Spot Welding with a MOT

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Richard Hull
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Re: Spot Welding with a MOT

Post by Richard Hull »

The spot welder that fusioneers need are generally all of the Capacitive discharge type. (Small element spot welders)
It you are putting together automobile skins or industrially working large pieces, a far less elegant and more beefy welder is demanded. The Capacitive discharge welder is a precision controlled source of small energy delivery with no error in how many joules are brought to the weld. The joule delivery is just dialed in. It is rare to see a 100 joule unit in most of the apps we would be interested in. (grid making and hyper thin foil metal welds.)

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Richard Hull
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Re: Spot Welding with a MOT

Post by Richard Hull »

Again, precision spot welders of the type that are used in small welding, (tube elements, grids, etc.), virtually all have capacitors discharing into transformers they are full electronic welder systems. Such welders are most often equipped with hand pieces to apply the welds unless on a vacuum tube assembly line.

Big, giant clunky auto welders are made cheap and need little precision. They are most normally just transformers with inductive limiters and dialed in settings via taps, etc.

My spot welder (Raytheon tube element welder) is limited to 60 joules with stops at 10, 20, 30, 40, 50 and 60 joules with a fine vernier between stops. Capacitors charge to 600 volts and are thyratron switched into a transformer, whose secondary puts out under 20 volts into a 5 ohm load. Most weld instances are tiny fractions of an ohm and the weld current is in the 100amp range for some milliseconds.

Welders for sheet automotice steel and fine precision miniature welding are not cross comparable....different beasts altogether. We are speaking of grids and other fine interior component attachments.

Of course, it is not possible to assemble a functioning fusor with a spot welder, as most folks here know.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
DonaldE
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Re: Spot Welding with a MOT

Post by DonaldE »

You are obviously correct it’s not the resistance, but the complex impedance that matters, because that is what limits AC current.

To be honest, I find your story a little hard to believe. Assuming the surface area of a man’s hand is about 0.02m^2 (10*20cm), a glass thickness of 1.5mm, a relative dielectric constant of 10 (range is about 5-10, assuming worst-case), the capacitance works out to approximately 1.25nF, which at 50Hz will have an effective impedance of just over 2.5 mega-ohms. Assuming a line voltage of 230Vrms, this would allow a maximum RMS current of about 90 microamps, assuming there is no other source of impedance in the circuit. The resistance of the human body can easily be much less than 2.5 mega-ohms, so we can probably ignore it in this case.

If this man indeed had a cardiac arrest, I’d think he must have had a pre-existing heart condition, quite possibly unknown to him. Obviously, I’m not a cardiologist, so I wouldn’t claim it’s impossible, but 90 microamps seems very low to trigger any serious problem.

On the other hand, it would be VERY foolish to assuming anything under 15 milliamps can’t kill you!
Last edited by DonaldE on Fri Oct 09, 2015 3:58 am, edited 1 time in total.
DonaldE
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Re: Spot Welding with a MOT

Post by DonaldE »

I’ve seen a bunch of these homebuilt spotwelders, usually built with transformers from old microwave ovens and yeah, they all do sort of work a bit.

None of these seem to put much effort in the reliability of these things however.

For a good spotweld it’s necassary to control the amount of power which is put into each weld, the pressure in the melting zone has to be high enough to press your material firmly together and the pieces of matiearila may not move during cooling of the weld.

A foot peddle can easily be added to increase the pressure ( And will keep both you hands free for positioning yor 2 work pieces).
It’ also pretty easy to add a timer (100Hz, 1/2 cycle counter) and a beefy solid state relay (use your favourite microcontroller) to adjust the engergy put into a spotweld. Every half decent spot welder i’ve seen has a timer to control the energy inpot to the weld.

Adding the foot pedal and timer also has another important benefit.
This way you can apply the clamping force on the spotweld untill the weld has cooled down enough to get out of the plastic region and this greatly reduces the chance of disturbing the hot welding zone.

Checking you weld:
If you torqe your to work pieces from each other on a single good spot weld your work piece breaks and not the spotweld. This will leave a hole in one of the 2 pieces.

Electrical safety:
Secondary is a “non issue” exept for (double) isolation from the mains voltage. Becasue of the extreme turns ratio secondary voltage is usually < 2V and perfeclty safe to touch. I haven't seen any spot welder with isolaton on the secondary side yet. So just put a box over the primary side.
Dan Tibbets
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Re: Spot Welding with a MOT

Post by Dan Tibbets »

I built a spot welder from a MOT by removing the secondary windings and replacing with about 5 windings of light starter cable wiring. About 3-4 volts and it heated well but not evenly. The major impediment was building a good contact arm structure with, as pointed out, small contact area and ability for high pressure. I finally abandoned it and bought a cheep Harber Freight spot welder.

With thin sheet or wire metal, the burn through of the material is difficult to avoid (at least at my skill level). I placed a MOT as as an input ballast (secondary shorted) and this decreased the heating enough that small wires may be manageable. I have not used this much. Here the big thick contact arms is the frustrating limitation.

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Steven Sesselmann
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Re: Spot Welding with a MOT

Post by Steven Sesselmann »

Here is an old link to a post where I made a MOT spot welder, I am sure I still have it in a box somewhere.

viewtopic.php?f=6&t=2897&p=12407&hilit= ... der#p12407

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG
Jerry Biehler
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Re: Spot Welding with a MOT

Post by Jerry Biehler »

Dan Tibbets wrote:I built a spot welder from a MOT by removing the secondary windings and replacing with about 5 windings of light starter cable wiring. About 3-4 volts and it heated well but not evenly. The major impediment was building a good contact arm structure with, as pointed out, small contact area and ability for high pressure. I finally abandoned it and bought a cheep Harber Freight spot welder.

With thin sheet or wire metal, the burn through of the material is difficult to avoid (at least at my skill level). I placed a MOT as as an input ballast (secondary shorted) and this decreased the heating enough that small wires may be manageable. I have not used this much. Here the big thick contact arms is the frustrating limitation.

Dan Tibbets
Way too many windings. One turn is all you need all you want is about 1v open circuit for a standard resistance type spot welder.
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Richard Hull
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Re: Spot Welding with a MOT

Post by Richard Hull »

Again, as stated above the tube element spot welder associated with fine wire and thin sheet metal work is capacitively discharged into a weld transformer. As the capacitor has a finite charge in it, it has finite and well controlled energy delivered to the weld. An MOT can easily be made into a capacitive welder using this scenario.

How? Make the MOT with two or three turns of insulated 8 gauge copper ground wire. Construct a very weak 200-300 volt DC supply. Use this to charge a 300 uf 400 volt cap and thyratron or SCR blast this into the 120 volt primary of the MOT. The energy is varied by variacing the AC to the linear 300 volt supply. My Raytheon welder used a special transformer and a 600 volt DC supply. A clever and determined amateur could easily assemble a capacitive spot welder.

However, it is also realized that virutally 100% of the folks working this angle will just use this thing once and never use it again. A major effort is just not needed to make a grid or two and then have the finished spot welder never used again.

My first grids, made back in 1998, used a 120 volt, 5 volt 50 amp transformer that I bought at a hamfest in the 1980's. The key was that I went to a welding supply store and bought two of their smallest replacement tips created just for spot welders. (special alloy). This worked great until I found the Raytheon tube element spot welder at a subsequent hamfest. I have used this for many non-fusor related welds of small parts and thin sheet metal, so it does still solve problems for me.

As for that old giant 5volt 50 amp transformer....It is now used to drive several Marinov motor demos that freak folks out.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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