My new pickup magnetizer

[Joe Gwinn]

And I also went the "brute force" path, no useful capacitors available in quantity here .

Do you have three phase power available?

[Rick Turner]

Joe, I do; I have tons...well, many amps...available @480 V or 208 V; I've used both on various machines in my shop.

How would that help given that you're slamming the magnetizer coils with DC? Is there an interesting rectifier circuit for 3 phase to big DC? And a circuit for capacitive discharge would be great.

[Claus H]

Schematic and mechanical drawing please

Just curious: what´s the frontal area of the bevelled adjustable ends of the yoke?

I hope it´s equivalent to (or smaller than) the 25 mm dia. rod area.

And, are you running 3A through those 0.4mm wires?

Rectified unfiltered 240VAC into thay coil which I calculate around 120 ohms would provide around 2A.

Adding supply caps would add some 50% more, if ripple is low.

Schematic

[Joe Gwinn]

Joe, I do; I have tons...well, many amps...available @480 V or 208 V; I've used both on various machines in my shop.

How would that help given that you're slamming the magnetizer coils with DC? Is there an interesting rectifier circuit for 3 phase to big DC? And a circuit for capacitive discharge would be great.

What comes out of a single-phase fullwave rectifier bridge is halfcycles at 120 Hz, which has a lot of harmonics, and will heat the iron and is somewhat self-defeating for a magnetizer. A filter capacitor helps a lot. Not that people don't just brute-force it and life goes on.

Juan said that it was hard to get big filter capacitors in Buenos Aires. Well, if you have three phase, a three-phase bridge rectifier bridge (which can be bought or made at home using six rectifier diodes) will make pretty clean DC all by itself, without the need for filter capacitors. I'd use 208 Vac, not 480 V (which requires all manner of special handling for safety) And make sure that the case is made of metal, and is solidly grounded to the safety (green) ground. If something shorts out, it's best if the breaker pops, not the user.


As for the capacitive discharge magnet charger, that's a very different kettle of fish. While I did publish the patent number of a good circuit, it came with a draconian warning. Just to be clear, a capacitive discharge magnet charger is quite capable of boiling the grease out of the unwary. There is a lot of energy stored in that capacitor bank. Small ones have the same stored energy as the defibrillators used in hospitals. This really does require someone with formal training in electronics to implement, and it isn't clear that there is much money to be saved by DIY approaches because most of the money is in big power components. I'd try to find a used unit instead.

[David Schwab]

Just to be clear, a capacitive discharge magnet charger is quite capable of boiling the grease out of the unwary.

That's quite a visual right there!

[Leo_Gnardo]

Just to be clear, a capacitive discharge magnet charger is quite capable of boiling the grease out of the unwary.

Like that turn of phrase Joe! With that in mind...

For cap discharge types would it not make sense to use film caps not electrolytic, also a "hockey puck" SCR or Triac as a switch? Lab upstairs from mine in college was trying to do some basic experiments in fusion @ 1975 - they invested in a truckload of surplus el caps, and turned their mechanical switch into vapor the first time they tried to run a pulse thru their magnet coils. The guy who operated the switch survived all right but couldn't hear for a week (sounded like a small cannon went off from my lab) and got splattered with red hot metal beads. Later they found a lot of the el caps going bad fast due to instant discharge. OK, a pickup magnetizer need only be 1/1000 as powerful but I'll never forget that debacle. Trying to think safety, plus a good service life for the caps.

[Joe Gwinn]

For cap discharge types would it not make sense to use film caps not electrolytic, also a "hockey puck" SCR or Triac as a switch?

For film caps, one cannot get as much capacitance, so one instead goes for voltage, as stored energy varies with the square of voltage. Commercial units use from 2,000 to 5,000 volts. The voltage is chosen by a tradeoff between the cost and capacitance of capacitors versus voltage.

