Looking for easy circuit to handle high amps to make a strong magnet stronger.

Thread Starter

Darkstar

Joined Sep 3, 2010
177
I have a 13 pound solid iron(?) "C" magnet that used to be a precision industrial magnet used in chemical research. One leg is weaker than the other which is probably why it was discarded. I was told the max field strength for this kind of magnet was about 11,000 Gauss. When it was discarded I measured the strength as 6,500 Gauss. That was 40 years ago. I'm sure it's less than half what it could be and I'd like to make it stronger. I was thinking of using a car battery to run a few hundred amps through a coil a few times to strengthen each leg of this magnet. I'm looking for some guidance to be on the safe side. Not to mention I'm concerned about the flyback effect when I open the circuit. Normally a diode can shunt the reverse current but I don't have any 500 amp diodes, so how would I prevent the reverse current from un-doing what the forward current did? I'm sure I'd be overlooking something if I tried to do this without asking for some advice. I don't know how to determine the field strength I'd need to generate with the coil I made. The magnet appears to be solid iron but appearances can be deceiving. I haven't got any means of measuring field strength directly. The way I do it now is to measure the time it takes for a strip of metal to drop between the poles, while being slowed by eddy currents, Lenz's Law. It's just a relative comparison. Even so, I don't know if it is even possible to calculate the field strength needed to give the results I see. Anybody have any ideas?
Thanks.
 

Sensacell

Joined Jun 19, 2012
2,837
The current will not reverse, the voltage will rise to sustain the current in the same direction.

Just enjoy the big arcs.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
The current will not reverse, the voltage will rise to sustain the current in the same direction.

Just enjoy the big arcs.
In diagrams I've seen, current flows in one direction and builds the coil's field. When the circuit is opened the field collapses, the polarity of the voltage and current reverses and the voltage can spike from the sudden change to many times the input voltage.
 
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crutschow

Joined Mar 14, 2008
27,241
In diagrams I've seen, current flows in one direction and builds the coil's field. When the circuit is opened the field collapses, the polarity reverses and the voltage can spike from the sudden change to many times the input voltage.
Yes you get a reverse voltage spike, but that's from the inductance trying to maintain the current flow.
The current never reverses.
The mechanical analog of inductance is inertia.
 

schmitt trigger

Joined Jul 12, 2010
441
What I have seen done in the magnetization process, is once that you have reached the maximum current, a high wattage low ohm resistor is placed in parallel with the coil.
When the current supply is interrupted, the stored inductive energy has a means of being safely dissipated.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
Why not reduce the current to zero gradually instead of a break? No High Voltage. No arcs to enjoy.
I actually did think of that. I've used water ballast when making carbon arcs when I was a kid but the current was 20 amps at most. I can't help but wonder if it would be so easy as pulling 2 electrodes apart under water. The water could boil explosively from the current arcing in it and splash me with hot water while I'm holding the wires. I could hold the electrodes by long PVC pipes for distance and insulation.

That's the only other idea I've had.

I have 1/4'' copper tubing to use for the coil and I could estimate the inductance and resulting magnetic field strength using various online calculators. But it seems like it will be trial and error to find the ideal number of coil turns for max current and field generated, and guesswork to estimate if the pulse is stronger than the existing permanent field. I want a uniform field, as I'd get from multiple turns of conductor. Too many turns = more resistance and inductance and more field, but it also limits the current and the field generated. With only trial and error to go by, that can take a long time to figure out the best combination.

Another concern is anchoring the magnet leg, or the whole magnet, so it doesn't get propelled through a wall by the strong fields. Almost like a rail gun. I'll have to bolt it down. I'm also wondering if the field spike generated might cause problems for electrical devices nearby. This is so much higher than anything I've worked with in the past I will have to work up to the values I want gradually, but it'd be nice to know what to expect.

