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Old 27th Mar 2020, 8:54 pm   #1
Electronpusher0
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Default A home brew meter re-magnetiser.

Following from this link
https://www.vintage-radio.net/forum/...60#post1228160
where I posted about my experiments with re-magnetising an AVO 2 panel meter I decided to go ahead and build a full re-magnetiser.
The basis is the RFL-440 referenced in this thread which has the manual and schematic as an attachment.

https://www.vintage-radio.net/forum/...ad.php?t=76514
The RFL-440 uses a bank of capacitors with a total capacitance of 7200uf which are charged at a selected voltage up to a maximum of 350V. This bank of capacitors is then discharged using a thyristor into a coil wound round the object to be magnetised.
I raided my junk box and found 3 off 2,200uF 500V capacitors which will become my capacitor bank (photo attached) The linking straps are copper bar with heatshrink insulation, again from the junk box.

For the Thyristor I decided to use a IXYS, MCO150-16io1, Thyristor, which has a rating of 1600V and 158A (continuous) and a peak current rating of 2000A! It is available from RS Stock No. 125-8053 at £20.29 incl VAT. (thanks for the heads up Bobger).
For the transformer I found one with an input of 240 or 415 and output tappings of 12V, 24V, 48V, 110V and 230V rated at 100VA. It is made by JFK electrical of NI and is available on e-bay at £28-99.
By selecting across the output tappings the following voltages are available:
12 V, 24 V, 36 V, 48 V, 62 V, 86 V, 98 V, 110 V, 120 V, 182 V, 206 V, 218 V, 230 V
I used an Excel spreadsheet (attached) to calculate the DC voltages available after rectification and the energy in joules available when this voltage is used to charge my capacitor bank. The sheet also shows which connections to make on the secondary to get the listed voltage.

In the RFL-440 the selected ac voltage is passed through an Inductor to limit the current before being rectified and charging the capacitors. I propose doing the same using a 10H choke again from the junk box.

I will be using relays to charge the capacitor bank and to fire the Thyristor. A second nc contact on the fire relay will open the charge relay in case I have a brainstorm and try firing the thyristor while charging!.
A further relay is engaged whenever the unit is powered up. nc contacts on this relay connect a discharge resistor across the capacitors thus when the unit is powered off the capacitors are discharged.
Additional safety features include having a moving coil meter permanently measuring the voltage on the capacitors and a mains indicator type neon also across the capacitors which will indicate a voltage of some 100V plus present on the capacitors.
I am undecided whether to float the whole circuit or ground the positive output, I cannot decide which is safer if the output terminal is accidentally touched.

The gate of the thyristor requires a trigger voltage of 1.4V and a trigger current of 150ma, this is provided via a current limiting resistor by a 100uF capacitor charged to 6.8V.
The supply for the relays and the gate trigger is provided by a 24V SMPSU, I have a box full of these so it is zero cost.
I attach a sketch circuit, this is work in progress and I will update it as I go and eventually draw the final circuit neatly.

Peter
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File Type: pdf Sketch cct Magnetiser.pdf (10.1 KB, 261 views)
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Old 27th Mar 2020, 9:45 pm   #2
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Default Re: A home brew meter re-magnetiser.

Scary!

I think I'd opt for floating. And I'd probably have a key operated switch just in case it got into the wrong hands.

How about DINSE type electric welding connectors to go to the coil?

I'd also wonder about sensing coil continuity before enabling a pulse, just in case one coil plug is left out. I'm an absolute coward

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Old 28th Mar 2020, 1:38 am   #3
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Default Re: A home brew meter re-magnetiser.

Interesting I'll be watching. A useful addition would be a demagnetiser, parts of my lathe around the tool post have become magnetised and it is a royal PIA when swarfe sticks to it, my puny Akai tape head demagnetiser is ok for tweezers and small screwdrivers but it is not not man enough for the job when it comes to the tool post.
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Old 28th Mar 2020, 9:09 am   #4
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Default Re: A home brew meter re-magnetiser.

Thanks for the tip about DINSE type electric welding connectors David, I have just ordered some.

