I have seen a few projects which use an old microwave oven transformer re-wound with a low voltage secondary to make a hobby spot welder, and I decided to have a go at making one. I ended up designing my own using copper sheet to make the secondary, following an unsatisfactory experience attempting to use wire I made from about 100 strands of stripped down CAT5 cable, which at nearly 6mm diameter wasn't thick enough and melted its insulation, causing the winding to short circuit.

The copper strips were cut from an old domestic hot water tank, of the sort usually equipped with an immersion heater, though this one had a heat exchanger (which looks like it really needs to be made into a tesla coil primary... hmmmm....). The steel parts came mostly from an old rotary washing line which had lost it's line, and the electrodes were made from a brass arm from an old ball cock (still trying to think of something to make from the ball). The wooden parts came from an old pallet and a bit of old shelf. The brass electrodes tend to soften and deform and really need to be made of something tougher. I used the wire from my experimental winding, doubled over, to connect the secondary to the electrodes.

My original experimental winding (100 strands of CAT5) gave a cross sectional area of less than 28.3mm2. Standard wire of this thickness would be rated at only 104 amps in free air,

Using 5 x 0.5mm copper strips, 25mm wide gives a cross sectional area for the winding of 62.5mm2 , which if it were standard wire would be rated at about 185 amps. Retrospectively, I should have used 6 layers of copper, giving a rating of approximately 212 amps, however I had anticpated an extra turn of my winding and 6 layers would have taken up too much space.

Today it blew a 5 amp fuse, showing it to have been pulling at least 750 watts. Unfortunately I have no way to measure current in the primary as my meter only measures up to 1 amp, so I don't know what the actual power being delivered is.

Afterword - If I were building this thing from scratch again, I would position the transformer to the side of the jaws, so using shorter wires, and house the transformer to cover up the mains wiring. I'd connect the wires to the where the electrodes emerge from the jaws too, so the current has less electrode to go through.

Step 1: Tools and Materials

For the welder
1 microwave oven transformer
0.5mm copper sheet for re-winding, taken from an old hot water cylinder
Scrap wood
10cm (4") hinge
19mm steel tubing from old rotary washing line
Steel rod taken from old rotary washing line
Brass or copper rod
Nuts, bolts, screws
Copper tubing offcuts
3 core mains cable, connectors
Cross dowels
Steel sheet offcuts

For the footswitch
Microswitch from microwave oven
Scrap wood
A long bolt
Spring from liquid soap bottle

Wood saw
19mm drill or hole saw (same diameter as steel tube if different)
small wood drill bits
high speed drill bits
taps and dies

Step 2: Overview of Construction

The welder uses a microwave oven transformer with the secondary cut off and rewound with 4.5 turns of 5 layers of 0.5mm copper strip, mounted on a plywood base-board. The ply I used is an inch thick, but that's just what I happened to have. The winding should have 6 layers, but I was anticipating an extra turn and the extra thickness would not have left enough room.

The jaws are made from short lengths of steel tubing I cut from an old rotary washing line which had lost it's line. These are mounted in wooden blocks, cut from the thick parts of an old pallet I found. The advantage of this method is that the jaws can be precisely adjusted before securing in their final position, can be made fairly slim and are insulated from each other.

The two wooden blocks are connected by a hinge. I chose a 10cm (4") hinge for stability. This gives a nice spacing between the ends of the jaws.

The welding electrodes are made out of a piece of brass rod which I took from the ball-cock of an old cistern. I made connectors for them using cut up and hammered flat bits of copper tubing, washers, and M8 nuts. They are secured into the ends of the jaws using cross dowels from old self-assembly furniture and cut off lengths of the bolts which went with these. Brass isn't a very good choice of material since it doesn't conduct as well as copper, and has a lower melting point, but it's all I could find. Tungsten or elkonite cored copper bar would be far superior.

The welder is operated by a foot-switch which I made using one of the micro-switches from the oven, some bits of scrap wood, one of the long bolts from old self assembly furniture, and a spring removed from a liquid soap dispenser.

