Introduction: Very Low Cost Sheet Metal Spot Welder (Portable & Convenient)

Picture of Very Low Cost Sheet Metal Spot Welder (Portable & Convenient)


This instructable is a how-to guide on transforming a spoilt microwave, a plank of wood, some 3-pin plugs, T-brackets and wiring into a Very Low Cost Sheet Metal Spot Welder that is also Portable & Convenient

Please note that as the label of the transformer in my picture states "DANGER, HIGH VOLTAGE", kindly take the necessary precautions and not electrocute yourself in due course of this instructable

Step 1: Materials

Picture of Materials

The low cost of this spot welder can be further reduced to nothing if the following items can be salvaged instead of bought, most of the time i settle for a trade off between the two.

1. Very old or spoilt microwave X1
(junkyards are overflowing with these)

2. Plank of scrap wood X min 2m long
(bed frames are nice usable sources of wood)

3. T-brackets X 1pair
(L-brackets are fine too, but i just had T-brackets handy)

4. Screws X A LOT
(you'll never know when you need these little fellas)

5. 1cm diameter cable X min 1m long
(preferably solid core, however if it is multi-stranded, ensure each strand is min 1mm diameter)

6. Misc wood working tools and electrical connectors

7. 3-pin plugs X2 (optional)

8. PC PSU power connector X1 (optional)

9. Length of metal chain or the like X min 15cm (optional)

10. Terminal strip (1cm inner diameter) X 2pieces

Step 2: Stripping the Microwave Transformer

Picture of Stripping the Microwave Transformer

This step describes how to obtain the transformer from the microwave

1. Dismantle the microwave without touching anything on the circuit boards

2. Look for the high voltage capacitor, it should be attached to the transformer and looks like a pistol magazine with 2 wires coming out of 1 end.

3. Proceed to short out the capacitor with a screwdriver, DO NOT LOOK AT IT, cos the spark is very bright.

4. PURELY OPTIONAL (SAFETY): take a fistful of used staple bullets and sprinkle them all over the exposed connections of the circuit board, this should render any dangerous voltages nullified

5. Remove the transformer (as seen in the picture) and leave it aside, you may also want to keep the magnetron as it contains some awfully strong magnets, but is hell to take apart (for another time)

Step 3: Structural Skeleton

Picture of Structural Skeleton

This entire structure is made from a single plank of wood and the only modifications made to it, is to cut it down to length, thus all the wood pieces share the same height and breath

Lengthwise you will need
short piece X1 (almost square)
average piece X2 (about 1.5times the length of the transformer)
long piece X2 (length totally dependent on how much 1cm diameter cable is left over from the coiling)

1. As seen from the picture, the 2 average pieces form the base which the transformer screws onto, in between them is the PSU power connector.

2.Attached onto the front are the 2 long pieces connected by the T-brackets (do not tighten the top screws,as it should be a flexible joint for use).

3.At the front end, just shy of the 2 electrodes, on the underside of the bottom long piece, attach on the short piece for added stability and support.

Step 4: Electrodes

Picture of Electrodes

Anyone and everyone who has dealt with welding knows that at such extreme temperatures, electrodes get eaten away like no one's business, thus i brain-stormed over it and realized that the ground pin of 3-pin plugs could  be used as electrodes, they are widely available and cost next to nothing, next i devised a way of attaching them to the spot welder such that they could be changed out as easily as a drill bit of a drill. Below is how to build your own electrodes for this welder.

1. Take apart 2 3-pin plugs and retrieve the ground pins (the longest pin)

2. Take apart the 2 terminal strip pieces and reassemble the metal parts

3. Screw a ground pin onto a piece of copper scrap and insert that scrap copper into the terminal strip's metal pin, screw down the metal pin to tighten.

4. Screw the terminal strip's metal pin onto the wooden plank with the free end pointing towards the transformer, as these will be attached to the ends of the 1cm diameter cable

Step 5: Electrical Modifications

Picture of Electrical Modifications

For a spot welder to work successfully, the key lies in ensuring that a huge amount of current is transmitted though the weld spot, as that coupled with the inherent resistance in the work material, produces the heat necessary for a weld to be achieved.

