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Old 17th Apr 2020, 3:47 pm   #48
Electronpusher0
Heptode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 856
Default Re: A home brew meter re-magnetiser.

I am now completely convinced that I have reached saturation of the magnet with a meter sensitivity of 700uA.
I tried again, this time with a diode across the coil. The diode was well under rated at only 8A 600V but what the hell. Anyway the magetiser put the pulse in and the diode prevented the reverse emf, dying as it did so.
It failed short circuit so it still prevented the reverse emf and the capacitor had no residual charge.

Still the magnet was stuck with a meter sensitivity of 700uA.
I attach a photo of the set up.

SO, I gave in and made my Neodymium magnet replacement with iron pole pieces.
I used a 30mm X 30mm X 20mm magnet I bought on ebay and used mild steel for the pole pieces, not ideal I know, but its all I had.
I had to space out the 20mm to the required gap of 31.7mm using some iron from another scrap meter.
I attach photos of the assembly.

And the result? The meter now has a sensitivity of 320uA!!! It is very heavily damped and now requires a bit of tapping to get it to finally settle however.
I have shunted the meter with a 47ohm resistor and now have a meter that is 584uA fsd.
I will now calibrate the AVO tester and see how it performs.

A shame about the remagnetiser but I will try it again on an AVO meter movement when the chance arises.

Oh well, its been fun.

Peter
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