Step 3: Decontruct the Transformer

The microwave oven transformer is factory wound to give a secondary voltage of around 2000 volts. I noticed the fuse connected to the secondary on my transformer is rated at 800 milliamps, so this would give an absolute maximum power output of about 1600 watts, however the usable power should be half this, more like 800 watts - the output of a powerful domestic microwave. Owing to the change of purpose for the transformer, some de-rating should be expected (I have no idea how much). Removing the transformer shunts increases the available power but introduces a risk of core saturation and overloading the primary.

Unfortunately the transformer core cannot be easily opened up for easy modification as the laminations are welded together, unlike old fashioned designs. I believe this is normal construction for a microwave oven transformer. The intrepid builder could grind off the welds attaching the "I" pieces to the "E" pieces, but a means of fastening the parts back together would need to be found, and there is a risk of de-lamination. IF you can put it back together satisfactorily, this would be a better way to deconstruct and re-wind the transformer as the old secondary could be pulled off undamaged and the new secondary wound on a bobbin, separately from the transformer frame. Perhaps I will try this for the next one...

The secondary will consist of maybe a couple of thousand turns of fine wire. Cut this off on one side with a hacksaw, then knock out the rest with a hammer and a bit of wood the same size and shape as the winding. If there is any gap in the fit of the piece of wood, strands of wire will get stuck in the gap and jam the whole thing up, at which point it will become very difficult.

This is tough going, and you still have to handle the transformer carefully. The primary is easy to identify as it is wound with fewer turns of thicker wire. The transformer I used had aluminium wire, so this was easy to cut through. Be very careful that you don't damage the primary or (less importantly) the laminations. There will be lot of insulation around the core - you can safely remove most of this. My transformer also had a low voltage winding which I removed, and two shunts consisting of small sets of laminations, which lay between the primary and secondary. The purpose of these is to waste some of the magnetic field, and prevent the core becoming saturated. Removing them will allow more current to be produced (which we want), however also increases the possibility to saturate the core and overload the primary (which we don't want). I left the shunts in for the first, trial version of this, using heavy wire, but took them out to make more room for the copper strip winding, and it seems to work fine without them. Take out most of the insulation, but leave a couple of layers intact.

Step 4: Prepare the Copper

You need enough copper strips to produce a conductor with an adequate cross sectional area, so aim to use all the available space in the transformer core. They need to be reasonably clean so that they will act as a single conductor. Sharp edges must be kept away from the insulation.

The first attempt at this used wire 6mm in diameter, made from strands stripped from CAT5 cable fastened together with tape, however it got too hot, melted the tape and short circuited. The current capacity of a wire depends on its cross sectional area, and this wire gave about 28 square mm. Doubling this is 56 square mm, which can be made out of a flat wire 1 inch wide and 2.5mm thick, actually giving 63 square mm. Interestingly, the equivalent area in a round wire is specified for less than twice the current. Presumably this is because of diminishing heat dissipation as the surface area to volume ratio decreases.

Make a test winding with normal wire to see how many turns you need to get about 5 volts. Measuring this wire will give you a very rough estimate of the length you need. I based this on a wikipedia article saying this is the voltage at which industrial spot welders operate, but it should work at a lower voltage than this. Mine gives about 4.4 volts.

My transformer has space for a winding an inch wide, and removing the shunts gave an extra 1/4 inch, which I left as 1/8 inch clearance either side of the winding.

I made a fat wad of off-cut copper sheet, measured the thickness with a ruler and divided this by the number of layers to get the thickness of the sheet - 19 layers measured 9.5mm, showing the sheet to be 0.5mm thick - hence my arrival at 2.5mm thickness - 5 layers.

To obtain a more exact figure for the length you need, measure the transformer inner core. The sum of all the sides gives you the length of the the first layer of the first turn. Add 4 times the thickness of the total winding for the next turn, and then the same amount again for the next, and so on. You also need to allow a bit extra for the corners. For me, the longest length possible was 48 inches, though I worked out I need at least 55 inches, leaving me half a turn short. It would be possible to add a bit on, but probably isn't worth it.

