Last update Dec. 5th. 2014

DIY Watch Demagnetiser
 

 
   
Image to the right shows a very simple mains operated device.

It has a coil which when alternating current is let through creates alternating magnetic field. 

The object that needs to be demagnetised is placed inside the coil, pushbutton is pressed and the object is withdrawn as far as one can get it, and the object is hopefully demagnetised.

If not careful, one can easily magnetise objects instead of demagnetising them. 

  
 More modern devices use simple electronics means of eliminating need for object removal away from the magnetic field.

This is achieved by creating decaying alternating magnetic field as shown at the diagram on the right. 

 
The electronic circuit which enables it is quite simple, consisting of a voltage doubler rectifier followed by an LC resonator circuit.

There is actuating switch, a LED indicator of charge, and a bleeder resistor which has a task of discharging capacitor in voltage doubler circuit, to prevent shock when the unit is unplugged.

 
To make this device I have started from an old  unit made in SU, the Soviet Union many years ago. 

Apart from it, I have also checked designs of few other devices made by Vigor, C&E Marshall and some of the no-name units.

All are very similar to each other. The only difference is between units operated from 220 VAC as opposed to 110 VAC. 

The latter have two stages of voltage doublers, ending with 600 V.

 
The SU original circuit diagram found in a users instructions along with the list of components.
 
To make inductor I have used an old transformer core made up from "E" and "I" laminations, using only "E" laminations. 

I removed old copper wire windings to make room for new windings with larger diameter wire, in order to end up with inductance value between 6 and 8 mH, which was the value used in most of the commercial units tested, including the SU unit.  

 
My transformer had core cross section AxB of 20x20 millimetres. The coil former took 220 turns of 0.8 mm diameter lacquered copper wire which produced inductance of 5.8 mH.

Close enough to get unit to work. Finding a suitable capacitor was a bit harder. Limitation was set by a high working voltage of 630 V.

The closest that I could find was 8 uF at 630 V, which  with the 5.8 mH inductor produced oscillations at about 750 Hz.

 
Unit packed in a plastic box, with an aluminium cover with a round sticker to mark the centre of the magnetic field action, a LED to show "Charge Ready".

Unit is normally in the OFF state until switch is set to a charge position. Only a few seconds later, not more than about five, unit is ready to demagnetise the watch, accomplished by setting switch to the ZAP position.

Complete process of  demagnetising a watch lasts only a fraction of a second, about 50 ms i.e. 1/20 of one second.

 
 
  
It should be noted how nowadays the cost of commercial unit is so high that it warrants validity of going ahead and making one's own unit which in materials used does not cost more than about fifty to seventy dollars. The cost of making one is ten to twenty fold lower than buying a commercial unit, the midrange costing in excess of five hundred dollars.
 

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