Rather than destroy a good toroid core, why not just add pole pieces to a wound coil solenoid magnet?? Or even just bend a section of steel rod into a "U" shape?? Or wind a bunch of wire around an old "horseshoe" magnet that has lost it's magnetism. Or even pull the "I" section off of a transformer made wit"E-I" laminations??

Not sure what you mean about the "E I" laminations. The "E" does not form a circular path with a narrow gap.

As to the steel rod, it may work but steel that is not made for magnetically active magnetic circuits is inferior to steel made for that purpose. The permeability is far lower.
I suggested the "C" core because it consists of two "U" shapes already, so putting the two "U" shapes open faces together forms a circular magnetic path. One of the tips of one (or both) of the "U" sections could be ground or cut off probably with a hacksaw.
A similar case for the horseshoe magnet. The metal used for that would be for a different purpose than for a magnetic circuit like a transformer. That would mean high coercivity (wide BH curve) which may get in the way when we go to energize it.

Bear in mind that it may be possible that a rock from the garden might work too, I am just suggesting what are probably the best ways of doing it. There are other ways. Take a spool of wire and run a steel rod through the center, that may work. The object though is to get as strong a magnetic field as possible so as to exceed the magnetism of the target object, and by concentrating the field this is easier to achieve.

I've gotten toroid cores less than $1 USD. I've cracked several of them and glued them back together to measure the change in characteristics. You can get some with very high permeability and that means less current to operate.

I think maybe the hardest part of this project is not the core though, I think it is the electronics. The current has to be reversed many times as the level of the current is decreased. That's the only way to overcome the target metal's coercivity. A sine wave would normally be used but maybe a triangle current wave would work too as long as it reversed too. Lucky it does not have to be high frequency, even 50Hz would do it.
One quick solution might be to use an audio amplifier with enough output current. It can be driven with a frequency generator maybe a triangle wave. The output would be capacitively coupled to the winding on the core.

So although there may be other ways to do this, I think a circular path with as small a gap as needed would work the best, and that means require the least current flow to get it to actually work. And, because less current is required with high permeability, get the highest permeability you can find (magnetic material made for these purposes).
As an example, you can get magnetic cores with relative permeability like 3000 or even 5000, while a steel 10 penny nail may only exhibit 100. Maybe if we bent the nail around in a circle it might work better.
Note that 3000 compared to 100 means we need 30 times the level of current for the nail in order to get the same field strength as that of the 3000 magnetic core. The electrical analog is the 'resistance' (which is reluctance in magnetic terms) would be 30 times higher for the nail than for the 3000 core which it comes to conducting the 'current' (flux in magnetic terms). That's kind of a big difference.

 

To reduce the current slowly, over the course of a few seconds, get a higher power rheostat, 20 watts or so. OR use a variable ration transformer, such as a VARIAC or a POWERSTAT. That will result in much less heat generation.

 

You can probably find an old bulk eraser which was used to quickly erase magnetic tape. Usually, they were nothing more than a modified version of a mains voltage transformer and depended on the 50/60Hz AC to do the job.

Small ones draw 5-6A, larger ones can be much more. The ones used in radio stations were quite large and it was standard practice to leave your watch and wallet in the other room when operatiing them.

eBay or whatever “bulk eraser”.

 

Does anybody still have the old 100 wat weller soldering iron? I used it to magnatise screw drivers and even degaused old color TV sets.

 

Does anybody still have the old 100 wat weller soldering iron? I used it to magnatise screw drivers and even degaused old color TV sets.

And with that you reminded me about the way it used to be done way back when.
The degaussing coil would be energized and held close to the tube, then gradually moved back away from the tube in order to get a decrease in the magnetic field, which is one of the requirements. That could work with a toroid or other type of core also.

I suppose an E I core could work if we only used one section of the "E" and crossed only one opening with an "I" only part way, then used the gap that remains to place the screwdriver tip, the move it farther and farther away very slowly.

I had luck with a permanent magnet on a computer CRT monitor a long time ago when I still had one. That only requires a lower field strength though.

 

Use the “I” core from the transformer and the 27 AWG enamelled copper wire. Put as many turns as you can get, 200-500 turns.
Try with a 25-40W incandescent light bulb in series to reduce the current.

The red line I draw on top of the picture represents the coil size under the black isolator.
I made this today:

And it made a short while my protection fuse blow up when I connect this to 240VAC.
I made some calculations: I=V/R=240/7.7=31A ... hmmm?!? is this correct?
How to make it not blow my fuse or making any shorts ?
My immediate thought is to add more coils. But how many? Any formula possible? or general right thumb rule?