If you are using a hot water cylinder like I did, unscrew and remove the immersion heater if it has one. Strip off the tank insulation if it is the sort that is bonded to the metal. I found a wallpaper scraper to be ideal for this job.

I used bypass secateurs to cut the tank open since I didn't have my tin snips handy, however this worked quite well as it kept my hand out of the way. The secateurs weren't damaged by the soft copper. Cut the ends off the tank, then cut alongside the long seam down the side. If it has a heat exchanger, cut around the pipe fittings.

Cutting the copper
Cut the strips (with tin snips) so that they have an "L" shaped end. The leg needs to be 1.5 inches long. This goes at the inner end of the winding so that you can connect to it. Clean the strips with a wire brush (a wire brush fitting on a drill will shift the worst dirt, but you still need to use a hand one to avoid too much damage to the copper. I got the strips nice and shiny, but this is actually unessesary except for where you are going to connect them together.

When cutting the copper, a sharp edge is created on one side of the cut edge. Cut each edge from alternate directions so that you end up with the sharp edges on the same side (see illustration). That way you can ensure the sharp edges are on the inside of the bundle where they won't cause damage. Ensure that you have at least one left handed and one right handed piece in terms of how the sharp edges lie, for the outside pieces.

You are going to use bolts to connect the winding, so align the pieces carefully, clamp them together, and drill two holes through the end of the leg. I did mine about 1/2 inch apart and 1/2 inch from the end, though this doesn't have to be exact. Put bolts through the holes and fasten them up nice and tight.

I originally intended to use oiled paper as insulation, but eventually settled on using baking parchment. There is no need for high voltage insulation, but it does need to be able to conduct heat, and without spending money on special transformer tape this was the best I could come up with. Cut strips so they are twice as wide as the space in the transformer and fold them in half lengthways. This is to make them slightly more durable.

If you damaged the original insulation down to the metal, like I did, wrap a couple of layers of new insulation around first. Align the strips of copper as exactly as you can, ensuring the two outer strips have the sharp side of the cut edges towards the inside of the bundle, and clamp the leg-ends together in a vice. Measure the face of the core of the transformer frame and subtract the width of the strip legs, and divide difference by 2. Treating the strips as one, make a sharp right angle bend, the distance you just worked out, past the legs, so that the legs will end up in the middle of the core. Use your square if you have one to ensure the bend is exactly at right angles to the length of the strips. All the other bends should follow on correctly if this first one is accurate.

Step 5: Rewinding the Transformer

The layers of copper need to act as a single conductor, have adequate insulation between the turns, and be tight so that heat can be conducted away. The transformer primary needs protection from damage during the winding process.

As my first attempt at winding this had a short circuit and required re-winding, I was able to try two methods of winding the copper. The first method was to feed the strips through and bend them one at a time, the second was to treat them as a whole. The one-at-a-time method gave a tighter winding, and the strips were easier to bend, but was also trickier as the first strips to go through would fill the space and make it hard to get the later ones through. The all-together method made it much easier to get the strips through, but it was harder to get good bends and the winding seems looser.

Feed the bunch of strips in through one side of the core, with a layer of insulation under them, and clamp the leg flush against the centre. Tightly wedge the strips down the side with some flat scrap wood (this is important to prevent bowing of the turns) and feed the strips, one at a time, down the other side. Remove the clamp (the wood should hold the copper in place) and start to bend them tightly against the core where they come out. Feed them through the other side of the core and finish the tight bend you started, gently hammering the winding down.

Make a second flat wedge and champher one edge of one end to about 45 degrees, and drive this down in against the windings to flatten them down. You will probably need to start with a thinner wedge, drive it in, then follow through with the thicker one. Again, it should be a tight fit. This should cause a tight bend where the copper goes back in. Hammer the winding a bit to shape the bend. Knock the first wedge out, then bend the copper over the L shaped part where you started and feed it and the insulation up the first side, again gently hammering it into shape. Clamp the winding again, thin your first wedge by the thickness of the winding and champher it, and drive it back in - again, you may need to use a thinner wedge first. Shape the bend slightly with a hammer. After this first turn is complete, there should be no need to use the clamp. Keep repeating the process until you have as many turns as you need, adding layers of insulation as needed.