However, the secondary winding of a microwave transformer is designed to do the direct opposite, it is designed to greatly increase the voltage of mains electricity at the expense of decreasing the current, thus has to be modified if the spot welder is to work, stated below is how to do so

1. Remove the secondary windings of the microwave transformer (it is the winding which has no connection to the mains, has smaller diameter wire and more windings), to do this i used an angle grinder with a cut-off blade to slice through the entire chunk, however i would advice caution as the primary windings cannot be damaged in any way if the spot welder is to work.

2. Using the 1cm diameter cable, make as many loops as possible through the space where the secondary windings used to be (in my case that's 3), then extend the rest of the cable to the front where the electrodes are and attach them on, after screwing the finished transformer onto the structural skeleton base plate

3. Some of you may notice the PC PSU power connector under the transformer, i salvaged that from a spoilt PSU and added it on for convenience and portability (just imagine a spot welder with a long trailing cable), it connects to the primary coil as seen in the next step

Step 6: Others

Picture of Others

Notice how i used the metal chain to secure the middle piece of wood, i added this for structure strength as you will not believe how heavy that microwave transformer is

The Brown and Blue wires that are attached onto the primary coil are connected to the PSU power connector mentioned in the previous step

An improvement that can be made would be to encase the transformer and to add a fan to the encasement, for safety and prolonged usage as it does heat up with use, however i personally prefer the raw steampunk look of the spot welder as it is now


danww2 (author)2016-12-08

This is great, I like "basic" tools and equipment. This is basic and it will get the job done. Thanks!

yaimousa (author)2013-04-10

Yours is the easiest one I have ever seen in the internet yet , good job and thanks for your help.

throbscottle (author)2011-12-06

I just wanted to say yours was the first design I looked at for one of these, that inspired my version - great work, and thanks!

jayfrigginjones (author)2010-05-05

Since it seems that you are connecting directly to 220V/50Hz (assumed) via the PSU connector, I wonder if there is a noticeable difference in the magnitude of current discharged when built with american components 120V/60Hz.  One might assume that the design of the transformer for each specific region would compensate, and that the output would essentially be the same?

brady911 (author)jayfrigginjones2011-01-02

The output would be much different. for 120V/60Hz, it would be more voltage and less current compared to 220V/50Hz, which would have more current and less voltage than the other. The differences shouldnt be too much though.

onesuperdon (author)2010-10-21

Its always a good idea to discharge a capacitor using a rasistor to avoid death and blindness and other desieses, use this as a guid...

nasferatu (author)2010-05-27

make sure that whatever you are using to short out the capacitor is insulated.  that thing will mess you up.

ironsmiter (author)2009-12-29

BEAUTIFULLY designed. Simple, cheap, and a whole lot safer than the other two spot welders I've seen on-line.

you have room on the transformer board.
 you should consider using a breaker, sized to whatever rating your transformer is designed for. Welding is dangerous enough(thought very very fun), no need to make it MORE so.

By placing your breaker on-board, you're no longer relying on the buildings breaker to do the work of protecting. you can set it to trip at 30 amp, instead of, well, one of my breakers in the house is a 500 amp(for the electric kiln)!
Plus, if you trip it, it's a lot easier to reach up, and throw the toggle, instead of having to go to the electric service panel to find the tripped breaker.

mrjohngoh (author)ironsmiter2009-12-29

NICE! i love that idea, guess i kinda wasn't considering about the safety aspect of this spot welder when i was building it, that tends to happen to me a lot, when i get into a build i end up with a single minded intent of making it work and nothing else.

But come to think of it those breakers don't come cheap, as it stands i have only invested only 2 bucks into the above working model (most of the materials were salvaged), excluding the electricity consumed while in operation, but if you're willing to spend some money, a breaker is definitely a worthwhile investment.

devonfletch (author)mrjohngoh2010-05-25

I'm thinking, as the business end of the welder is only a few volts, it's not going to electrocute anyone.... nobody has been killed by a car battery! Though I have a handsome scar around my ring-finger from when I shorted out a car battery thru my wedding-ring when I was spannering a terminal bolt.

ironsmiter (author)mrjohngoh2009-12-29


In keeping with your "2 dollar spot welder" idea,

Unless the Fuse blew ini the microwave, leadinig to it's discard, it should still be good. You could salvage it, and slap it in-line wih one of the AC poles. Not as nice as a breaker, but the first time it blows(though it may never) you'll appreciate it.

