Magnet Charger to re-magnetise.

[WME_bill]

pmmunro's comment in the thread "How accurate is your Avo" made me think about how to re-magnetise. A rummage in my boxes of old manuals produced details of a RFL Industries Magnet charger. It looks a very uncomfortable unit to use. Discharging 7200 uf charged at 350V through a coil.
1. Does anyone have experience of re-charging magnets.
2. Is the circuit in my manual typical of these things.
3. What about the actual coil around the magnet. Must be fairly hefty.
4. Surely it will oscillate. How are they damped out.
5. If pmm builds one, what will his look like.
I attach the RFL information. Bill m0wpn

[Herald1360]

Without some details about the charging coil L & R it's difficult to say whether it would ring or not. The coil does actually transfer energy into the magnet it's charging where it's stored in the magnet's field, so the whole thing may look fairly resistive, much like a loaded transformer does.

The whole device is definitely similar to the "FBS" Box we use at work for a totally different purpose, though still magnet related.

That's "..... Big Splat", an apt if inelegant description of what it does.

[kalee20]

Originally Posted by WME_bill

4. Surely it will oscillate. How are they damped out.

It will oscillate. But thiuis could be prevented by a pair of hefty diodes, one to stop the current reversing, and one to allow it to freewheel in the coil, till losses cause it to decay.

The material that's being magnetised will absorb a lot of energy, and quite possibly could itself apply enough damping to prevent oscillations. But, as a general-purpose instrument, that's not to be guaranteed.

[Herald1360]

Been cogitating this.... Basically, once it's fired it's a series tuned circuit but with a diode (the SCR) in series with it. The current through the coil will have a single large positive peak, and the voltage across the coil will rise instantaneously to the capacitor voltage, then discharge following a half sine to zero at the current peak and on to - capacitor voltage when the current is back at zero, at which time the voltage snaps back to zero.

[WME_bill]

Thank you kalee20 and Chris (Herald1360). Using the thyristor as if a diode to kill the reverse swing makes sense, as it seemed to me that any reversal of current as it oscillates would annul the magnetisation that was just put there. Now what about phase lags, or would that be too small an effect. Has anyone tried putting a scope on it to see.
Would the problems with a radar pulser be similar I wonder. Firing a thyratron to put a voltage pulse across the magnetron. Bill m0wpn

[Herald1360]

Some waveforms from a quick and dirty simulation- a switch connects 300V to the capacitor then opens shortly before a second switch closes to discharge the bank via an ideal diode into a 100uH inductor.

Red trace is inductor current, green is inductor voltage, blue is capacitor voltage.

The circuit pdf doesn't show any polarity markings against the bank capacitors, so presumably they're reversible types.

[Billy T]

Originally Posted by WME_bill

pmmunro's comment in the thread "How accurate is your Avo" made me think about how to re-magnetise. Bill m0wpn

A simpler way is to attach a tiny neodymium disk magnet (the sort that comes on novelty items like fridge magnets etc, but not the lesser strength ferrite variety) to the meter magnet and it will restore the field strength. No need to glue them on or anything like that, they won't fall off.

Cheers

Billy

[neonlamp]

I like the idea of building a remagnetiser. Could someone confirm whether the central core piece in an Avo is the magnet (as opposed to the outer ring)? To build a work coil for the Avo needs a pretty careful coil shape. I assume it would have to fit into the space for the moving coil. And you would have to limit the pulse so that the coil did not act as a secondary and blow to bits when you mag'd it?

[pmmunro]

Bill,

Thanks very much for posting the manual. The information in the manual could be very useful in assisting the design of a magnetiser, possibly a retirement project in a year or two.

Did you know that these magnetisers (magnetizer in US and Oxford spelling) are still on sale, although hardly at a price many private users could justify?

The movement in the Model 8 Mark V is described as a "centre pole movement" which confirms that the magnet is in the centre.

Neonlamp,

You raise a salutary cautionary note. I take it that you mean the moving coil but this would be open-circuit during magnetising and so should not carry any current. There would however be current in the aluminium former which is intended to act as an induction damper as it cuts the lines of flux in the gap in which the moving coil rotates.

I understand that the field is applied externally by pole pieces which fit the outer ring of the movement with the movement fully assembled. If this is correct then it can be assumed that the induced current in the coil former is not harmful.

The pulse from the magnetiser saturates the magnet and in this condition it is not stable - it tends to demagnetise to some extent with time. To ensure a stable level of magnetic flux, the magnet has to be "artificially aged". This is done by subjecting it to an alternating field weaker than the original magnetising field. In this way the movement sensitivity is reduced to the required figure.

PMM