It's a good idea to test the transformer for short circuits after every turn or half turn. Do this by connecting it to the mains. If it hums really loudly and gets hot, you have a short. Undo the turn or half turn you just did and test again until you've undone the shorted winding. Check the insulation and insert an extra piece where the hole is, and investigate the copper for sharp edges which will have cut it, and file them off.

If you need to add extra length to your winding, trim the pieces so that the joint will end up on the outside as it will be very bulky. Idealy you would solder them, but be careful not to burn the insulation - you may need a heatsink.

Step 6: Make the Jaws

For the jaws, I used two blocks of wood measuring approximately 3.5" by 1.5" by 5", a 4" hinge, and two lengths of 19mm steel tubing about 10" long. The blocks of wood were cut from an old pallet, and the steel tubing came from an old rotary washing line.

Although I could have run the wire through the tubing, I chose not to for reasons of simplicity, heat dissipation, and strength of the jaws (as extra holes would need to be drilled). Also I would like to leave open the option to use thicker wire at some point. I have seen a design which uses copper tubing for the jaws which then form conductors between the transformer and electrodes, however I thought the conducting area would probably not be enough. I used cross dowels to enable quick and easy removal of the electrodes for renewal and re-shaping.

Hinge blocks
Lay the two blocks of wood with the long edges together, lay the hinge on them with the pin along the edges which meet, and draw around it.

Using a chisel, cut a rebate just inside the line you drew on each block for the hinge. Slope the rebate off a bit towards the edge of the wood to allow for the slope up to the hinge pin. Check that the hinge fits correctly, and screw it into place.

Measure the centre line at right angles to the hinge pin, and mark the centre at the outer edge of each of the two blocks. Fold them together and check the centre marks meet. It isn't important that the marks are in the exact centre, but it is important that they are aligned. Use your square to mark lines across ends of the blocks at these marks, and then find the middle of each of these lines.

Remove the hinge and make a 19mm hole (or whatever size your tubing is) in the end of each block at the place you just marked. Go a couple of inches for each one - they do need to be about the same depth. The exact method you use depends on the tools you have available. I used a hole saw, first making a pilot hole with a wood boring bit. Drilling a pilot hole enabled me to check that the hole was square (to the face of the wood), as I was doing this using a hand-held drill. Apply the hole saw in stages, pulling it out frequently to remove sawdust. Break the core of the hole by inserting a drill bit into the centre hole and pushing it sideways to snap the wood across the grain.

Drill a pilot hole in each piece on the hinge side - this is where the screws will eventually go which lock the tubing in place, so they need to be partway down the depth of the big holes you made. Don't drill the tubing yet. I used a wood boring bit for these, with the tubing in place. This gave a nice shallow hole with a clear centre point, but not drilled through, precisely positioned as the drill doesn't wander. Offset the holes from each other along the pipe so the heads won't short the pipes (which will have the welding voltage across them)

Insert the two pieces of tubing into the holes as far as they will go, and fit the hinge back on.

Position the jaw assembly in place in front of the transformer, and mark where the electrodes will be. They both need to be exactly the same distance from the hinge, so use your square to mark both tubes together.

Make a second mark on each tube far enough from the first to allow for the thickness of the electrode, a gap of a few mm, and a cross-dowel, then a few mm again to allow enough steel to retain the dowel. Screw the retaining bolt into the dowel and hold it and the rod you intend to make the electrode out of, against the tube to check the position of this mark. This is where you will cut the tube to length. Don't cut it yet though in case you make a mistake or decide to change something.

Mark the position for a hole in the bottom wood block so it can go from the hinge-pin side to the tubing side without fouling the hinge screws, and also misses the mounting screws which will be underneath. The hole needs to be big enough for the wire to the bottom electrode to pass through (I used 13mm)

At this point, you might want to tidy up the ends of the wooden blocks so all is flush and nice.

Step 7: Make the Electrodes

I made the electrodes out of a piece of brass rod I retrieved from an old ball-cock. Copper rod would be better, but is probably harder to obtain - for free, anyway. The rod was just thick enough to take a shallow M8 thread, so I used M8 nuts to clamp the wire. The wire is held in shape by a retaining nut, which is locked against a piece of copper soldered onto the rod.

