Bit of a suck it and see exercise since we don't know the inductance of the modified transformer or the resistance of the lamp ( it varies with filament temperature ). I would guestimate that the original charger primary current was around 200 milliamps so this is your target. I would suggest maybe start with a 40W lamp, measure the current and see how hot it gets then adjust as necessary. Nothing will melt instantly.

Incidentally I would enclose the lamp in a substantial cage or tin can with ventilation holes. With the current setup, if you break the glass, you have wires carrying 240V waving in the breeze. I just hope the author has a fully functional RCD on his supply.

Mike

Here's my demagnetiser, the coil in the box is one removed from the transformer from a micro wave oven, the transformer that powers it came from an electric blanket, the HI setting is 12V, and LOW is 6V.

Ian S C

The inductance with the laminations removed must be minimal, so the DC resistance of the primary must be of the order of 500 Ohms if the current is limited to just 0.5A – any means of measuring with a multimeter to check? The load regulation if the original transformer must have been pretty poor. That's a simpler and neater solution than messing about with bulbs etc.

Muzzer

The measured inductance of the demagnetiser is 936mH and the resistance is 231R. A similar sized unmodified transformer measures 232R and 2.4H.

Got a photo here of the windings as removed from the micro wave transformer. There is no need to use more than 12V in a small (or even quite a large demagnetiser of this type. Either coil will do

Ian S C

If only life was so simple Jason

There is no real or simple why the end result will be any less safer but a lot depends on the skill and knowledge of the person doing the modification.

A lot of transformers are just not suitable for being adapted because of such things as all windings on one bobbin, laminations bonded or welded together, presence of a built in thermal fuse etc.

One thing in favour (from a safety aspect) of the current MEW project is that if all else fails the lamp will prevent any damage. Removing half the laminations will significantly change the magnetic circuit so its a guess how much current the demagnetiser will draw from the mains. The one shown in MEW looks to be quite a low power demagnetiser and I would be tempted to use the transformer direct to the mains via a push button switch that is then only held down for a few seconds at a time allowing the transformer to cool down between objects.

Ian P

Hi Andy,

If there is an open circuit across the original primary connection then I think you are correct in assuming the transformer is NBG.

Don't eat to much cow pie it might make you desperate!

Mike

The component you describe is almost certainly a non resettable thermal fuse. Set to open at 125 degrees C and rated to carry 2A continuously.

What is does tell you though is that at some time your transformer has been hotter than it should! The fuse though will have prevented any winding damage and certainly if you have the limiting resistor (lamp) then it should be quite safe to use.

I originally suggested it might have a fuse but you had said that the photo showing the mains terminals and wiring was original but I could not see any connections.

Ian P

I started to construct the demagnetiser suggested by Mike Cox in MEW but on disassembly found that the primary was open circuit (tho thermal fuse seemed ok) – the source being one of those 25W small halogen lights on telescopic arms sold (and possibly still are) by WH Smiths, B+Q etc – didn't check the transformer as thought the failure as a lamp was due to the failure of the non-repairable on/off switch embedded in the hemispherical cover

however the secondary winding was fine – the base was removable and was already provided with screwfittings for the transformer which just needed a mdf spacer to replace the removed I laminations

Powered from an old 6V battery charger with a 1ohm 10W resistor(ex Maplins) fitted to limit current to about 2A – the 3 bits of spare MDF easily cut to form a case – the only got-cha I realised was that the terminals needed to be isolated from the case just in case came into contact with on of the charger leads and shorted the rectifier hence the use of 2 4mm terminals rather than the planned barrel connector

No problems with mains or the use of empty soup tins!

sorry thought obvious I merely tapped across the 8v winding on the transformer (hence my comment re possible short circuit of one arm of the bridge rectifier (an old selenium rectifier the smell of which when fried is not pleasant) – thus fed by low voltage 50Hz!