mrjohngoh (author)ironsmiter2009-12-29

Yup that idea did cross my mind when you 1st mentioned the breaker, but ironically, sadly perhaps, spoilt microwaves with multiple usable parts tend to be those with burnt fuses, less technically inclined people throw them out for us DIY-er to salvage, though usually all that is needed is a fuse change (most of the time its an in-line voltage spike or a magnetic flux vector change that causes the fuse to burn to begin with, which doesn't occur too often)

But, i know those 3-pin plugs i salvaged the ground pins from for my electrodes, each have an in-built fuse rated for 5A, so if i'm looking for a 30A rating, i'll just rig 6 of these in parallel, solution found, cost nothing, what do you think ?

2ManyProjects (author)2010-04-09

Nice instructable, I am going to build one. It may be obvious, but you could just go and by spot welding electrodes from your local welding supply. They run $2 or a little more depending on size. Now where did I put that old mictowave...:)

rimar2000 (author)2009-12-29

Excellent instructable!

Can you weld galvanized sheet (iron sheet, coated with zinc ) with this spot welder?

If yes, what thickness max?

papatait (author)rimar20002010-04-08

I would be very careful welding galvanized steel. The fumes produced from welding galvanized will kill you.  We would not want to see that happen to anyone.
Make sure you have plenty of ventilation, and you are up wind of what you are welding.

rimar2000 (author)papatait2010-04-08

Thanks for your concern, but reading the other comments, it seems it is not so dangerous.

Anyway, when I weld galvanized sheet I will consider your suggestion

NachoMahma (author)rimar20002010-04-08

It IS dangerous! Please be careful.

mrjohngoh (author)rimar20002009-12-29

Any conductive metal can be welded with a spot welder, however i highly recommend stripping the zinc off the area being welded using some sort of abrasive, when welding galvanized sheets, as the molten zinc won't mix well with the molten steel, thus forming a poor joint bond, but a joint none the less.

Personally i haven't tried any stock material over 1mm thick, but from my experience  this spot welder should be able to handle stock material up to 2mm, you may want to push it to about 3mm, but i am certain i won't do more than that.

bloomautomatic (author)mrjohngoh2010-01-04

We spot weld galvanized steel on a regular basis in our plant.  The galvanizing does not need to be removed.  The heat of  the weld will vaporize the zinc and allow the steel to weld.  The remaining zinc will around the weld nugget will help to give some corrosion protection to the spot weld.

Welding galvanized requires higher amperage than welding carbon steel.  The main variables in spot welding are weld current, weld time, tip diameter, and tip pressure.  The weld current doesn't appear to be adjustable.  The weld time and tip pressure are dependent on how hard and how long you hold the tips down.  The tip diameter is adjustable only by modifying the tips.

If you are getting cold welds, you could try using a smaller tip.  This will concentrate the current through a smaller area and produce more heat. 

I would also recommend some sort of handle on this to multiply your torque to provide more pressure at the tips.

Based on the size of the transformer, I would suspect that 1mm would be the limit for this machine.  Do you know how many watts or amps it is rated for?

rimar2000 (author)bloomautomatic2010-01-04

Thanks, your comment is very didactic and useful. I saw some weld spots of 1 mm diameter, very strong, on stainless steel. I think it is the same thing on galvanized iron. My needs are very modest, only hold trays for a dehydrator of vegetables and fruits. Each tray must bear no more than 1 kg, three or four weld spots on each side will be sufficient.

rimar2000 (author)mrjohngoh2009-12-29

Thanks for the explanations.

1 mm is enough for me! I want make an dehydrator for fruits, vegetables and foods in general.

As for the mixture of zinc and steel, I meant something like

mrjohngoh (author)rimar20002009-12-29

No problem, glad i could help, your reply seems kinda broken though, weird...

BTW vote for me thanks

rimar2000 (author)mrjohngoh2009-12-29

The last paragraph must be: "With respect to the mixture of zinc and steel, I expected something like this".

I speak Spanish, and use on line translators.
Sometimes I forget to review the outcome...