 

The DC resistance is not important. What you need is higher inductance, i.e. more turns.

Have you tried putting an incandescent light bulb in series?

 

Have you tried putting an incandescent light bulb in series?

I know you mentioned it before, but I dont have one. Only LED light bulbs.
I also do have a Variac ! if that will help with anything.... It should.

 

And, again, it made a short while my protection fuse blow up when I connect this to 240VAC.

 

And with that you reminded me about the way it used to be done way back when.
The degaussing coil would be energized and held close to the tube, then gradually moved back away from the tube in order to get a decrease in the magnetic field, which is one of the requirements. That could work with a toroid or other type of core also.

I suppose an E I core could work if we only used one section of the "E" and crossed only one opening with an "I" only part way, then used the gap that remains to place the screwdriver tip, the move it farther and farther away very slowly.

I had luck with a permanent magnet on a computer CRT monitor a long time ago when I still had one. That only requires a lower field strength though.

An "E-I core with only the "E" in place can work very well, which is why I suggested it earlier.

 

An "E-I core with only the "E" in place can work very well, which is why I suggested it earlier.

So next, is to use the E core instead of this I one. Ok...
By the way, I measured a small transformer primary coil and give me 4.5R, as in CC measuring with my DMM.
I know the Inductive Reactance ( XL ) is the resistance of a coil in an AC current and it also depends of the fv, in my case 50Hz.
So Im aware about these things.
I will unwind the I core and I will rewind a E core, getting close to the first example answer in #2.

 

Use the variac starting at a low voltage.

 

An "E-I core with only the "E" in place can work very well, which is why I suggested it earlier.

Are you talking about magnetizing/demagnitizing a screwdriver?
I would not bet an E I core only using the E would work for this purpose. The tips of the "E" would be too far apart, unless you want to wind a ton of turns onto the core. With an "E" and additional "I" forming a small gap, the number of turns would be less, much less, and the level of current required would also be much less.

I am not sure if you realize this or not, but even EI cores used as you normally would for a transformer could change the amount of current needed for an application just because of the way the "I" is placed on top of the "E".
For examples:
With a gapping material more turns are required, and the larger the gap, the more turns required.
Even with no gapping material, with a butt stack more turns are required, but less than with a gapping material.
With interleaving 3x3 more turns are required, but not as much as with a butt stack.
With interleaving 1x1 more turns are required, but not as much as with 3x3 interleaving.

So you see how very, very sensitive this is to the coupling method. A gap always means more turns, and even when the "I" of the "E" is placed directly on top of the tips of the "E" more turns are required than interleaving.

NOW imagine having a gap as wide as the space between any two tips of the "E" section. That would be a HUGE gap. The number of turns to get the same magnetization would be a LOT more than with a small gap. I suggested using the "I" with the "E" but leaving a SMALL gap just so we could get high magnetization with much fewer winding turns. If anything was going to work with an EI core, that would just have to be the best bet. Not only that, but there would also be some adjustment range for the size of the gap, although it would have to be limited.

 

So next, is to use the E core instead of this I one. Ok...
By the way, I measured a small transformer primary coil and give me 4.5R, as in CC measuring with my DMM.
I know the Inductive Reactance ( XL ) is the resistance of a coil in an AC current and it also depends of the fv, in my case 50Hz.
So Im aware about these things.
I will unwind the I core and I will rewind a E core, getting close to the first example answer in #2.

Hi,

Using an "I" section from an "EI" core would require thousands of turns to work at 220vac, maybe more, especially if the cross sectional area of the core is small. Using an "E" core alone is about the same deal.
The best way is to use an "E" section with an "I" section by placing the "I" section across two of the tips of the "E" section, and sliding it across so you get just a small gap near the center leg. That means you will have the "I" section sticking way out on one side. When limited to using an EI core this would be the best method because it would require the least number of turns.
As I said above, you might get an "I" section or an "E" section to work alone, but it requires more turns. The idea is to not have to wind a million turns on the core

The link you provided is not the best way to wind the "E" part of the core for this purpose where the intention is to only use one half of the E section. That way will require the same number of turns, but each turn will be twice the length as required meaning you'll have to use two times the wire length.
The best way is to wind it on the leg you will be using with the "I" section, or winding it on the mid part of the "E" core where that leg connects to.