I'd upload some demonstrations if I could.
If I could put my video files into a form accepted by this website I'd show you a piece of 0.025'' aluminum in freefall between the magnet poles separated bu ~0.030'', falling at the rate of 0.9''/second. By using my larger magnets, I can drop a stack of aluminum sheets 4'' high x 5'' long x 1.25'' thick through a gap about 5'' high and 14'' long and 1.5'' wide and it will fall noticeably slower than normal. These are strong industrial ceramic magnets. Those are huge distances and the aluminum sheets are individual, not a solid piece so the eddy currents aren't as strong as they could be. What they lack in surface field density vs Neodymiums, they make up for with their much larger area. They can reverse a compass needle almost 10' away. Neodymiums are dangerous in the near field. These are dangerous much farther away. With these big ceramics, I was able to out perform about 5 different demonstrations done online by people with regular size Neodymiums. If I didn't keep full control as I removed one magnet from another, it could suddenly decide it was feeling enough force from the magnet behind it and it could jump up and back to the magnet it had been attached to. If my hand was there it would be like a 31 pound block wrapped in steel with sharp edges suddenly slamming into my hand. I have to wear protection for my hands. Sometimes it takes all my strength to manipulate the individual parts. If I got my arm trapped between some poles, I'd have to be conscious to direct rescue personnel on how to remove the poles without losing control & causing more damage.
Thanks.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
Yes you get a reverse voltage spike, but that's from the inductance trying to maintain the current flow.
The current never reverses.
The mechanical analog of inductance is inertia.
Doesn't the polarity of the magnetic field change though because the direction of the collapsing field is opposite to the direction of the growing field?
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
What I have seen done in the magnetization process, is once that you have reached the maximum current, a high wattage low ohm resistor is placed in parallel with the coil.
When the current supply is interrupted, the stored inductive energy has a means of being safely dissipated.
Not counting inductance, coil resistance would be 12V/400A = 0.03 ohms. If I^2R=P, I have 12V, 400 A, so a 100 ohm resistor x (400 A)^2 amps^2 * 100.03 ohms = 16 Megawatt resistor. Do you have one I can borrow? The water mentioned earlier acts as the resistor. A tank of water would be a less likely to boil than a cup of water. Electrodes would be touching under the water, then separated. Distilled water has a very high resistance, it is almost non-conductive. By adding a little salt I could vary the rate of resistance rise with electrode separation. The greater the separation the more the resistance thus reducing the forward current gradually. Or would the power be (12V * 400 A = 4.8KW?) Will I have to dissipate 5 KW of heat or 16 MW of heat? It's only a difference of 3333 times more or less. Which is correct? (Edit: I think 4.8KW is correct. A flyback diode only has to handle the current put into the circuit as it comes out, not the square of the current.)
 
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crutschow

Joined Mar 14, 2008
27,241
Doesn't the polarity of the magnetic field change though because the direction of the collapsing field is opposite to the direction of the growing field?
No, because the current direction is what determines the magnetic field direction, and that does not change.
Whether the filed is expanding or collapsing has no effect on the field direction.
 

Hymie

Joined Mar 30, 2018
976
Many years ago, the company I worked for produced a magnetic part for the product they were making. Rather than use a continuous dc current to produce the induced magnetic field, the rig used a capacitive discharge current to achieve this; since it was only required to produce a magnetic field long enough to induce the permanent magnetism within the manufactured part.

Although I only worked on the periphery of this operation, from memory the capacitance and voltage were quite large – of the order of 1F at hundreds of volts. I’m also reasonably certain that the electrolytic capacitors were exceeding their specified maximum voltage change rate when discharged (di/dt), and the switching device was switching a very high peak current - but the rig lasted for many years.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
No, because the current direction is what determines the magnetic field direction, and that does not change.
Whether the filed is expanding or collapsing has no effect on the field direction.
I found the diagram I saw and attached it below. I looked up flyback snubber diodes. When the coil charges the current flows through the coil only. The diode is wired in reverse so it does not conduct. It only conducts when the field collapses due to the polarity change. This way it shorts the current from the coil to protect the load.
In the Wikipedia example below notice how current flows clockwise in the top 2 pics without a diode snubber but in the bottom pic the current flows counter clockwise through the diode path. Both show current flowing from the top down through the coil, but only in the top left pic is the top of the coil the same polarity as the battery. When the coil is discharging (with or without the diode,) the top of the coil is the opposite polarity. That's the back EMF from Faraday's Law.

FlybackExample.gif
Another way to view this is with a demonstration done in schools. When a bar magnet is moved through a coil connected to a meter with zero in the center, the current flows one way when the North pole enters the coil then the current flows the other way when the South pole enters the coil from the same direction (or if the North pole is removed from the coil). Either way the moving field gets reversed compared to when the North pole originally entered the coil.
 
Darkstar:
Don’t be ironic. Contrary to your speculation, I have actually seen this system in operation. In a commercial operation.

The key is to connect the large resistor in parallel for a very brief moment prior to removing the source current.

The resistor was one hefty nichrome rod mounted on ceramic bushings. The peak watts were very high, but its duty cycle low enough that it properly cooled down with forced air.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
Many years ago, the company I worked for produced a magnetic part for the product they were making. Rather than use a continuous dc current to produce the induced magnetic field, the rig used a capacitive discharge current to achieve this; since it was only required to produce a magnetic field long enough to induce the permanent magnetism within the manufactured part.

Although I only worked on the periphery of this operation, from memory the capacitance and voltage were quite large – of the order of 1F at hundreds of volts. I’m also reasonably certain that the electrolytic capacitors were exceeding their specified maximum voltage change rate when discharged (di/dt), and the switching device was switching a very high peak current - but the rig lasted for many years.
Yeah, I thought of that too. A capacitive discharge will give a very brief pulse. I don't want to hold it long and have the magnet warm up, but I do want to allow time for the magnetic domains to shift. While I have capacitors good to 15KV and others good to 120,000 uF, they are not in the same package. Each leg of the C capacitor I want to boost is 2.5'' in diameter for about 3'' before it turns and begins to taper. That's a lot of metal to push as close to saturation as I can. One early researcher found that he could help shift domains by using a hammer to hit a metal bar in a magnetic field. I tried that a little bit by putting a leg from this magnet between the poles of larger magnets and concentrating the field with metal. It didn't seem to work for me but then I was afraid I might damage the test leg.