DirttrackinRedneck (author)2010-03-05

i used a angle grinder and accidentially cut  one wire on the primary and messed it up, long story short  there ant no fixen it, i wound up finding out a good sharp chisel will cut through it like a hot knife through butter. I used a bunch of small wire and made my own primary. It has bout 140 to 160 windings on it. I used 4 gauge for the secondary and got bout 4 turns on it.           I like your design. Its all neat and organized. Im 15 and built one, no problem.  

twofouroh (author)2010-02-21

That's a nice one in England I'm sure, but in America, our ground pins are too flimsy.  Anybody got an idea for an American equivalent?  Also, wouldn't copper work better than brass?

rimar2000 (author)2010-02-19

This question is addressed to several authors of welding related instructables.
Some time ago I was excited with the possibility of constructing a spot welder, but here in my city there is no way to get an
used microwave transformer: nobody throws away something as that.
Then I tried with my 220-volt electric welder, but I could hardly weak solder some iron wires of 2 mm, even though I was a good time trying.
Today I decided to uncover my welding machine, and found that in the secondary winding there is no place to put
even a loop of thin wire. But in the primary, yes!
I think I can easily add several turns of thick wire, by way of a "bis" secondary winding. Now come the doubts, and related questions: whether the primary winding has 248 turns (more or less, it is what I could count), and get 220 volts, it is assumed that each round of "my" coil will produce 220 / 248 = 0,887 volts. What for me? Put only one or two turns, or try to reach five or six? A more laps, more volts but less amps. I suppose that losses play an important role in the case of small voltages, and who knows what is best.
Maybe you has an answer and save me the work of trial and error, which can become very tedious. Thanks in advance!

Dipankar (author)2010-01-06

Hi mrjohngoh,
This is really good welding machine.
Can it weld two 5 mm mild steel rods together?
I need a small welding machine to weld only 5 mm or less then 5 mm MS wires.
I could not find a small spot welder in my town.
Please Help.

stephenniall (author)2009-12-28

I use the same conecpt but without the transformer and i use it as a soldering Iron

mrjohngoh (author)stephenniall2009-12-28

That's a good idea, you care to post an instructable on that?

But come to think of it, what benefit does a dual tipped soldering iron provide?

Either way, thanks for your comment and kindly reply.

stephenniall (author)mrjohngoh2009-12-28

Ill post a ible soon ! and the only reason its got two tips is when they touch they short and Create the heat ! but you put the wire inbetween the tips and put the solder then close it ! like a desktop soldering iron

mrjohngoh (author)stephenniall2009-12-28

Oh, i get what you mean, they have those things on the market already, they're called cold heat soldering irons. Their only plus point is portability due to their ability to use batteries for power. As they rely on brittle and costly graphite tips to work and have the chance of shorting out electronic integrated circuits, i personally don't like using them.

However, if somehow you're able to make them more cost-effective and usable, i wouldn't mind building one and giving it a go, in summary give your idea a go!

masterochicken (author)mrjohngoh2009-12-28

Some on here actually made one and posted it.

mrjohngoh (author)masterochicken2009-12-28

Thanks for the link, guess someone thought up the idea long before you stephenniall, oh well, anyway as i said the graphite tips are a serious drawback, using mechanical pencil leads seem to serve only to worsen that problem.

Either way i know its kinda late, but good job photozz (the author of the linked instructable), and stephenniall if you could build it better or build upon to improve it, go ahead cos tools are a staple for any DIY-er and the soldering iron could be the most important yet.

masterochicken (author)mrjohngoh2009-12-29

One way to make it better would be to make it portable. I want see someone try using a stun gun. If I had one, I would do it.

mrjohngoh (author)masterochicken2009-12-30

Actually come to think of it, a stun gun wouldn't work, stun guns use extremely high voltages to shock a person, the trade off being that the shock is of extremely low current, as V=R*I, and frankly it has to be cos anything over 50mA will kill a person!

Cold heat soldering irons, like spot welders make use of the inherent resistance in materials (in this case the graphite tips), coupled with extremely high currents to generate lots of heat, the trade of being a very low voltage

In short, a stun gun has the total opposite operating characteristic to cold heat soldering iron or a spot weld, thus will never switch jobs

masterochicken (author)mrjohngoh2010-01-03

Oh well. Just an idea. Thanks for letting me know.

mrjohngoh (author)masterochicken2010-01-03

No problem, nice try though.

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