I made retaining nuts for the wire using cut off bits of 15mm copper tubing - though thicker tubing may have been better.

Experience has shown that the brass electrodes wear and deform quite quickly, so something better is needed. Elconite would be nice. In the meantime, I'm going to try to fit steel tips to my electrodes.

For each electrode, allow enough rod to reach a bit more than halfway between the jaws (to allow for wear), plus the thickness of the steel tubing, plus at least 15mm to make the connection. Don't cut the rod yet though - then if you mess up you minimise the amount you waste.

Cut a M8 thread (or whatever size you are using) for about 15mm at the end of the rod. Cut two copper washers with a hole which will give a nice snug fit on the rod - ideally you should be able to screw them onto the threads you cut. Fit each piece of copper onto a rod, get it nice and level, and solder it in place using a blowtorch, If you are using plumbers solder and flux (like I did), make sure you wash off the flux thoroughly as it is corrosive. (You could miss out the copper washer and cut a longer thread and screw on a nut to perform the same function, however this method does use more of the rod).

Retrospectively, it would be better to make the connection at the point where the electrode emerges from the jaw, so the current has less electrode to go through.

Retaining nuts
Get a short bit of copper tubing about 2 inches long and give it a good clean with a wire brush or wire wool so it's nice and shiny. Hammer it flat but without creasing the edges. Mark a centre line down it's length, then fold it in half down this line, again hammering it flat, creasing hard this time. This will be the wire retaining nut.

Mark a portion about 15mm long, in the centre of this piece and bend the ends over. Find the centre of the middle section, and drill a hole in it and cut a M8 (or whatever you are using) thread.

Hold the rod against the tube and mark where you need to cut, then cut off the electrode you just made. File the cut end to a short taper (or round), then fit the piece in the jaw of a drill and file it whilst it's turning to even out the shape. Try to get a rounded shape where the shaped part is quite stubby, but the area of contact will be small.

Screw the wire retaining nut you made onto the rod so it's nice and tight against the piece you soldered on. Cut the ends off the nut so they come just below the end of the rod, and file off any sharp bits.

(Alternatively, you could fit the wire without the retaining nut, then fit the nut on top, prongs down, and screw the electrode into it, then use a standard nut as a lock nut, however I'm not sure this would be sturdy enough using the materials described.)

Step 8: Fit the Electrodes

Shape the wire
I used the 6mm wire I'd originally made to wind the transformer with, stripped and double thickness - though at around 1cm thick it still gets pretty hot. Position the jaws and transformer in their final positions and bend a length of wire so it can loop around an electrode and connect to one end of the transformer winding, then cut it to length. Allow a little bit of spare wire for the moving jaw so you can make it slightly wriggly. Part strip the wire and tape the ends separately, then tape the two parts together, removing more insulation as you go, until you have a single very thick wire, forked at one end and with a loop at the other. Shape the loop so it will fit the electrode. Remember the bottom wire needs to pass through the hole in the bottom block, so allow for this. If you have thinner wire, build up as many thicknesses as you need.

Drill holes
I thought tightening a nut on the electrodes directly onto the tube would crush the tube, so I opted to use cross dowels to clamp them in place laterally.

Drill a pilot hole through one of the jaws for the electrode, and enlarge it so that the electrode is a tight fit. On my welder, the hole needed to be slightly bigger than 7mm. Drill a second hole in the tube, this one at right angles to the electrode (so it is parallel to the hinge pin), large enough to fit a cross dowel (10mm, in my case) so that there is a small gap between the edge of the dowel and the edge of the electrode. Drill right through the tube. Allow a few mm clearance from the outer edge of the dowel, then cut the end of the tube off. Find a short screw to fit the dowel, or cut a section off the bolt that comes with it, and cut a screwdriver slot in the end (or cut off the end with the head, if it is threaded all the way up).

