A home brew meter re-magnetiser.

[Electronpusher0]

Originally Posted by merlinmaxwell

Yes they are, but backwards (the keeper was the wound upon bit), the modern magnetisers hold the (soon to be) magnet twixt poles of the machine and then zap.

Thanks gentlemen!
definitely a light bulb moment. I checked the manual for the rfl-440 again and
they suggest a keep across the poles when magnetising a horseshoe, see the attached.

The idea of having the keeper as the wound bit is a good idea.
I will try again with a keeper.

Peter

[John_BS]

We're all interested in the result!

John

[chriswood1900]

I recently came across this online and thought it might give few clues on re-magnetising technique.

[Electronpusher0]

Thanks Chris, thats very helpful, the diagram on page 2 is showing magnetising a horseshoe magnet is very interesting.
The recommended ampere turns is quite frightening, 65,000 to 70,000 AT!
I will try and make a suitable U shape with coils and adjustable pole pieces.

Peter

[Electronpusher0]

I have been thinking about having the magnetising coil wound on the keeper as suggested by Peter (Merlinmaxwell) and as illustrated in the article linked by Chris.
I attach the relevant diagram.
Would the magnetising iron U shape have to be solid soft iron (which could be a challenge to obtain) or could I use laminations and a bobbin from a transformer?
I was thinking of cutting down the E laminations to make them C shaped instead, perhaps remove one one of the side arms leaving the centre which would take the bobbin and the other side arm, or maybe remove the centre arm and make new bobbins.

Peter

[John_BS]

Peter: As you're pulsing the field, you should use laminations of transformer-grade steel. I suspect the U shape was purely a pragmatic shape for use with magnetos: the actual topology should be dictated by the shape of the magnet you're trying to reform.
BTW, I think the 70,000AT figure was assuming quite large air-gaps in the magnetic circuit, e.g. where the magneto magnet was "buried" inside the casing.

[Electronpusher0]

Thanks John, I have found an old transformer that was never varnished so it has come apart easily. It is made of C and T cores.
I am using the C cores to make a suitable magnetising assembly for my meter.
I will post photos when done.

Peter

And the T cores cut in half for the adjustable poles?

[Electronpusher0]

I am making a dedicated core / coil for the AVO 2 panel valve tester, the meter uses a horseshoe magnet with its ends 1.25in apart and 3/8 thick material by 1 inch wide. The core I have made is an exact match to butt against this.
I cut down some of the C sections into L and I pieces and assembled them interleaved. They are varnished and clamped up waiting for the assembly to dry.
I will flatten the ends with wet and dry paper on a flat surface.

Meanwhile I am making the bobbin for the coil.

I will look to making adjustable pole pieces if this works.

Hopefully coil winding tomorrow.

Peter

[Radio Wrangler]

Don't forget insulating bushes on the clamp bolts, otherwise they form a shorted turn on half the flux and will spoil the impulse amplitude. Importnt for those going through holes in the lams.

David

[Electronpusher0]

Thanks for the reminder David.

Peter

Looks like a proper job to me. It may be worth levelling off the lamination's where they contact the meter magnet. A half hour with wet and dry should do the trick.

[Electronpusher0]

Originally Posted by merlinmaxwell

Looks like a proper job to me. It may be worth levelling off the lamination's where they contact the meter magnet. A half hour with wet and dry should do the trick.

Thanks. I do intend levelling the lams with wet and dry when the varnish has set.

I shall only make a single coil rather than the two shown in the Lucas manual.

Peter

[Electronpusher0]

I have wound the coil and fitted it to the core. I confess I reused the wire from the transformer as I did not have anything as thick in stock. I wound 2 windings Bifilar of 75 turns. This enables me to either series or parallel them.
The wire is 14 SWG.

I now need to determine the North / South poles for a given direction of current. I will use a compass for this. I can then try it on my AVO valve tester meter.
Testing tomorrow, fingers crossed.


Peter

I now need to determine the North / South poles for a given direction of current.

