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Old Jan 04, 2005, 07:30 AM
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Australia, VIC, Cranbourne East
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Warske That reminds me

the gentleman also stated that when measuring with a gauss meter one should place either side of the magnet being tested pole pieces wich poke out in front and then place the probe between ,them not on the magnet face.
he then went into a lengthy explanation how the probes need to be treated with the utmost care as any pressure or bending will throw the calibration out.he also mentioned that you need big old oil paper capacators built for those sorts of applications.he then mentioned a pulse type magnetiser that does not use capacators mmmm he would not go into ant detail on that tho' i thought when i saw the rat trap that was what you were going to do cheers.
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Old Jan 04, 2005, 08:31 AM
Fidler & twidler
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' Lots of mistakes

Warske
Most of the advances in most fields are the result of mistakes e.g. the transistor - the guy was trying to make 2 identical diodes for radar recievers - the clever bit is building on the results of the mistake

Your dismembered rat trap looks as though you could have got round the bounce problems by getting a cold weld on the 2 copper contact strips.

I just found 20 6800 uF 450 volt Caps from the old pulse laser - will start reforming some today - now where did I put those sandbags

Mike
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Old Jan 05, 2005, 09:56 PM
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Just how fast will this rat trap go?

Quote:
Originally Posted by olmod
he then mentioned a pulse type magnetiser that does not use capacators mmmm he would not go into ant detail on that tho'
Thanks for all the info. The more the better! I have read that some magnetizers just used a half cycle of the mains voltage. If you had a 220 volt service, a half cycle of that would give you an 8 mS pulse (in the US) of 311 volts peak (root 2 times 220). That would work quite well. My capacitors were only charged to 330 v.

Many years ago, I worked in a lab at Texas Instruments in the motor controls div. The back room had special 440 VAC service piped in from the power grid with very high current capability. It was for testing their motor protector designs. They had SCRs set up to apply the full 440 to whatever they wanted for a few cycles. On slow days you could hear explosions coming from back there. The techs got bored, and were using it to blow up capacitors.

Quote:
Originally Posted by empeabee
Your dismembered rat trap looks as though you could have got round the bounce problems by getting a cold weld on the 2 copper contact strips.
I have hopes!
Quote:
I just found 20 6800 uF 450 volt Caps from the old pulse laser - will start reforming some today - now where did I put those sandbags
My puny capacitors pale by comparison! Each one of yours is more than 4 times the energy of my entire bank.! Happy magnetizing!

---------------------

Now that I've got it souped-up, lets see just how fast the rat trap will go!

Actually, since I silver soldered a stiffening bar to the business end, it won't be as fast as the rats experience, but it will be interesting to see how close it is to the desired 30 MPH.

I added another switch by attaching a bent paper clip to the side (see picture) which contacts the arm as it goes by. That signal goes to the 'scope trigger input. (Note that, instead of a scope, an inexpensive counter and frequency source could have been used to get the data.)

Then I captured a trace of the arm hitting the upper contact, and finally the upper and lower contacts coming together (see picture, and note the contact bounce later in the trace). The contacts are made out of flattened copper tubing.

I don't have a storage scope, but it worked pretty well to stick the camera on a tripod and point it at the scope. I set the camera for half second exposures, then clicked the shutter button and pulled the cord to release the rat trap.

With the scope set to 0.1 mS/division, you can see that the time for the contacts to close is 0.12 mS. With a contact gap of 0.048 inches, this works out to 0.048 / 0.12e-3 = 400 inches/sec. or 22.72 miles per hour. This is less than the 30 MPH I was hoping for, but it should still be a big improvement from before.

By the way, in the first picture you will notice some big C clamps holding the switch to the table. Without these, I found that my readings were somewhat random. The board would jump when I released the spring, and that affected the results.

Warske
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Old Jan 05, 2005, 10:18 PM
go flyin or die tryin
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[QUOTE=Warske] Perhaps eventually we'll even get to the flux capacitor!
[QUOTE]

what is a flux capacitor
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Old Jan 06, 2005, 01:39 AM
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Flux Capacitor

Quote:
Originally Posted by new flyer
what is a flux capacitor
For my concept, see the picture below. I was spinning off of Andy W's post:
Quote:
Originally Posted by Andy W
Now, can you make a flux capacitor?
..a
You can also read more about it here, and there are other references which Google didn't pick up.