A demagnetiser is probably harder since it requires a continuous high ac current whereas a magnetiser only requires a pulse of high current dc.

It will probably be about a week before I can report much progress as I am waiting for parts on order and doing metal bashing.

Peter
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Old 28th Mar 2020, 5:35 pm   #5
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Default Re: A home brew meter re-magnetiser.

I remember one of our local auto electrical repair shop had a magneto re magnetiser. I never tried it but it looked like a large transformer. It had two very large coils with a space at the top in which to put your magneto, there was some special shaped spacers which fitted around the magneto ,filling up the air gap. As far I can remember the device was battery powered with just a contactor and push button switch.
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Old 29th Mar 2020, 4:12 pm   #6
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Default Re: A home brew meter re-magnetiser.

ex 2 base,
I have similar memories of going to the local Wipac agent about 45 years ago to get magnetos re-magnetized particularly the flywheel ones common on Villiers engines.
They usually did it while you waited, I didn't see what was done as they went out the back where there were load clunking sounds, it achieved the desired effect with a much better spark from the magneto.
This made me think though whether it might not be easier and safer to generate the magnet field with very high current low voltage from a lead acid battery than to pursue the discharging of a high voltage charge.
Does anyone have any knowledge or circuits of how these old units worked.
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Old 29th Mar 2020, 5:59 pm   #7
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Default Re: A home brew meter re-magnetiser.

I thought that magnetisers used a very small number of turns in order to reduce the initial inductance to a minimum and hence allow the peak current to flow rapidly? Perhaps that's the topolgy for industrial machines where some other practical contraints need to be met?
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Old 29th Mar 2020, 6:02 pm   #8
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Default Re: A home brew meter re-magnetiser.

I don't see why a high current low voltage system would not work.
The advantage of a high voltage capacitor discharge system is that the energy available for the pulse is 1/2 C (V)2 (that Voltage squared) so the total energy available from the system I am building is 1/2 X .0066 X 325 X 325 = 349 Joules.

Peter
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Old 30th Mar 2020, 9:57 am   #9
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Default Re: A home brew meter re-magnetiser.

I have on subscription a motor bike magazine "Old Bike Mart" and Villiers Services will re magnetising flywheel magnetos as described by chriswood1900 above. Also, I have had a look through my motor bike info and have found details of a home brew magnetiser. I also have a home made "Growler", with which I have magnetise various things for friends and demagnetised things with it as well. Basically it is a large transformer with the secondary winding removed. I milled away some of the laminations forming a vee in which to place an armature ready for testing. These were common in garages when vehicles had dynamos. If Electronpusher0 would like to send me a PM with his email address, I'll send him a copy. Ted
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Old 30th Mar 2020, 5:24 pm   #10
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Default Re: A home brew meter re-magnetiser.

PM sent, thanks Ted.

Peter
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Old 7th Apr 2020, 11:51 am   #11
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Default Re: A home brew meter re-magnetiser.

Well the bits finally arrived and I have built it.
Entering the tesing phase now. I have only had the capacitors charged to about 85V so far but the thyristor fires and discharges them into the coil.
The charge time is very long so I think I need a smaller inductor than the 10H one I am using at present, or perhaps change it for a resistor?

I am testing into a 16 turn coil wound round a steel bolt at present. Of course the proof will be when I try and get the sensitivity of my 2 panel AVO meter back to 600uA.

I attach a few photos, a bit crowded, but I used a case I already had.

I will report on further testing.

Peter
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Old 7th Apr 2020, 12:14 pm   #12
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Default Re: A home brew meter re-magnetiser.

Looks a very pro job: well done! Hope it works!

John
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Old 7th Apr 2020, 12:58 pm   #13
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Default Re: A home brew meter re-magnetiser.

Probably needs some polarity markings on the output connectors and a little diagram of a coil to show the polarity of magnetisation that will be set.

David
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Old 8th Apr 2020, 1:15 pm   #14
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Default Re: A home brew meter re-magnetiser.

Your work is impressive - looks professional, I'm sure it will work well.
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Old 8th Apr 2020, 1:50 pm   #15
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Default Re: A home brew meter re-magnetiser.