For electrolytic caps, the max working voltage is something like 400 Volts (using caps rated for 450 Vdc), but one can get a lot of microfarads per dollar, but reverses voltage causes instant destruction.

Hocky-puck SCRs (not triacs) are the standard way to go, due to their surge capacity.

Lab upstairs from mine in college was trying to do some basic experiments in fusion @ 1975 - they invested in a truckload of surplus el caps, and turned their mechanical switch into vapor the first time they tried to run a pulse thru their magnet coils. The guy who operated the switch survived all right but couldn't hear for a week (sounded like a small cannon went off from my lab) and got splattered with red hot metal beads.

Glad no permanent damage. Sounds like some back of the envelope calculations were needed. Not to mention a series of tests starting at 5% voltage.

Later they found a lot of the el caps going bad fast due to instant discharge. OK, a pickup magnetizer need only be 1/1000 as powerful but I'll never forget that debacle. Trying to think safety, plus a good service life for the caps.

Yeah. There are a number of considerations, and risetime is one of them (too quick and eddy currents in the alnico prevent full magnet charging). As is peak surge current.

As is resonance: The charger is a big capacitor connected to a small coil (which generates the magnetic field), and so rings, the voltage on the big capacitor reversing periodically. Reversal will blow an electro immediately, and greatly shortens the lifetime of film caps if done at full power, so the charger circuit must prevent reversals. The patent whose number I published some time ago does just that, and is intended for electros.

[Rick Turner]

Joe, something like this?: 80A 80 Amp 3-Phase Three Phase Bridge Rectifier AC to DC

Or bigger?

I would imagine that I've got several of these inside my CNC machine...

[Joe Gwinn]

Joe, something like this?: 80A 80 Amp 3-Phase Three Phase Bridge Rectifier AC to DC

Or bigger?

I would imagine that I've got several of these inside my CNC machine...

That sounds like it will work, although no datasheet is provided.

It would be difficult to run at anything resembling 80 amps at 220 volts (17,600 watts) because the coil would melt. Actually, boiling copper would spray out the cooling vents.

Anyway, a proposed design is needed before any real evaluation is possible.


Do not try this at 480, even though the rectifier is rated for 1000 volts, because handling 480 requires real engineering knowledge to use safely.

Let me put it this way: Below about 300 volts, if an arc starts (say by a tin whisker), there will be a pop and the arc will self extinguish. Above, the arc will not self-extinguish, and the arc will continue until something external stops the drama. This has many consequences, one of which is that one does not have panel switches controlling 480 V, one has a full-size contactor controlling the 480, the contactor (running at 110 or lower) being controlled by a front-panel switch, all in a well-grounded stout steel box.

And then there is arc flash: Arc flash - Wikipedia, the free encyclopedia


More generally, one should follow the National Electrical Code (NEC), NFPA 70. Not to put too fine a point on it, but the NEC is written in blood.

War story from my Grandfather: There was an industrial concern where my Grandfather worked. A workman touched an electrical box, and was instantly electrocuted. The brass gathered to investigate, and one of the dead workman's fellows explained that what had happened was that the unfortunate fellow had only touched this box. Unfortunately, the second fellow demonstrated what the first fellow had done, and he too was struck dead. Two men down. Not a good day.

Root cause: In the old days, people believed that one could isolate the stuff inside from the box, and this would be good enough. There were many stories like this one, and eventually the NEC (or its predecessor) was changed to require that all electrical boxes be solidly grounded. This has worked far better.

[Rick Turner]

I'm pretty good at red wire to red wire, white to white, black to black, blue to blue, green to green, but I do not want molten copper anywhere near me! Obviously there has to be some current limiting built in there somewhere. I would imagine that the lower voltage would let the coil resistance (and inductance) work favorably to prevent a major mishap. I've got 208 available, too. I do like the idea of the smoother DC from 3 phase rectification. There must be timer switches that one could calibrate for something like this.

