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Old 15th Apr 2020, 4:46 pm   #41
retailer
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Default Re: A home brew meter re-magnetiser.

There is a short description on magnetisation in the attached document starting on page 16 if may be of interest. I haven't looked into my two panel tester but is it possible to remove the magnet and place it on the magnetiser, after giving it a jolt place a keeper across the poles before removing, I recall reading somewhere that you will lose some magnet power if a keeper is not used or is it practical to wind an auxiliary coil around each leg of the magnet perhaps split the capacitor bank in two one for the main magnetiser and other for the auxiliary coils on the magnet legs as in the diagram on pg16.
A bit disappointing when you do a lot of work on a project and don't get the results you want.
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Old 15th Apr 2020, 5:02 pm   #42
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Default Re: A home brew meter re-magnetiser.

Stupid sideways thought....

If you can wind some turns around the Avo's magnet, you could sneak in a regulated low voltage power supply to provide a bit of extra mains powered field. It could even be adjustable for future re-calibration.

A different approach to adding a little PSU and an opamp!

David
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Old 16th Apr 2020, 7:48 am   #43
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Default Re: A home brew meter re-magnetiser.

Quote:
Originally Posted by Radio Wrangler View Post
Stupid sideways thought....

If you can wind some turns around the Avo's magnet, you could sneak in a regulated low voltage power supply to provide a bit of extra mains powered field. It could even be adjustable for future re-calibration.

A different approach to adding a little PSU and an opamp!

David
Not a stupid thought. When I first tried my magetiser I wound 20 turns round the meter magnet itself. To determine which direction to pass current through it to remagnetise I passed enough current through the meter itself (via a resistor) to give half scale deflection, I then attached the (magnetising) coil to a bench power supply and passed 2A through it. Passing it one way reduced the reading, ie reduced the magnetism but passing it the other way did not increase the sensitivity at all.
This increases my suspicion that I have reached saturation on the magnet.

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Old 16th Apr 2020, 7:57 am   #44
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Default Re: A home brew meter re-magnetiser.

Thanks for the article you attached Retailer, very interesting reading, especially the statement concering the method of attaching an electromagnet to the ends of a horseshoe magnet, as I have been doing, that "it is practically impossible, however, to obtain complete saturation in this way".

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Old 16th Apr 2020, 8:42 am   #45
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Default Re: A home brew meter re-magnetiser.

I really like your project!

I have asked a few magnet manufacturers for re-magnetizing of a few different types of magnets from some different meters and also for speakers and Trochotrons and always gotten a negative answer on re-magnetizing as they all have claimed that the magnets needs to be heated up to a specific temperature to be able to re-magnetize them to their original strength, they also claimed that they had to be magnetized above the level you want and then brought down to the specified level - not sure how this is done though but I guess that you can Google some and find out. The magnets I have asked about have been metallic magnets, mostly Alnico and similar types, none have been ceramic/ferrite magnets.

Having six new Alnico magnets manufactured to specification, size and strength for a tubular magnet for Trochotrons, would cost at least USD $4000 from the cheapest source I have found that would guarantee the magnets specifications- I could have them made in China for a tenth of that cost but then the companies I have found have had to large tolerances for my needs, and even that was to much money for me.

Adding magnets at strategic points or adjusting the magnetic shunt, or a combination, have always been the magnet manufacturers recomended procedure if it is possible, so that's what I have done by adding Neodymium magnets. Or in the case with the Trochotrons I have bought a lot of dead Trochotrons (leaky or smashed glass envelopes and with pins damaged beyond repair) so I could extract the tubular magnets, the same for meters where I have bought damaged meters and salvaged the magnets and parts.
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Old 16th Apr 2020, 2:40 pm   #46
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Default Re: A home brew meter re-magnetiser.

My first brush with meters and magnets was when I was a 14yr old in high school, my multimeter had somehow got iron particles in the magnet/coil assembly (probably when I changed the battery) I tried unsuccessfully to get them out with a pair of tweezers and eventually hit on the idea of removing the magnet to make it easier, it was a rectangular steel block that slid into the back of the movement, held in place with tabs, it all went smoothly until I replaced the magnet to find the meter had lost around 1/3 of it's sensitivity, it was my pride and joy, a 'proper' 20K ohm/volt multimeter with a switchable ranges, I was heartbroken.

Here is the circuit of a Lucas magnetiser, the 2 way switch at 38 will charge or discharge the capacitors 44&45 through resistors to the required voltage indicated on the meter 48. A push switch at 57 discharges the capacitors through the magnetising coil 26, coil polarity can be reversed with switch 53/54, a commutating diode 60 prevents possible reverse polarity from coil oscillation and also back emf both of which adversely affect the magnetisation process. Not sure what 46 is, a varistor or transient suppressor diode ?

