
Discharge power supplies can be designed, even simulated in LTSpice as mentioned above .... magnetizing yokes-fixtures not that much (except the very crude very simple "first principles" ones used in a Lab/University setting):
http://www.vatmag.com/products_50.html
That said, a clever and dedicated experimenter such as oldaudioguy will certainly succeed.
But he´ll have to rewind yokes many times (notice RFL440 instructions are vague , as in "20 to 200 Turns", etc. nd burn a few SCRs ... let alone exploding capacitors, dynamite sticks in all except colour.
Important/safety hint: if using Electrolytics (most do), avoid by all means getting a reverse polarity back pulse into them.
Monster inductors (what yokes are) can also store huge energy and be hard to turn off (same as Power Supply chokes do, and for the exact same reason); one of my early "large" machines , capable of 150mm magnets and weighing some 250kg of iron and 80 kG copper, was fed from a 6 diode bridge out of 3x 380V line (our Mains dual purpose standard: 3 x 380V Phase to Phase "Triangle" - 3 x 220V Phase to Ground "Star" so same line feeds Homes (220V single phase) and shops-small Factories:three phase 380V), I got the equivalent of 550-560Vdc, no filtering needed. just 50Hz waveforms overlapping.
Problem was since I still didn´t have a proper switch but was itching to test it (wouldn´t you?) , I used a trick I found in a textbook: I suspended a piece of wire between two posts, and calculated it to blow on its own after a few seconds, it was the old trick o0f using an underrated fuse wire as an auto-off switch.
It DID blow .... that part "worked" ... the problem was that an arc jumped terminal to terminal and kept conduction (my single "switch" was obviously on the DC side) feeding a monster over 300kg inductor which has its own ideas about being switched OFF
I had to RUN to the street mains pole and one by one pull the big fuses feeding not only my own shop, but others sharing the premises (an old semi abandoned Industrial building),drawing BIG sparks from each of them.
Oh well.
PS:
Ok, I mentioned them, all the time because I make ferrite magnet speakers ; have no clue (at last no experience) on rare Earth Magnets.
For the near future, no need either.
Nor experience on electric motors, hard disk head positioning, alternators, generators, not even on fridge type publicity magnets, all of which are legitimate fields, of course..
Yet feel confident that IF I had to design a junkyard type ferrous materials pulling electromagnet would be able to
Whoops - switching high power DC is a completely different ballgame to AC - DC contactors are complex pieces of equipment, which are carefully designed not to be arc-lights!!
Just a thought - if you are making your own magnetizer I'd suggest adding a warning sticker about it not being safe for pacemakers.
Just a thought - if you are making your own magnetizer I'd suggest adding a warning sticker about it not being safe for pacemakers.
Revisiting this thread I am realizing with regards to the sheer size of those speaker magnetizers available that this is not an ordinary level of a DIY project to realize for a few DIY speaker projects, imagining here having one of these in my home.. hah
But nonetheless these manly gears are intriguing stuff, and after having watched through some more material with regards to their innards, they way of use.. one question arise in my head, why are most speaker magnets magnetized through such a relatively large air gap instead of attaching both pole sides with a magnetic field conducting steel clamp, I can't really fathom that, air has such a poor permeability, Fahey and other experts on this topic what's up with that??
But nonetheless these manly gears are intriguing stuff, and after having watched through some more material with regards to their innards, they way of use.. one question arise in my head, why are most speaker magnets magnetized through such a relatively large air gap instead of attaching both pole sides with a magnetic field conducting steel clamp, I can't really fathom that, air has such a poor permeability, Fahey and other experts on this topic what's up with that??