I think its a bit like the concept of a brass magnet. My dad used to tell the story like this (I think he actually saw this happen):

A little after the new guy Fred was hired, one of the foremen asked him to go down to supply and bring back the brass magnet, knowing of course that there was no such thing. When Fred asked supply for it, the fellow there told him to wait, and took a regular magnet and plated it with brass, which he gave to Fred without comment. Fred took it to the Forman, who was bewildered and exclaimed: "There really is a brass magnet!"

My picture is in reply to Andy's question, though I'm not sure he realized it!

Warske
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Old Jan 06, 2005, 11:34 PM
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Transient Analysis with switch arcing

I've measured the parameters of my capacitor, coil, and switch, and used those in a transient analysis program to model the behavior of the circuit.

The only parameter I didn't measure was the arc resistance. To get that I used the previous assumption of 2k amps peak, and adjusted the resistance until the simulation came out right. That gave me an arc resistance of 117 mOhms.

Attached is the schematic, the simulation output, and the circuit file I used.

For a simulator I downloaded and installed WinSpice3 v1.05.01 which is fully functioning shareware. This one runs under Windows, but there are probably other versions of Spice3 for other systems.

To run the simulation, you would need to rename the circuit file to have a .cir file extension. Then in WinSpice3, you click File, Open, and open the circuit file. It should plot the data automatically. To modify the file, open it with Notepad, make the changes, and save it. The new plot should be generated automatically.

Looking at the attached plots of capacitor voltage and coil current, notice that the capacitor voltage never goes negative. Thus, my flyback diode wasn't actually needed, and I didn't model it in this simulation.

Warske
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Old Jan 06, 2005, 11:35 PM
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Transient Analysis with low switch resistance.

With the new rat trap switch, the resistance is about 1mOhm. Notice what this does in the simulation.

We get lots more magnetizing current, but the capacitor voltage goes negative, which in real life could cause it to explode. Its time to put in a flyback diode.

In case anyone asks, these curves are known as exponentially decaying sinusoids.

Warske
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Old Jan 06, 2005, 11:35 PM
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Transient Analysis with low switch resistance and a flyback diode

In this simulation, I added a flyback diode across the capacitor to keep it from reverse charging.

Notice that the peak current through the coil (red) is the same, but the diode current (green) gets up to nearly 4,000 amps. The Capacitor current is shown in blue.

Its a good thing I didn't actually test my hardware this way, since my diode is only rated at 3 amps continuous. But for short pulses of current like this, the diode will take more than three amps. With this pulse width, it might even take up to 400 times its continuous rated current, based on absorbed energy ratings. That would be 1,200 amps. Still not enough.

I can get a bigger diode, or I can add some resistance back into the circuit until it is critically damped so that the voltage doesn't try to go negative.

Next, I want to explore how the number of turns of wire in the coil affects the circuit behavior.

Warske
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Old Jan 07, 2005, 07:24 AM
Fidler & twidler
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Cranfield U.K.
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Quote:
Originally Posted by Warske
simulation
Oh wow - Tha puts the Kybosh on my guestimated 100A inrush doesn't it

No wonder the early experimenters blew things up in comprehensive ways !

Thank you for your elightening work.

Will play with the sim b4 I try my 6800 uf caps - looks like I will need the sandbags! ( got a healthy spark from 9 volts so who knows what will happen on 250v from an old ' tube ' sig gen I found)
Time to work Very Slowly indeed.

Mike
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Old Jan 08, 2005, 01:53 AM
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Transient Analysis with more turns in the coil

empeabee,

Those monster caps of yours probably have quite a bit less resistance than what I've been working with. They should give you even more current!

I read that microwave oven transformers step up to around 4000 volts, and can be salvaged from dead ovens. If you need more than 250 v ...

---------------------

What happens if we keep the size of the coil the same, but change the number of turns?

