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Old 18th Nov 2020, 8:04 pm   #1
Dennis M
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Default 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.
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Old 19th Nov 2020, 6:09 pm   #2
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Default Re: Home made pulse magnetiser

Nice job, love it. By the way, whatis a magneto magnet?
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Old 19th Nov 2020, 6:40 pm   #3
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Default 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.

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Old 19th Nov 2020, 6:41 pm   #4
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Default 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.

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Old 19th Nov 2020, 6:55 pm   #5
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Default Re: Home made pulse magnetiser

I am wondering what is the switch to dump the capacitors charge into the coil.
Quote:
Powerful magnets are rotated past a stationary high voltage coil
Or the other way round, especially for the old type of weaker magnet where you need a lot of it to get enough field, rather impractical to spin a huge hunk of magnet. This (I think) is designed for the stationary horseshoe ones. Like the picture below.
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Old 19th Nov 2020, 7:11 pm   #6
Dennis M
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Default 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.
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Old 19th Nov 2020, 7:20 pm   #7
Dennis M
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Default 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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Old 19th Nov 2020, 7:33 pm   #8
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Default Re: Home made pulse magnetiser

Quote:
Originally Posted by pmmunro View Post
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.

PMM
Have you seen this thread about the magnetiser I built?
https://www.vintage-radio.net/forum/...d.php?t=165159

Peter
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Old 19th Nov 2020, 8:32 pm   #9
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Default Re: Home made pulse magnetiser

Quote:
Originally Posted by Dennis M View Post
I have got an 800A thyristor ready to fit...
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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Last edited by Al (astral highway); 19th Nov 2020 at 8:34 pm. Reason: cross posting
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Old 19th Nov 2020, 9:15 pm   #10
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Default Re: Home made pulse magnetiser

To clarify, wrong terminology there. I mean back-EMF.
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Old 19th Nov 2020, 9:30 pm   #11
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Default 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.
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Old 19th Nov 2020, 10:06 pm   #12
Dennis M
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Default Re: Home made pulse magnetiser

I have used a bridge rectifier so should not need to add another diode.
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Old 20th Nov 2020, 8:48 am   #13
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Default Re: Home made pulse magnetiser

Quote:
Originally Posted by kalee20 View Post
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.
I had to put a diode in as I found that there was quite a significant reverse voltage across the capacitor. The first diode blew itself to bits, I had to uprate it to a stud mounted rectifier.

Peter
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Old 24th Nov 2020, 8:35 am   #14
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Default 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.
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