Repeat for the other jaw, using the first holes you drilled as a drilling guide to mark the centres of the second holes. Only use this method to make an initial mark though, as you will enlarge the first hole you made. If you are using a hand held drill you also risk further errors by leaning the drill. Fit the electrodes and dowels, screw the screws in and tighten them to retain the electrodes.

It would be nice to be able to insulate the electrodes from the jaws, but they do get very hot - even the tubes get quite warm and they aren't even carrying any current.

Adjust for short cross dowels if necessary
If the dowel is shorter than the width of the tube (which mine were), drill one side slightly bigger, and make a sleeve to fit the cross dowel, which will fit through the enlarged hole. Fit the dowel in the sleeve and drill a hole in the sleeve to match the hole in the dowel. Shape the end of the sleeve to fit inside the pipe, allowing the end of the dowel to project, and trim the other end so it doesn't stick out too far. I made sleeves out of a bit of sheet steel (from the microwave oven's casing) by cutting a strip, squaring one end, rolling the dowel along it to get the circumference, cutting it off squarely, gripping dowel and steel in a vice, hammering the end of the steel to fit the dowel, turning it over and bending the rest of the steel to fit all the way around, and finally hammering the other end into shape.

Step 9: Assemble the Pieces

Make a baseboard big enough to hold the transformer and jaw assembly. Allow it to extend most of the way along the jaws, and allow a couple of inches behind the transformer for wiring. Allow clearance at either side - I only left half and inch, but the more the better since there needs to be plenty of room for air to circulate if you build and enclosure. My board is about 6 x 17 inches, though it would have been better if it was wider.

Make clamps to connect the wires. These are made out of steel. Cut a rectangular piece the same length as the width of the strips the transformer secondary, and wide enough to hold the two forks of wire with some 4mm bolts between them, plus a little bit over. Shape one long side by bending it around the shank of a drill bit (I used 5mm) to make a scant half circle, then bend it back against the jaw of the vice. Take it out and turn it around, make another bend back in the vice 4 or 5 mm away from the first one, then shape the remainder around the drill bit again. Drill two holes in the middle portion to align with the holes you drilled in the leg of the transformer winding. Make a second clamp the same, but this time you can drill the holes first and use it as a guide for where to drill the holes in the winding.

Get everything in position and make small marks to identify the edges of the bottom jaw block, remove the jaws, and drill 4 holes in the baseboard to take 4 wood screws which will secure the block. Position the holes so that they don't interfere with the pipe inside the block, or with the hole you made for the bottom wire, or the screws for the hinge. Countersink the holes on the underside of the board then clamp the jaw assembly in place and screw in its mounting screws from underneath.

Fit the transformer into place, and screw it down onto the baseboard using it's original mounting holes. Screws of the non-countersunk kind, eg pan or dome head, are better for this, as well as looking better.

Fit the electrodes in place if you haven't already done so, push the ends of the wires onto them, fit washers and nuts and tighten securely. You may need to file the washers a bit to fit inside the retaining nuts. Feed the bottom wire through the hole in the bottom block before connecting it.

Take the tape off the ends of the wires and use the clamps you made to connect them to the transformer. Fit them so the head of the screw is next to a washer, next to the back of the winding, then the wire goes against the front of the winding, then then clamp, then two more washers PER BOLT, then the nuts. Tighten as much as you can, and re-tighten after the first few welds. Use wire wool to make sure each layer of the winding is nice and shiny on both sides before you connect the wire, and the wire strands are nice and clean too.

Step 10: Footswitch and Cables

A simple footswitch is easily made using one of the micro switches (normally open type) from the microwave oven and some scrap wood. A small spring (obtained from a liquid soap dispenser bottle) reduces the chance of accidental operation (two springs would be better). Far better designs are possible, but I made this for speed and simplicity.

Cut a piece of plank for the base and a shorter, slightly narrower piece to operate the switch. Cut a small block of wood which will be narrower than the planks after the switch is fixed on, a little taller than the switch allowing for connections, and deep enough to mount securely and project forward a bit to hold the spring.

Make side pieces out of thinner board. These will support the top plank as well as cover the electrical connections and protect the switch.

You need a long rod or bolt to go through the top board - this will be the pivot. It passes through the two side pieces and a hole drilled across the width of the top plank.