See attachment. The upper drawing is the EI core the way it is normally used, the lower one is the best way to use it for this purpose although if you want to grind a little we can make it much better too.

 

I made a coil with 568 turns over the middle column of this E core. This is different size than the I core alone. For comparison, those 500 turns on the I core, here on this new coil was 386 turns - you can see it mentioned on my paper. So this coil is having a bigger internal area than the I one.
The right wire is broken in the picture. It was wounded on the right most column of this E core, like 4 turns for the wire not to unwind itself from the main coil. I plug it into 240VAC, I clicked the button, sparks come out of the coil and immediately after <1second I close the button. At least this time, the fuse didnt burn ! Thats a good sign. What is not a good sign is the coil got extremely hot to my fingers, like 80*C perhaps, from my finger testing, and all this under 1 sec of activation. The right wire broke, while I was unwinding it. It must got hot, and made some contact with the metal core? most probably the cause for the sparks Ive seen? Hmmmmmmmmmmmmmmmmm !!! Im not sure where the sparks come from.

With an "E" and additional "I" forming a small gap, the number of turns would be less, much less, and the level of current required would also be much less.

Very interesting observation ! I agree with you at this point. I will do your idea the next time.
Now.... Im a bit worried if my coil got burned. The thing is, this wire already was scrapped from somewhere, and Im not completly sure how stressed already was, especially if it got holes in the isolation - from high temperature like it just did now in my experiment here.
Anyway... lets close our eyes and say its still a good wire. Hahaha...oh boy...
-

Using an "I" section from an "EI" core would require thousands of turns to work at 220vac, maybe more, especially if the cross sectional area of the core is small. Using an "E" core alone is about the same deal.

Now.... following your rule you just described, wouldn't this transformer tutorial have the same consequence as your rule? With such a large gap, as you just said, wouldnt had to have 1000's of turns to actually work properly? I wonder if this hacked transformer is getting very hot .... like my test I just did, simply as you put it, because of this large gap in its core being the most probable cause.

 

I remade the exact same experiment as before but this time I didnt wounded the wire over any exposed metal column of the E core like before. And, bingo, the fuse didnt blow up, and the entire core started vibrating and dancing on the table with a loud buzz. This experiment, like before, I conducted it in 1 second button press-depress. I actually run it 3 times specifically to see where the sparks come from and no sparks anymore. This tell me the previous winding on metal was the most probable cause for sparks. Hmmmmm. Stupid mistakes, right? Hehehe.
The biggest problem is overheating of the winding. With my finger test (unpowered coil) I can say probably it was 100*C. Because I made 3 tests this time. Nothing smelled as burned, but I could smell a very faint hot metal smell. This experiment told me the coil, or the wire itself is FINE ! Which I was worrying actually for it. That's very good news ! No fuse burned, no burned coil, only highly overheating for the moment. I say this is progress.

 

The inductance of an open loop core is much lower than that of a closed loop core.
I say your input voltage is too high. Use the variac to reduce the voltage.

 

I used black sowing string to tie in place those I core together and then the I core to the E core as well.

I left a small gap this time as you mentioned.

--------
But the entire coil got extremely hot in 1 second switch. Actually, very funny, a white dense smoke come out this time - hahaha. Originally I thought its the enamel on the copper wire. But I quickly realized it was the sawing string that smoked instantaneously from the intense heat of the coil. Here is the proof:

So.... this method didnt solved the over heating of the coil. Actually with this string method, I can check the temperature of the coil for very short time duration. If its getting in smoke, then the coil had reached that 100*C I estimate. If not getting in smoke, then project succesfull. Hmmmmmmm...
- how to make it not overheat? -My immediate thought is to add even MORE wire to the coil. Right?
I start to believe I will actually reach that 1000 turns you mentioned.

I say your input voltage is too high. Use the variac to reduce the voltage.

Yes, its a good solution but.... I want to plug it into 240VAC, like a normal transformer or (serious) electrical tool.
I will get a very strong EM field this way, so im counting on that. Im not sure how strong it will be from the variac, with a lower AC voltage. I also want to find a solution to 240VAC to really calibrate it and make it work reliably.
This idea with the variac is plan B or if anything else fails.

 

What about inductively heating the tips if the screwdrivers? This one is overkill, you could design a smaller one.

 

heating the tips if the screwdrivers?

any kind of heating will blacken the metal surface. I dont want that. I thought of it and no.