At one of my jobs I found that putting a magnet in liquid nitrogen made it stronger, but putting one of these big legs into a LN2 bath with all the defects I can see on the bottom, would probably cause them to crack. I'd still have to lock the domains somehow since they return to normal as they warm up.

I did have a little luck by using metal to concentrate the field from the bigger magnets and positioning the legs of this magnet at the high field points and leaving them in pace for days to months. I did about 100 design variations. The best was only a tiny increase.

I recall a good way to measure the lifting strength of each individual pole was to put a pile of playing cards (60-70) between a 402 gm chunk of iron and magnet pole. The more cards used as spacers when the magnet could still lift the metal off the table, the stronger the field was. It's easy and gives a numerical answer for easy comparisons.
Thanks
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
Darkstar:
Don’t be ironic. Contrary to your speculation, I have actually seen this system in operation. In a commercial operation.

The key is to connect the large resistor in parallel for a very brief moment prior to removing the source current.

The resistor was one hefty nichrome rod mounted on ceramic bushings. The peak watts were very high, but its duty cycle low enough that it properly cooled down with forced air.
How was the resistor moved into position and back so quickly? I'm alone here doing most of this by hand, I can't turn the circuit on and off and connect a resistor to it before the field collapses. Connect the resistor just before the current is shut off? I'll have to think about it. I can either work in my living room or on my garage floor. I don't know how any of this will be ultimately wired together so I'm not sure if I can do it or not. I'll keep it in mind though. Thanks for giving me another option to consider.
 

nsaspook

Joined Aug 27, 2009
8,393
How was the resistor moved into position and back so quickly? I'm alone here doing most of this by hand, I can't turn the circuit on and off and connect a resistor to it before the field collapses. Connect the resistor just before the current is shut off? I'll have to think about it. I can either work in my living room or on my garage floor. I don't know how any of this will be ultimately wired together so I'm not sure if I can do it or not. I'll keep it in mind though. Thanks for giving me another option to consider.
We solve this problem by using multi-quadrant power supplies. They can both source and sink magnet currents.

https://www.electronicdesign.com/po.../21806098/power-supplies-can-absorb-power-too
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
We solve this problem by using multi-quadrant power supplies. They can both source and sink magnet currents.

https://www.electronicdesign.com/po.../21806098/power-supplies-can-absorb-power-too
Never heard of such a thing. Now, if you want to talk about quadrapole or high resolution, double beam, low voltage mass spectrometry or scanning electron microscopes, that I can do.

The best I can do here is do test runs to look for problems before I risk my irreplaceable magnet.
Thanks for the help.
 

nsaspook

Joined Aug 27, 2009
8,393
Never heard of such a thing. Now, if you want to talk about quadrapole or high resolution, double beam, low voltage mass spectrometry or scanning electron microscopes, that I can do.

The best I can do here is do test runs to look for problems before I risk my irreplaceable magnet.
Thanks for the help.
Bring it on, I've worked with most of those technologies in one form or another in the last 30 years.
 

Thread Starter

Darkstar

Joined Sep 3, 2010
177
Bring it on, I've worked with most of those technologies in one form or another in the last 30 years.
Amazing! It's rare to find someone with similar experience. In my whole career only 4 times have I met other chemists who happened to be at the same, non-chemistry, social function.

I recently did some demonstrations at lunch with friends. Did you ever see the one with 3 polarizing filters? Cross 2 and the light coming through dims, as expected. Take the third, put it at 45 degrees to the other 2 and put it in between them. Most people would guess it would dim the light more because it is a third filter, but instead the light coming through is bright again. Nobody has a good explanation for that. The most popular touches on quantum mechanics and probabilities. The last explanation I heard I can't even come close to recounting, even if I remembered the details, but it's supposed to be the best so far. I heard it in a lecture given by a physicist on Youtube within the last couple years.

I enjoy quantum mechanics. I built a dye laser in my spare time senior yr in college and aced the class in which I lectured about it. At my last job I was allowed to take electron micrographs of some insects just for fun. It was a good job.

You wouldn't by chance know of any companies that can reclaim the gold and platinum cleaned out of our lab vacuum cleaner? Anything going in the garbage we could take home, as long as I approved it, so I have a couple pounds of silicon wafers, metal, and dust.

I also have a fused quartz tube from a diffusion furnace I'm selling. It's about 4-5'' ID and about 6' long. The stuff I have doesn't sell at garage sales. If you know anyone looking for technical hobby stuff I can send you a list. I have 2 rooms I just use for storage of this stuff.

I had a couple vacuum microwave capacitors but didn't think I'd find anyone who could use them. Two weeks after I threw them out I saw a video from a ham radio guy who uses them to tune his antennas. $500 each new. $200 each used on ebay. I had two. I could have sold them in a day but they were gone. Arrrg I hate it when that happens.

I know there are hobbyists who do photolithography and I have a bunch of nice flat glass plates they can use if I can find a way to strip the chrome masks. I don't have the chemicals I'd like to have so it hasn't been easy.

Enough for now, nice talking to you.
 
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