Nail the microswitch to the side of the small block of wood so that the button projects above the top, being careful not to damage the casing. Fasten the small block of wood to the base, and drill a hole in it to retain the spring. A small nail driven through the end will stop the spring bouncing out.

Position one of the side pieces next to the small block, and hold the top board in a position where it depresses the switch, as though it were pivoting at one end. Mark the position of the end which pivots. Clamp both side pieces together so the edges are aligned, and drill a hole through both pieces which will take the bolt or rod you are are using, as a tight fit. Drill a long hole through the end of the top board through which the pivot will fit comfortably.

Connect a length of wire to the microswitch, which will reach the transformer. Use wire with an earth for when you eventually make a better footswitch with a metal casing.... In the meantime just fasten the earth to the wood somewhere.

Fit the two side pieces, fit the top board between them and pass the pivot through. You may want to enclose the wires at the back.

Step 11: Electrics

The transformer I used has an aluminium rivet which was used for an original earth connection to the secondary winding - I prised this out, fitted a ring terminal onto a short length of wire, and fitted this using the rivet, having crimped the rivet with pliers a few times to create raised bumps.

Cut a four way piece of connector block, connect some short wires to the transformer primary and connect these to the block. I don't know which end of the primary should be live though I'm guessing it should be the outer end. If the transformer frame has an earth pin, pull this out and use it to secure a new ring connector with a length of earth wire on it. Crimp the pin a bit before you put it back in. i also used one of the transformer mounting screws as an earth point since it was more sturdy.

If you can get a mains socket of the sort used for kettles, PC's etc, make a bracket for it and connect it to the other side of the connecting block. Otherwise, just connect your mains lead straight into the block. Offset the live connection so the mains in and transformer out have separate connectors.

Connect a piece of mains cable to go to the footswitch, between the two live connections. Code it so that the live side of this wire goes to the live mains connection, so neutral will actually be switched live. If it's 3 core wire, connect the earth wire as well, even if you don't use it, in case you make a fancy footswitch with metal parts. Fit connectors to this to connect to the footswitch.

I used a piece of aluminium to make the mains socket bracket, cut with about 1/2 inch past each side of the socket and 1 inch longer than it's total height. Cut down it parallel to the edges of the socket body, then bend each of the two outer "legs" at right angles. They must be bent at the same level. Cut off the middle bit which is now sticking out. Fit the socket and mark it's holes, then drill them, also drill a couple of holes to screw the bracket down to the base board.

I made a P clip for the footswitch wire using a cut out bit of plastic milk bottle top wrapped round the wire, with a countersink type screw put through close to the wire to make it nice and tight.

Step 12: Operating Lever

I used a metal bar from the old rotary washing line to make the lever to operate the welder, and connected it to the top arm with a sliding sleeve to allow it raise the arm. The bar is 6mm thick or thereabouts, but this isn't quite thick enough as it tends to twist a little at the pivot. I made a handle by shaping a bit of old pallet wood, which I fitted with a ferrule made from a bit of thin steel.

The steel bar I used already has a bend at both ends, so one of these is straightened out, and the other one just nicely makes the end pivot. Meaure up to a point about an inch above the top tube, and mark it on the bar. Make the first bend here. It will go much more easily if you heat the bar up with a blowlamp - it just needs to be a dull red glow. Whack it with a hammer a few times to tighten the bend. Don't bend it the full 90 degrees - the lever needs to be slightly V shaped. Make the next bend an inch further along, to the same angle, then two more bends to create a second pivot aligned with the first one. Offset the final bend so that the lever arm is tilted forward a bit.

Make two staples to hold down the bar at it's two pivot points to the base board, drill, and screw them down using dome or pan headed screws. Test that the lever pushes the top arm down nicely. It's very important that it doesn't touch the bottom arm.

I made a handle because I didn't have anything to use for one, but use whatever you have.

