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 Post subject: Antique speaker re-magnetizer
PostPosted: Feb Fri 17, 2017 2:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16984
Location: Warner Robins, GA
Would something like this be good to use for making a magnet re-magnetizer for antique speakers?

http://www.goldmine-elec-products.com/p ... ber=G17930

Maxwell 3000 farad 2.7VDC Boostcap(R)
Maximum power rating 3020 watts
Maximum stored energy 10944 joules (3.043WH).


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 Post subject: Re: Antique speaker re-magnetizer
PostPosted: Feb Fri 17, 2017 5:20 am 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 8945
Location: Santa Rosa, CA
I helped to repair a magnetizer in the late 1960s. The capacitor bank was under 1000 uF, but the voltage was higher... I think over 400 V. They used an ignitron to pulse the magnet coil.

It's all about ampere-turns.

Rich


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 Post subject: Re: Antique speaker re-magnetizer
PostPosted: Feb Fri 17, 2017 7:29 am 
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Location: In the village, 19115
The stored energy increases by the voltage squared, so higher voltages are generally used to get more bang for your buck.

_________________
I like things that light up, tubes, bulbs, LEDs, whatever, and things that make noise.


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 Post subject: Re: Antique speaker re-magnetizer
PostPosted: Feb Fri 17, 2017 12:49 pm 
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Location: Warner Robins, GA
Based on the data provided is there any way to figure out the ampere turns with for instance a 10 turn coil of wire?

Don't know if this is right, but if I take the cap voltage and the power and use ohms law I get 1,118A of instantaneous current.

Another issue I would have is ensuring the cap voltage never goes above 2.7Vdc.

At one point I used an EICO power supply/battery eliminator with 30,000uF of built in capacitance (added that myself) plus around 8,000uF of external capacitance and wound a coil around the magnet of my AK E3 speaker and it seemed to do a good job.

Wonder if for instance a larger external flash unit for a camera would be better to use with the coil connected in series with the flash tube or maybe just manually connecting the coil across the cap when it is charged in order to get a higher energy than what could be achieved with the flash tube in the circuit? I could then parallel several photoflash capacitors of the same voltage rating to increase the available power, although the high voltage would be quite hazardous with the available instantaneous current I would have.

Just looked on Mouser's website and I can get a brand new one for $58.44 versus the $44.95 Electronics Goldmine wants for a used one.

http://www.mouser.com/ds/2/257/Maxwell_ ... 341196.pdf


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 Post subject: Re: Antique speaker re-magnetizer
PostPosted: Feb Sat 18, 2017 10:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1998
Location: Saskatoon
As mentioned, you'd be better off with a higher voltage capacitor with lower capacitance. The amount of energy stored in a capacitor is:
E=0.5*C*V^2

Using that formula, you can work out which combination of capacitance and voltage will store the most energy.

Once you know how much energy is stored, then you can figure out the peak current in the inductor. If we assume for the time being, that there is no resistance in the circuit, then the peak current is determined completely by the energy and the coil inductance. The energy stored in an inductor is:
E=0.5*L*I^2

Combining the two formulas we get:

IPEAK = sqrt(C/L)*VI

where VI is the initial voltage on the capacitor.

With no resistance, the circuit is oscillatory, and peak current occurs at the exact instant that the voltage across the capacitor is zero (just before the current flow causes the capacitor to start recharging in the opposite direction).

Now, if you include circuit resistance and assume zero inductance, the peak current is simply:

IPEAK = VI/R

The true peak current will be less than the smaller of the above two IPEAK values. You can combine both IPEAK formulas in order to calculate the true peak value, but time now enters into the relationship, and it becomes a differential equation, which is a bit messy to solve.


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 Post subject: Re: Antique speaker re-magnetizer
PostPosted: Feb Mon 20, 2017 6:43 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Warner Robins, GA
Yes way too much math for me.

Suppose I could try to find a camera flash that is either stand alone or built into a camera (pretty sure my local flea market has several to choose from on any given day for a few dollars) and see how well that works, although I'd bypass the tube and do a direct short with a coil of wire or some other switching device that can handle very high currents. Maybe then I could parallel several photoflash capacitors for even more current, but may have to modify the driver circuit due to the increased charging current of the caps in parallel.


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