If there were no resistance in the circuit, all of the energy in the capacitor would transfer to the magnetic field in the coil by the end of the first quarter cycle of oscillation. That is, the strength of the magnetic field would be determined only by the amount of energy in the capacitor, and the number of turns in the coil wouldn't matter.

For example, if we double the number of turns (from 20 to 40), the inductance of the coil goes up by a factor of 4. The oscillation frequency and the peak current are both cut in half. The number of amp-turns (and thus the strength of the magnetic field) remains the same.

As an aside, note that the oscillation frequency isn't really all that high. Its only about 1.3 kHz, so audio range test equipment is adequate for playing with much of this.

In reality, there is resistance in the circuit, and if we change the number of turns, we have to change the wire diameter so we can fit the new number of turns into the same space.

If we double the number of turns, the coil resistance (due to twice the length of wire with half the cross sectional area) goes up by a factor of 4, just like the inductance.

At first glance, this would seem to suggest that a larger number of turns is going in the wrong direction since the circuit resistance is increased. But I've neglected to deal with the resistance of the capacitor, which is large compared to the resistance of our coil.

Since the inductance goes up and the current goes down with more turns, the voltage lost across the capacitor resistance will also be reduced, and that's an improvement. The little bit of extra resistance added by the increased turns in the coil isn't enough to offset this improvement. Unless we go too far, of course. When the coil resistance is large compared to the capacitor resistance, increasing the number of turns will reduce the performance.

This simulation (see attachments) shows what happens when we double the number of turns in the coil (increasing both L and R by 4x). The peak current went down, but its still more than half. This means that the amp-turns in the coil went up. There are other advantages to the reduced current. The flyback diode and the switch don't have to be as rugged.

We can fine tune the number of turns and use the simulator to determine what the optimum is.
For my present circuit, I found that the amp-turns peaks when the coil has 38 turns (with correspondingly smaller wire) and its resistance is slightly larger than all the other circuit resistance combined (23.6 mOhms vs 21.0 mOhms).

Of course, the coil I used had thick insulation. Using magnet wire should work even better.

This is all assuming that we want to get the biggest magnetic field out of the charge in the capacitors. With more capacitors, it should be possible to use more turns (reducing the current) and still get a large enough field to magnetize the neo magnets.

Warske
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Old Jan 08, 2005, 09:17 AM
Fidler & twidler
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Cranfield U.K.
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Quote:
Originally Posted by Warske
empeabee,

Those monster caps of yours probably have quite a bit less resistance than what I've been working with. They should give you even more current!
Using your Sim files & upping the C to 6800 uf resulted in only 500 Amps more in the coil - the pulse just lasted longer !

doubling to 13600 uf again only upped the amps a bit more!

huh

Smelt an old old smell today - the old sig gen used selenium rectifiers, and they went to that big selenium factory in the sky - with apropriate bad eggs smell - & that was starting the geni on 110vac ( its 230v nominal) to re-form it's capacitors. Back to the drawing board

( uWave psu - dead ones difficult to get here in UK due to lunatic re-cycling laws.)

Instead of more turns in same size, how about making a C core out of an old torroid transformer with the magnet in the (small) gap ?
= more gauss's to the magnet.
Maybe that could be coupled with the half wave multi pulse from the AC supply Idea. No caps to go bang.
Mike
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Old Jan 08, 2005, 12:32 PM
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Quote:
Originally Posted by empeabee
Using your Sim files & upping the C to 6800 uf resulted in only 500 Amps more in the coil - the pulse just lasted longer !
That's interesting. I have to admit, it seems a lot easier to tweek a number and run the simulation than it is to heat up the soldering iron and reconnect 12 ga wire.

What will really shoot your amps up in the simulation is to lower the capacitor resistance. With about double the capacitance, I would guess you have about half the resistance. So you could set rCap in the simulation to a value of 10m. My guess comes from the idea that you can put two caps in parallel and get double the capacitance, but two resistors in parallel gives you half the resistance.

About capacitor resistance, the best reference I've seen so far is Bob Parker's ESR Meter Page. He did some research on what the highest resistance should be for a good electrolytic capacitor, and found that it didn't necessarily work like I suggested above. It probably depends on the way they are built. By the way, his ESR meter might not read low enough resistance to be useful here. It was designed more to detect capacitors that were out of spec.