Worst case, the wrong way round, all you have to do is swop the meters connexions.

[Electronpusher0]

I have now tested the new magnetising coil.
First try using the coils in parrallel, ie 75 turns, and 300V on the capacitor there was a loud thump, the current was dumped into the coil and the meter kicked despite the meter coil being open circuit, probably the induced current in the aluminium coil former of the meter coil.
After that the magnetiser would not charge again, clearly something had blown. I was afraid it was the thyristor but in fact it was the 1N4007 diodes of the bridge rectifier, all 4 were short circuit.

In the initial circuit posted in post #1 I switched the ac onto the bridge rec to initiate charge, the bridge was permanently across the capacitor. I assume that with the increased inductance the back emf suggested by Astralhighway in a previous post had taken out the diodes.
I changed the charge switching to after the bridge so it is no longer across the capacitor when zapping the coil. I also uprated the diodes to BY127s.

The remagnetiser now works without blowing up but there is a worrying reverse polarity charge of some 50V left on the capacitors after zapping the coil caused by the back emf. This can be removed by the discharge resistor or starting a new charge cyle. The capacitors seem unaffected by this reverse voltage but it is worrying.

Anyway, did it remagnetise the meter magnet?
Unfortunately not beyond the 700uA already acheived.
In my final attempt I had the coils in series, ie 150 turns, the cores were very close coupled to the ends of the horseshoe magnet and held tight by a large tie wrap. The capacitor was charged to 300V. Again a kick of the meter indicated a large surge of magnetising energy but no increase in magnetism after the pulse.
I think I really have reached saturation for the magnet.

This begs the question, was the meter ever the 600uA it was intended to be?
There has been discussion on the forum about whether the meter should be 700 or 600uA.
In this thread
https://www.vintage-radio.net/forum/...d.php?t=162366
I reported my own investigations and after analysing the schematic I came to the conclusion that the designed sensitivity was 600uA.
Indeed the meter in my other 2 panel AVO valve tester is 600uA.

Is it possible that the material the magnet is made from has lost some ability to be magnetised?

Anyway, the end result is like the cutates egg, good in parts.
The magnetiser clearly works but I have failed to restore the meter from my valve tester.

I will draw the final schematic more neatly including the transformer switching and post it should anyone else be inspired to try it.

As to the meter, I have nothing to lose so am going to try replacing the horseshoe magnet with a Neodymium magnet and iron pole pieces and see what sensitivity I can get. I will of course report the results.

Peter

[Radio Wrangler]

Sounds like you need a reverse connected very large diode across the capacitor bank

If your circuit resonates in a twang with an oscillating but diminishing current, it begins to sound like a DEmagnetiser! I wonder if you are remagnetising the magnet and then milliseconds later reducing its magnetisation?

The whole thing about making a permanent magnet is in taking it right up to full saturation, so you want the magnet-to-be to saturate before your coil cores and pole pieces, then you want it to come back down the saturated-magnetised limb of the B/H hysteresis curve and sit there.

David

[Refugee]

I would be tempted to do it like a tape recorder does.
You have got two coils on the former.
Connect one coil to a DC supply and feed it to get the meter at least twice the sensitivity you want and then add low voltage AC to the other coil at about 10 times the DC value you found earlier and see what happens.
A Variac and low voltage transformer should do the trick. Just turn the Variac up for a few seconds and turn it down again while avoiding switching if you can.
Put your capacitor bank across the DC coil to reduce and back EMF when cutting off the DC.

I souds like you are re-magnetizng with the meter intact, perhaps just doing the magnet will lead to a better result?

[Electronpusher0]

Thanks for the suggestions.
I will try and find a suitable diode to absorb the back emf.
The suggestion of just doing the magnet is worth a try. I will have to remove it if I want to try a different magnet so I can try remagnetising just the magnet.
I had hoped to use some ac to artificially age the magnet but that relies on getting the magnetism greater than you need and then reducing it. I can't even get to where I need never mind over.

Peter