Make a sleeve out of thin steel which will hold the lever to the top tube. It has to be wide so that it won't bind when it slides along. Cut a piece of steel about two inches wide, and long enough to wrap around the tube with about 3/8" excess at each end. It needs to be a close fit, but not tight. Shape it around the tube, take it off and align the ends. Centre punch a spot where the lever can go through at a point where it will just meet the top of the tube, and drill a hole through both thicknesses to take the lever.

Remove the lever from the baseboard, and the top electrode if it's in place. Slide the sleeve onto the lever so it sits on the middle section, then fit it onto the top tube. You will probably need to make the sleeve round again so that it will fit. Squish it into shape around the tube so it pulls the lever down close, then put everything back together.
Quick safety question. I have rewrapped the secondary of my mot with 2/0 with only a couple turns so I'm getting 2.5 v out. So at 12 amps in at 120 I have the potential of 575 amps out. So I'm just a tab bit nervous about holding the thing I want to spot weld....Even though I realize at 2.5 v it'd have a pretty hard time of poking me. You guys have had no problems?
<p>The secondary is isolated from the mains, so unless you did something wrong it can't hurt you. The current that flows is dependent on resistance - so even if you grabbed both electrodes firmly with your bare hands you wouldn't feel anything.</p>
Oh yeah that makes sense. Thanks!
I made this also, and I just cut one welding with a hacksaw and bent it open, stuck in a set of jumper cables, and super glued it back up, works like a charm!
<p>Interesting that you took the route of cutting a weld. I had thought about this but decided I didn't want to risk doing irreversible damage to the transformer. Brave you :)</p>
<p>i need technical specifications of the transformer</p>
<p>All I can tell you is that it's the transformer from an 800W microwave oven, and that the primary was wound with approximately 1 turn per volt. Everything else you need to know is in this instructable!</p>
<p>I am curious as to why most people are not using something like jumper cable or the like for secondaries.</p>
Tried it with expensive jumper cables, in about 10 seconds the outer plastic started to melt off xD
<p>Jump leads come in two types - cheap and expensive. The cheap ones tend to have very thick insulation to make them look better, so are not really a good type of wire to use, and with expensive ones probably the wire is too thick, and would you really want to cut one up anyway? Either way you would probably find it was simply too thick.</p>
<p>Is there any alternative for microwave transformer? how about other appliances do you have any idea?</p>
<p>Hi fburito</p><p>The only thing I can think of straight away is a &quot;site transformer&quot; - the ones that are used on building sites to provide safer power for the tools. They are pretty big and beefy. I see them on Ebay a lot for very little money, though they tend to be &quot;buyer collects&quot;. Of course they might not be available in your country. The next thing I can think of is a transformer from an arc welder - and it's already fairly close to what you want.</p><p>If you have access to some thin sheet steel you could of course cut out your own laminations. Would be a poor performer compared to proper transformer steel but it would do if you can't get anything else. I found this site to be quite informative: </p><p><a href="http://ludens.cl/Electron/trafos/trafos.html" rel="nofollow">http://ludens.cl/Electron/trafos/trafos.html</a></p><p>Good luck!</p>
Congratulations on an excellent piece of work. I&acute;ve been searching on the net for a MOT spot welder diy project. This one of yours is by far the best. Thank you. Keep up the good work. No need of a video step by step demonstration. Pictures, drawing, text, just right.
Wow - I'm deeply flattered! Thank you :D
taanks <br>
cool instructables!! but what about those chisels?? like them what is the brand?
may be a video instructional and step by step, how work well done thanks...
If I re-wind another transformer like this, I will certainly try to make a video of it.
Your ingenious solution to the secondary winding is terrific. An excellent design.
What a Great Instructable loads of pictures and well written with lots of information.....Thanks i've read lots of these as I want to make a spot welder,this is by far the best i've read..........If you want to make something out of the float ball from the cisten cock have a look at my 'Ball Mill' ...I have 4 ... I dont want to blow my own trumpet, but they are quick and easy to make and work really well......Thank for this again Phil..
I have no idea what I would mill though! Besides, it's a copper float and has great potential to make something *shiny* :o)
Very good work! Congratulations.

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Bio: Loving getting back into electronics as a hobby after a break of many years. Now I work as an EPOS engineer, so I spend my ... More »
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