How to "De-Magnetise tools

Tony Wells said:
The PTC John is referring to above is a special kind of thermistor with a rather sharp cutoff, like a switch.
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No, the whole point of the PTC is that it *doesn't* have a sharp cutoff. The current and therefor the field decreases slowly as it heats up, just as it does when you move the tool being demagnetized away. Otherwise it wouldn't effectively degauss the shadowmask in the crt.
 
That's not my understanding, John. Unlike most thermistors that are used to measure temperature, these used in degauss circuits act more as a switch. They are also used as thermal protection in some electric motors


therm_resistance.GIF
 
Tony Wells said:
That's not my understanding, John. Unlike most thermistors that are used to measure temperature, these used in degauss circuits act more as a switch. They are also used as thermal protection in some electric motors


therm_resistance.GIF
Click to expand...

There are different types of PTC thermistors with different curves.

The PTC thermistor in a degaussing circuit is relatively large and has a low cold resistance. When you first apply power it is cold and therefor allows lots of current to flow. However, as its resistance is nonzero, it begins to heat up. As it heats up its resistance rises and so the current decreases, eventually becoming too small to produce a significant field in the coil (but still large enough to keep the PTC hot). Due to the thermal mass of the PTC the change is not instantaneous. Thus the current ramps down over a period of a second or so.

http://en.wikipedia.org/wiki/Thermistor#Applications
 
PTC's used in degaussing circuits are capable of handling, in some cases, 30-35 amps initially, and some also contain an auxiliary heater. The standard rating time is 3 seconds for the cutoff, and the current flow at that point is only 200-300 ma. The idea is to pass the inrush current but then the resistance increased dropping the current. At only a few hundred ma, the coil only has a small field. Curve steepness is relative, and yes, I know there are many types of this thermistor, and consider a 3 second cutoff rather short. Not exactly fast switching as electronics go, but faster than a linear graduated thermistor as you be used to monitor temperature.

My only point was that I prefer to energize the coil for more than the typical 3 seconds, and withdraw the subject tool slowly from the field before the current is altered. Just a personal preference.
 
Can this coil be used as the demagnetiser. Ignore the smaller one it's just in the pic
What would the schematic look like ? AC power used I suspect.
What is going on in the "demagnetising" process
Thank you ... :))
dd

Coil.jpg
 
ddmunroe said:
Can this coil be used as the demagnetiser.
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No. That's a toroidal transformer. The magnetic field is all inside the donut.

What is going on in the "demagnetising" process
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The AC-driven coil produces a strong magnetic field that rapidly alternates direction and so causes the magnetization of the tool to alternate. The alternating field is gradually reduced to zero leaving the tool unmagnetized. Moving the tool away from the coil has the same effect as does gradually reducing the field.
 
John Hasler said:
No. That's a toroidal transformer. The magnetic field is all inside the donut.



The AC-driven coil produces a strong magnetic field that rapidly alternates direction and so causes the magnetization of the tool to alternate. The alternating field is gradually reduced to zero leaving the tool unmagnetized. Moving the tool away from the coil has the same effect as does gradually reducing the field.
Click to expand...

Thank's John Hasler
What would be the most effective build with regard to the AC coil demagnetiser / configuration EG : wire guage number of coils coiled over some thing maybe ?
dd
 
The shop that I apprenticed in had a big surface grinder (16 X 72) and we frequently ground hard steel, which takes on premanent magnetism from the magnetic chuck; to demagnetize, we flipped the reversing switch for the magnet and it would more or less cancel the magnetic charge so that work could be removed from the chuck, however this does not completely do the job, so we used a coil that was, I think taken from the generator end of a D.C. motor generator welding machine; the opening in the coil was about 4 X 5"; it was equipped with a cord to be plugged into a regular 110V outlet, and the part was passed through the coil several times and slowly withdrawn, and did a good job of degaussing the part. Now, I have a B&S Micromaster 6 X 18 that has a electromagnetic chuck with a Neutrofier controll that offers both variable holding strength and the demagnetising feature that can be used to degauss nearly anything; I think it works by alternating the polarity of the current to the magnet and lessening the voltage at each reversal until it completes the cycle. When I had the chance when the shop was finally shut down and sold off, I was able to make the demag coil my own ---
 
Tony Wells said:
Find an old CRT monitor and rob the degaussing coil out of it. Wire a momentary contact with of the proper rating and put the tool within the ring, hit the switch and slowly withdraw the tool. When it is a foot or more outside the coil, let go of the switch. Might take a pass or two, but will do it.
Click to expand...

Hi Tony,
I now understand your post ... "CRT" was not in my abbreviation vocabulary. I'm not electronically inclined.
Now I do have an old TV. I can take it apart to identify the degaussing coil and hook it up as you mentioned.
Will let you know how it works.
Thank's to everyone that posted I am considering everything you've said.
:))dd
 
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