The full patent can be found at google patents by searching for Method of magnetising the flyweights of a governor

You mentioned that the capacitors were being reverse charged did you add the, diode ? being able to charge the capacitors to a specific voltage and also reverse the coil polarity is useful as one could reverse magnetism a magnet to calibrate a magnet. The two way switch and resistors used to charge and discharge the capacitors is a very simple way to control the capacitor charge voltage, when I eventually get around to building one I think I'll use this idea.
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Old 17th Apr 2020, 7:56 am   #47
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Default Re: A home brew meter re-magnetiser.

Dekatron, your experience of losing 1/3 of the magnetism is what I have been worried about and is what has prevented me from removing the meter block from the arms of the magnet to try magnetising (so far).

Also the comment about heating the magnet is interesting.

Retailer, thanks again for posting the information. I have not used a reversing switch as I decided I wanted as few connections between the capacitor, thyristor and the coil as possible. The sockets and plugs I use on the front panel are welding types and not polarised so I can reverse them easily.

It is now clear from the Lucas circuit and comments by others that I need a diode across the coil as the coil oscillation could be defeating the purpose and de-magnetising again.
I just need to find a suitable diode!

Peter
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Old 17th Apr 2020, 3:47 pm   #48
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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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Old 17th Apr 2020, 5:25 pm   #49
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Default Re: A home brew meter re-magnetiser.

It's good to see you have had some success even thought it wasn't the way you planned, sort of lucky that the meter used a horseshoe magnet that could be replaced with a pair of rare earth types.

I'm playing with the meter that came with my MKI, it is not the original as I can see it has a hand made mounting bracket fixing the movement into the case.
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Old 17th Apr 2020, 5:36 pm   #50
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Default Re: A home brew meter re-magnetiser.

That's fixed it, good job too.
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Old 17th Apr 2020, 5:37 pm   #51
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Default Re: A home brew meter re-magnetiser.

Quote:
Originally Posted by retailer View Post
It's good to see you have had some success even thought it wasn't the way you planned, sort of lucky that the meter used a horseshoe magnet that could be replaced with a pair of rare earth types.
Actually its just one rare earth type, the block in the middle, the two small pieces are just packing pieces of iron to get the gap right.

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Old 17th Apr 2020, 5:41 pm   #52
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Default Re: A home brew meter re-magnetiser.

I have now calibrated the valve tester and checked it against my standard ECC82 which has a gm of 4. Both my AVO 2 panel testers now work and agree. Actually its just one complete tester and an extra meter panel. I have a patch panel to enable the spare panel to be used independantly.

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Old 17th Apr 2020, 8:52 pm   #53
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Default Re: A home brew meter re-magnetiser.

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Originally Posted by Radio Wrangler View Post

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?
That sounds like an interesting line of inquiry, David.

Electronpusher, how about capturing the waveform with a current transformer and your 'scope set to trigger in single-shot mode?

A while back, I posted some results from discharging a 3200uF capacitor from just 28V into a giant 2.6uH inductor. The waveform was very interesting and the peak current was nearly minus 200A. Here's the trace...

It would be easy for you to set up a current transformer; you could wind one quickly enough on an appropriate ferrite core, and it might reveal some interesting things?
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Old 17th Apr 2020, 9:59 pm   #54
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Default Re: A home brew meter re-magnetiser.

I had thought about probing the current in the coil. I do have a Tektronix P6021 current probe for my Tek scope. It is rated at a maximum peak current of 500A so it may do the job. I just don't want to risk blowing my scope, I just spent 100 getting it fully working.

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Old 18th Apr 2020, 1:50 pm   #55
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Default Re: A home brew meter re-magnetiser.

That is a seriously good result and a very nice restoration of the meter!

I do have a bunch of old AVO VCM CT160, MK III and Mk IV movements that I will try something similar with when I have some time. However most of these meters will need a refurbishing of the pivot points as they have all got problems at the resting position of the needle where I have been told that the pivots have been worn down with age and use. But this gives hope to being able to fix the magnet problem.

Thanks for showing how you solved this and congratulations to a job well done!

/Martin

Quote:
Originally Posted by Electronpusher0 View Post
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.
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Old 18th Apr 2020, 6:09 pm   #56
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Default Re: A home brew meter re-magnetiser.

Peter:

looking at the topology in photo 1 of your post (48), I think you've have suceeded if the meter movement was removed from the magnet prior to firing your magnetiser. The pole-pieces are shunting magnetic flux across the ends of the horseshoe, so reducing the energy available to do the job!


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Old 18th Apr 2020, 7:10 pm   #57
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Default Re: A home brew meter re-magnetiser.

You may be right John, I still have the horseshoe so can experiment with it.
My only concern was how to maintain the magnetism while transfering from the magnetiser and refitting the movement.

Peter
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