[Rick Turner]

Hmmm, a nice Dr. Frankenstein's Lab large three bladed knife switch would be utterly bitchin'! The bigger the better. Then make a nice Plexiglas cover for it... Tesla coil humming in the background... Or just discharge the Tesla coil into the pickup zapper coils.

[Rick Turner]

Here we go: Knife Switches by Filnor

[Leo_Gnardo]

Hmmm, a nice Dr. Frankenstein's Lab large three bladed knife switch would be utterly bitchin'! The bigger the better.

Love those old Frankenstein switches! So many have gone to the recycler, lotta copper there. But one might be practical for a reasonable size magnetizer, might be able to scour one up in the antique shops or ebay.

Yes important to get the colors right. One fine day in a South Bend theater the house 'lectrician wasn't too careful and swapped the blue and green. I was about to point out . . . when he threw the switch. Some serious welding went on, shooting BIG sparks. Woops. Maybe he had to replace a couple cam-locks but the show went on.

[J M Fahey]

Answering a few questions.

1) My home magnetizer is single phase.
As said before, 220V rectified (which amounts to some 180V "mean/average"? (in Spanish the same word "valor medio" applies to both, depending on context) ,25 Amperes.
No, no molten copper, even less flying droplets, in spite of the 220x25=5500W rating because it has a HUGE thermal mass (some 6 lb 2 mm copper wire) and duty cycle is less than 1% : 5 seconds magnetizing , batches of 10/15 speakers tops.
Of course, if I let it "on", it would probably catch fire in less than 15 minutes ... but that amounts to some 150 speakers or more ... not in my wildest dreams.
I was not referring to filter caps, magnetizers are the world's largest choke input power supplies with a huge time constant, measured in seconds.
Puny 100Hz ripple pales before that.
In fact, current buildup is above 2 or 3 seconds (hence the 5 seconds "on" time), then I can't pull the speaker after 5 or 6 seconds until the residual current lowers enough.
Yes, it's a big inductor.
I need *a ton* of capacitors not for filtering but for a capacitive discharge one.
The cap bank cabinet is small fridge size.

2) the three phase one lives at a friend's factory, who has 3x380V power.
Full wave rectified 380V has very little ripple, even if unfiltered.
I use a huge Siemens contactor, meant to start big motors, switching the **AC** side of things which is doable, switching DC would be close to impossible.
380Vx40A = 15KW but as before there's a lot of thermal mass, that machine has some 60 lbs copper tightly wound on some 400 lb core and duty cycle is also very low.
It has so much inductance that current rise is easily seen on a needle ammeter, so slow (say 5 or 6 seconds) that although max current is around 46A, we stop when the needle reaches 40A, both for uniformity and to minimize heating.

Both coils (home and factory) have flyback diodes whic make turning OFF much easier.
And both have been flawlessly in operation for 20/25 years, already lost count.

Now the next BIG one is scaring me, whether brute force or CD.

Although I'm mulling making it a different shape: instead of making the big monster (I actually need the same magnetizer as JBL or EV, magnets follow Physics Laws and couldn't care less I am a pygmy compared to them) which can magnetize a finished speaker inside the shipping box, while I'll probably make a closed box magnetizer, *only* for the magnetic assembly, no frame or anything else (this reduces needed power by 80% because of the much reduced magnetic path) and later bolt the pre-magnetized assembly to the frame.

[Joe Gwinn]

I'm pretty good at red wire to red wire, white to white, black to black, blue to blue, green to green, but I do not want molten copper anywhere near me! Obviously there has to be some current limiting built in there somewhere. I would imagine that the lower voltage would let the coil resistance (and inductance) work favorably to prevent a major mishap. I've got 208 available, too. I do like the idea of the smoother DC from 3 phase rectification. There must be timer switches that one could calibrate for something like this.

It's OK if you stick to 208 three phase. Things are a lot quieter. And an interval timer is a very good idea, for repeatability and equipment safety both.

But don't forget the grounded metal box.