Spot Welding with a MOT

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Charles Vorbach
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Spot Welding with a MOT

Post by Charles Vorbach »

I found a design for a microwave oven transformer spot welder at http://www.rtftechnologies.org/emtech/spot-welder.htm by Andrew Seltzman. He used it to spot weld his grid and it looks like it might be useful for making a lot of the necessary or convenient small welds and brazing involved in building a chamber/fittings. I believe you could even adapt it for induction welds.

My understanding is that he uses the MOT to step down mains to a low voltage/high current which dissipates a lot of power in defined point for a good weld. Now MOTs are infamous for not being current limited like a neon sign transformer so they can supply a lot current. This also makes them deceptively good at killing you.

I was wondering though, does Ohm's law still apply here and if the voltage is dropped too low the resistance will prevent a high current from flowing?
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Richard Hull
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Re: Spot Welding with a MOT

Post by Richard Hull »

This is true and is the reason a tremendous amount of pressure is classically applied by small surface area contacts by the welding machine on each side of the two piece to be joined. In this manner the highest resistance is usually at the contact of the two surfaces and all the energy is expended in melting the metal at that point. Another term for spot welding is resistance welding.

Most spot welders employ a capacitive discharge type of impulse welding such that there is only a temporary short circuit. In this manner, no welder circuitry, contacts or cabling is melted or damaged during the welding process.

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
Charles Vorbach
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Re: Spot Welding with a MOT

Post by Charles Vorbach »

Thanks for your reply. These little transformers seem pretty useful. It might be fun to get one - especially since it can be used as an inductive ballast in a real fusing power supply.
NicKolas Garrett
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Re: Spot Welding with a MOT

Post by NicKolas Garrett »

Mot's are actually current limited by the perpendicularly laminated shunts that you will find inside the cross section of the core.
if you want a true current monster, you must remove these. They will be between the primary and secondary of the transformer. typically they are wrapped in paper.
Expect to blow some breakers while causing dead shorts between a work piece and a few wraps of the core. Mot's kind of ignore some rules of transformer design. That being volts per turn. It is very important that you do not let the core over heat or it's magnetizing current will shoot up to quite high numbers.
So yes you can probably get some great tack welds if you do not wish for deep penetration or overall quality, Otherwise look at other for other sources of power, such as starter batteries or properly wound transformers intended for continuous duty.
Jerry Biehler
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Re: Spot Welding with a MOT

Post by Jerry Biehler »

The majority of the spot welders out there are not capacitive but just good old transformer tech. I have worked on small hand held units to big, several ton spot welders that ran off three phase. Some still use Ignitrons but a lot have been upgraded to SCR control and the most modern are switched mode power units. I have also worked on their relative, the stuff welder, they run at a much higher voltage, up to ~150v.

In general you want about 1 volt or so at the output, that is what most of the smaller hand held spots use and the gauge of the wire used, the tongs, and the tips will all make a difference in the weld current. For something like one made out of a MOT it is just going to take playing.

You can also build a capacitive type. Some people just charge up a bunch of big electrolytic in parallel and zap. The problem with this design is you need a real big switch to handle the weld current. To get around this you charge a capacitor to high voltage and then dump that though the weld transformer. This way you can use a mercury relay or a hockey puck SCR to do the switching. I use an old Hughes Capacitive Discharge unit to make filaments for my SEM.

I think there was an article on hackaday a few months ago with a guy building a capacitive welder.
Jeroen Vriesman
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Re: Spot Welding with a MOT

Post by Jeroen Vriesman »

Jerry, what is a "stuff welder"? sounds like a tool to weld stuff :)

For my own spot welder I used and old transformer with very high secondary current, 230 -> 12V, 50A, the seconday winding was on the outside, so I cut it in half and connected both halfs parrallel, making a 230->6V, 100A transformer.
It's a heavy old transformer, second hand, 10 euro.
The secondary resistance is in the milliohm range.

On the primary side I used some big electrolytic capacitors, 2x 1000uF, 500V.
Make sure to use capacitors which can handle the high current.
I can charge them with a 100mA/500V supply, usually I only need to charge them to about 150V to make small welds, I use a thyristor to discharge the capacitors in the primary side.

I would not recommend discharging capacitors directly without a transformer, usually the capitors are not made for such high peak currents.

So, look at the max low-voltage-side current of your MOT's, and the resistance of these windings to see how much MOT's you have to put in parrallel, my spot welder has a low-voltage-side resistance of <10milliohm, I don't know the minimum you need to make good welds, but initially the spot needs to be heated to become the point with the highest resistance, to make the energy dissipation happen in the spot the rest of the circuit needs a low resistance, wild guess: at least lower than about 80 milliohm.
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Richard Hull
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Re: Spot Welding with a MOT

Post by Richard Hull »

When I talked of capacitive discharge welding, it was with the tacit assumption that the caps were discharged into a weld transformer. I would never use direct capacitive energy to the weldment.

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
Peter Schmelcher
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Re: Spot Welding with a MOT

Post by Peter Schmelcher »

To my surprise some super capacitors are capable of starting cars. https://www.youtube.com/watch?v=z3x_kYq3mHM
In the video the guy put 2.7V 350F caps in series. Fully charged they will crank the car engine for about 4 seconds. I would think that a transformer free welder is not out of the question but a high current switch might be a gotcha.

-Peter
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Re: Spot Welding with a MOT

Post by Jeroen Vriesman »

supercaps have indeed improved, this one http://www.maxwell.com/images/documents ... 7105-4.pdf allows 250A peak.

The switching is another thing, maybe trigger a spark gap with HV to switch the low voltage, would be a very lightweight spot welder.
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Re: Spot Welding with a MOT

Post by Jerry Biehler »

Richard Hull wrote:When I talked of capacitive discharge welding, it was with the tacit assumption that the caps were discharged into a weld transformer. I would never use direct capacitive energy to the weldment.

Richard Hull
Even at that most spot welders out there do not have capacitors driving a transformer.
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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.
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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.

Dan Tibbets
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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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