Quote:
Smelt an old old smell today - the old sig gen used selenium rectifiers, and they went to that big selenium factory in the sky - with apropriate bad eggs smell - & that was starting the geni on 110vac ( its 230v nominal) to re-form it's capacitors. Back to the drawing board
Now that is sad. But maybe its repairable with cheap silicon diodes? The signal generator I'm using is an old tube type Eico hobby model, and so far I've been lucky.

If you were wanting to use the signal generator to check your coil inductance, another way to get the answer would be to calculate it. On page 3 of this pdf file, I found a formula for the inductance of "single-layer air-core coils with a length approximately equal to the diameter"

L = r^2 * n^2 / ( 24 * r + 25 * l )
where
L is inductance in uH
r and l are radius and length in centimeters
n is number of turns

The formula gave me 5.449 uH, vs the 3.825 uH I measured. Not too bad considering I had an oddly shaped coil with 4 layers.

Quote:
( uWave psu - dead ones difficult to get here in UK due to lunatic re-cycling laws.)
My original power supply was from the flash unit in a pocket camera that I had gotten for $5 from a thrift store. I've also heard that you can pull them out of disposable cameras that they throw away at photo shops. Or you could use a voltage doubler on your 230v, but that introduces yet another hazard to be careful of. There should be lots of other options, since you only need a few mA of current.

Quote:
Instead of more turns in same size, how about making a C core out of an old torroid transformer with the magnet in the (small) gap ?
= more gauss's to the magnet.
A while back, Olmod reported: "I had a chat today with the gentleman who knows about the magnatiser i previously mentioned,a handy thing to know is that it has removable focusing poles and they increase the power of gauss ..."

Which suggests there is room for improvement over the simple air-core coil I used. It would be a good area to explore. Your C core idea should work well, the thing to watch being that your material doesn't saturate. You want maybe 40k gauss in the air gap, and I don't know if you can do that with iron.

It would be lovely to have a magnetic circuit simulator for this. Does anyone know of an inexpensive one? Or maybe a link to some designs that have already been worked out? Or more info on the focusing poles?

Quote:
Maybe that could be coupled with the half wave multi pulse from the AC supply Idea. No caps to go bang.
It seems like that should work. An interesting design project...

Warske
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Old Jan 08, 2005, 05:45 PM
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Quote:
Originally Posted by Warske
What will really shoot your amps up in the simulation is to lower the capacitor resistance. With about double the capacitance, I would guess you have about half the resistance.
could be that they are low IR, 'cos they were to pulse a xenon flash tube int a 400w (output) YAG laser - all 20 of them in //ell ( 3in dia 9in long).
Sig gen (EMCO NY) was ( & may be - SI diodes on order) to provide charging current from its HT line.
Quote:
My original power supply was from the flash unit in a pocket camera that I had gotten for $5 from a thrift store. I've also heard that you can pull them out of disposable cameras that they throw away at photo shops.
humm must try local photo lab as well...
Though I've just now unearthed a little strobe flash that runs off 9v - will try first that as a charger tomorrow.
Don't like V doublers - more cap's to go BANG - I've had quite enough of those since 1950 !

Quote:
the thing to watch being that your material doesn't saturate
AH gotchya - Oh Flip
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C core - An interesting design project...
Later -

Mike
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Old Jan 09, 2005, 05:20 PM
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Another hall sensor fixture

For the next round of experiments, I decided I needed a more stable fixture for measuring the strength of the 5x5x1 mm magnets.

Its just like the one in post #15, but more ridged, and it uses an LM317 voltage regulator to keep the voltage at 1.50 volts.

Warske
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Old Jan 09, 2005, 07:20 PM
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Quote:
Originally Posted by Warske
For the next round of experiments, I decided I needed a more stable fixture for measuring the strength of the 5x5x1 mm magnets.

Its just like the one in post #15, but more ridged, and it uses an LM317 voltage regulator to keep the voltage at 1.50 volts.

Warske
How wonderfully crude - In the best tradition on the mad professor - where is the black board with E=MC2 on it ?

Think is it must work or you'd not have published.
Great.
Mike
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