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18th Nov 2020, 8:04 pm | #1 |
Pentode
Join Date: Jun 2020
Location: Bude, Cornwall, UK.
Posts: 182
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Home made pulse magnetiser
Made to recharge magneto magnets.
A cut down spot welder transformer was rewound wih 440 turns of 1mm enamelled wire. Power supply is an old valve radio transformer rectified and charging up a 2000uF capacitor. Aim was for 400VDC but the transformer I had to hand ended up giving me a 500VDC supply. That gives me over 90,000 amp turns which is well over what I need for the old magneto's. |
19th Nov 2020, 6:09 pm | #2 |
Hexode
Join Date: Mar 2014
Location: Fareham, Hampshire, UK.
Posts: 482
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Re: Home made pulse magnetiser
Nice job, love it. By the way, whatis a magneto magnet?
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19th Nov 2020, 6:40 pm | #3 |
Octode
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,813
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Re: Home made pulse magnetiser
Many thanks for posting this Dennis.
It's of interest to me mainly as it could be used as the basis for a magnetiser for the magnet assemblies of moving coil meters. Could I please confirm that the 1mm enamelled wire is 1 mm diameter and not 1mm sq. cross sectional area? Are your your magnetos the type with horseshoe magnets as used on early motor vehicles? What results have you had with these and do they need any ageing after being re-magnetised as your description would suggest that the are magnetised to saturation by your device. If I understand correctly, magnetising to saturation usually implies some gradual loss of flux below the saturation level. PMM |
19th Nov 2020, 6:41 pm | #4 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,864
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Re: Home made pulse magnetiser
A mechanically driven high voltage impulse generator used to operate spark plugs in the days before Kettering (coil) ignition. Powerful magnets are rotated past a stationary high voltage coil, and a contact breaker is used to interrupt the current. Magnetos are still used today on aircraft piston engines as well as small implement engines like lawn mowers. Magnetos don't need outside sources of electrical power to work and are considered very reliable, but output drops as their magnets weaken.
Also used to mean a hand-cranked ringing voltage generator used in early telephones. David
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19th Nov 2020, 6:55 pm | #5 | |
Guest
Posts: n/a
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Re: Home made pulse magnetiser
I am wondering what is the switch to dump the capacitors charge into the coil.
Quote:
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19th Nov 2020, 7:11 pm | #6 |
Pentode
Join Date: Jun 2020
Location: Bude, Cornwall, UK.
Posts: 182
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Re: Home made pulse magnetiser
Yes the wire is 1mm diameter.
The magneto's I want to recharge have the magnets cast in but with the right shaped pole pieces it will do horseshoe magnets or flywheel magnets. Need to get a gaussmeter before I can do much else, as I want to see just how much of an improvement the charger makes. I have been experimenting with it on a horseshoe magnet and have been able to reverse the poles a few times. The most important thing though is to make sure I don't get my fingers between the magnet and the charger if I want to keep them. |
19th Nov 2020, 7:20 pm | #7 |
Pentode
Join Date: Jun 2020
Location: Bude, Cornwall, UK.
Posts: 182
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Re: Home made pulse magnetiser
Currently switching it with a big contactor to try it out, but I have got an 800A thyristor ready to fit.
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19th Nov 2020, 7:33 pm | #8 | |
Nonode
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,294
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Re: Home made pulse magnetiser
Quote:
https://www.vintage-radio.net/forum/...d.php?t=165159 Peter |
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19th Nov 2020, 8:32 pm | #9 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Re: Home made pulse magnetiser
Have you worked out the reverse voltage when your thyristor switches off? Might be worth checking
Another forum member did an excellent project on the same lines... Ah, cross-posted by the time I wrote this - hi Peter, glad you saw the thread!
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Al Last edited by Al (astral highway); 19th Nov 2020 at 8:34 pm. Reason: cross posting |
19th Nov 2020, 9:15 pm | #10 |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Re: Home made pulse magnetiser
To clarify, wrong terminology there. I mean back-EMF.
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Al |
19th Nov 2020, 9:30 pm | #11 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,078
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Re: Home made pulse magnetiser
It shouldn't be very much, TBH.
If using a thyristor, the current will carry on by itself after triggering, because the thyristor latches 'on'. It will only let go when current has fallen to a small value - and then, the energy will be small. I'd put an anti-parallel diode across the capacitor, though, unless your rectifiers do this job. Otherwise, the current will cease after the capacitor voltage has reversed its polarity, not good in an electrolytic! Connecting the diode will allow current to build up to a peak, following a quarter sine-wave, and then die away, relatively slowly, with an exponential decay. |
19th Nov 2020, 10:06 pm | #12 |
Pentode
Join Date: Jun 2020
Location: Bude, Cornwall, UK.
Posts: 182
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Re: Home made pulse magnetiser
I have used a bridge rectifier so should not need to add another diode.
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20th Nov 2020, 8:48 am | #13 | |
Nonode
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,294
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Re: Home made pulse magnetiser
Quote:
Peter |
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24th Nov 2020, 8:35 am | #14 |
Octode
Join Date: Mar 2020
Location: Kitchener, Ontario, Canada
Posts: 1,294
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Re: Home made pulse magnetiser
One project at work back in the late 80’s was a switch to reverse the drive between a magnet driver and an electromagnet to magnetise transducers for telephone earpieces. They were having yield issues due to reversing the coil polarity on about 5%, so the switch would detect the coil polarity at a previous position of the turntable, then switch the polarity of the pulse from the magnet driver.
I think the magnet driver was manufactured by a company called Radcliffe. It was similar in operation to a flashgun but could deliver 300v at 300A, but was normally operated at about 150v. I was always very careful working on the switch circuit, remembering poking about in a camera flash with a screwdriver when I was 13. I thought I’d check if it was charging the cap, as I could here the whine, not a good idea to short the cap to see if it sparked. Screwdriver hit the wall at the other end of the livingroom, not sure if that was due to the charge or my surprise and pulling away. I was never tempted to try that with the radcliffe. The switch was controlled by an 8748, this would detect the output pulse of the radcliffe via an opto-isolator and then trigger one pair of four SCRs that were arranged in a bridge to select the direction to drive the magnet. I don’t remember using diodes across the magnet coil, in fact I don’t think it would be possible due to switching the direction of the current with the bridge. I think it relied on the SCR reverse breakdown voltage. The Radcliffe also used the same SCRs, or rather I used the same SCRs that radcliffe used. I did have quite a few SA5.0s in the interface from the 8748 to protect it from any stray spikes. |