Quote:
The charge time is very long so I think I need a smaller inductor than the 10H one I am using at present
What about a "proper" light bulb, that would have the perfect characteristics, limited current while it was lit going down to a much lower resistance as it cools down giving a full charge. Almost constant current.
 
Old 8th Apr 2020, 3:19 pm   #16
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Default Re: A home brew meter re-magnetiser.

Hi Peter,

A nicely detailed project and meticulous approach. I do love a pulse experiment; following with interest.

That's a lot of joules, right there, so I'm happy to see you precautions.

Just one thought. I haven't calculated it, but have you thought about the high and nearly instantaneous reverse voltage across the thyristor once the LC circuit is complete? Also what is the duration of the maximum pulsed current for that component...
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Old 9th Apr 2020, 8:14 am   #17
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Default Re: A home brew meter re-magnetiser.

Thanks for the encouraging comments.
David, I have taken your suggestion and marked the polarity, I knew the plus is on the left but in several months time I would have forgotten.
I have now tried it on my meter from my spare panel for my 2 panel AVO valve tester.
I wound 20 turns round the horseshoe magnet.
In order to decide which way to pass current through the coil I applied an external supply via a 10K resistor to the meter and increased the voltage to give about half scale reading. I then passed a small amount of current through the coil I had wound round the magnet and observed if the reading increased or decreased, this indicated which way to pass the current to attempt remagetisation.
As reported in the first link in post No1 I had already done some proof of concept magnetising on this using a bank of 4 X 15,000uF capacitors charged to 45V.

Well the good news is new magnetiser certainly works, I have had it running at 300V and the thyristor fires with an audible "thump". The coil round the meter (and the meter magnet) visibly jump due to the high magnetic field.
The bad news is that I could only get a marginal improvement in the meter sensitivity over my previous experiments.
To recap the meter had been bu**ered by my attempts to increase sensitivity by additional magnets and was now 2.5mA fsd! The proof of concept experiments improved this to a sensitivity of about 710uA. The new magnetiser only improved this to 700uA. It is supposed to be 600uA fsd.
The energy in the caps in the proof of concept was 61 Joules and in the new magnetiser was 300 Joules at the voltage I used. Certainly there was no "jump" in the coil in the proof of concept test so I am confident that the magnetising current was significantly more.
I can only conclude that I have reached the maximum magnetisation the meter magnet is capable of.

I will try the new magnetiser on an AVO 8 movement which has not been bu**ered about with and only needs a small increase in sensitivity.

Al, your comments about the reverse voltage are pertinent and have been discussed at length in the second link I posted in post No1 of this thread.
However the RFL-440 has no protection and the thyristor I used has a maximum non repetitive reverse blocking voltage rating of 1700V so I decided to just risk it. So far it is surviving. You also asked about the maximum pulsed current, it is rated at over 2kA for a half mains cycle.

As to the AVO valve tester meter I have ordered some neodymium magnets and am going to try and replace the horseshoe magnet with neodymium magnets and iron pole pieces. After all I have nothing to lose. If it works I will report about it.

Peter
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Old 9th Apr 2020, 10:57 am   #18
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Default Re: A home brew meter re-magnetiser.

I wonder if the magnets were originally charged with a magnetic link fitted between N and S so that the flux density generated would be maximum?

It's not something I've been involved in, so I'm only guessing.

David
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Old 9th Apr 2020, 11:46 am   #19
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Default Re: A home brew meter re-magnetiser.

Not sure if you have these or not - I forget where I copied them from - one is a translation by Google but still understandable, they may be of assistance. Circuit of a Meccano remagnetiser, and 2 home brew magneto magnetisers
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Old 9th Apr 2020, 1:24 pm   #20
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Default Re: A home brew meter re-magnetiser.

Quote:
I wonder if the magnets were originally charged with a magnetic link fitted between N and S
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. It's what I did for my Brown horn speaker rejuvenation. It must be quite effective, they do it to neodymium magnets.

See here https://www.vintage-radio.net/forum/...ad.php?t=65905
 
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