Instructables
Picture of Homemade Spot Welder
Anyone can build this simple to use light duty spot welder. I found all the parts I needed in my shop. I looked at some other plans on the Internet but they all seemed to require something I didn't already have.
 
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Step 1: The transformer (core)

Picture of The transformer (core)
transformer2.jpg
secondary_cut.jpg
I had three MOTs (Microwave Oven Transformers) collecting dust in a box with a bunch of adapters and transformers. The one was exceptional for the fact that the primary and secondary were separated by a shunt. Placed on it's base the secondary was on top and the primary on the bottom (pictured).

I used a pipe cutting blade (hacksaw blade) on a reciprocating saw to cut the secondary off the transformer core. Near the end of the cut I had to use extreme caution as I did not want to damage the primary coil. The primary and secondary can be identified by the number of turns and the gauge of wire. The secondary has many thousands of turns and uses hair thin wire. The primary is more like 18 gauge.

Once the secondary coil was cut away I used a short metal bar and a hammer to pound the trapped portion from the core. It was wedged in there pretty good. I cleaned out everything except the shunt and the primary.

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-CCCP-3 months ago

COOL.

-CCCP-3 months ago

COOL.

mstone102 years ago
I have a problem with mine, basically the work piece will get red hot almost melting but will not actually weld together.

Anyone experience this? or anyone know how to resolve this problem?
I think maybe the electrodes need to have an immense amount of clamping force to actually work. I could be wrong, I would appreciate some insight from anyone who knows whats happening here.


Thank You.
jds1969 (author)  mstone102 years ago
It does take a certain amount of pressure but usually just enough to make contact through the two pieces which is what it sounds like you have.

I'll take a few stabs in the dark on this one:
1) clean and sand both side of the weld prior to starting
2) ensure the weld stays unstressed and clamped until cold
3) certain metals/alloys are notoriously hard to weld
4) sharpen the welding tips to concentrate the heat at a point
5) I always count to time my weld (experiment with longer times)
6) try thinner material (I couldn't make a weld in anything over 1/16 inch thickness)
fidgety22 years ago
i have a semi-unique issue i have built a spot welder and it occasionally works very well but the majority of the time what happens is when i place a piece of metal in between the electrodes and plug in the machine the secondary wire insulation begins smoking and getting very hot unfortunately the metal in-between the electrodes (contact points) stays cold could someone please help me or tell me what i am doing wrong and how i can remedy this problem i really hope that this is just some silly beginner mistake and not something more serious that will require a considerable amount of work and effort

thank you,
fidgety 2
In my opinion it sounds like you have used thin wire for your secondary winding what gauge wire did you use?
jds1969 (author)  fidgety22 years ago
Sorry for not getting back to you until now. This sounds very serious and dangerous. I would be extremely cautious about using this primary winding as it sounds like there is a short somewhere in it. With enough heat internal to the unit like this, the core can blow-apart. If it is possible, try to replace your transformer and core. A lot of your secondary can likely be re-used. At the very least I would ensure a metal box around the troublesome primary core for your own safety. One way to test for a short is to accurately measure the ohms of the primary coil and compare it to a good one of equivalent size.
Thank you I appreciate your insight
xXxFelipexD2 years ago
it works by giving short circuit? because I think it would be necessary to place a resistor
My multimeter is saying is that my primary resistance of 3.5Ohm
I think it strange
I would like to say thank you for your instructable - yours is one of the designs I looked at before coming up with my version, and gave me some inspiration!
jds1969 (author)  throbscottle2 years ago
Thank you for the compliment.
narpas3 years ago
So is there any electrical danger from the actual arms? Or is the voltage not high enough to be a problem. Is it just a danger from the parts that are running at 110 V?
wiseboy narpas2 years ago
Current (less than 1 Amp.) may kill . Be sure to ground and NEVER touch any parts when experimenting ...
Dry air and floor are vital as electricity resistant gloves ...
Electricity is no game ...
Be sure you don't bet your life !
jds1969 (author)  narpas3 years ago
If the device is working properly and everything is electrically isolated the only issue with the arms is an unintended short-circuit on the work piece that might prevent the weld.

The DANGER comes from unforeseen / unexpected paths for the electricity. In theory the voltage at the arms should be around 1.5V but if the transformer or any of the other circuit shorts out you may be exposed to the full voltage.

Sometimes safety devices are there to limit your exposure to situations that are rare but still possible. When it comes to AC power take no chances!
magiccowy3 years ago
Having a problem with mine. I have two transformers with the input in parallel and the output in series. I get a great spark between the wires while testing but when i joined it all to my welder the secondary wires get really hot and it doesn't make the weld. I used non-copper contact screws by the way.
Any help would be appreciated
jds1969 (author)  magiccowy3 years ago
The key is low voltage, high current. I've never tried two transformers at once but I think you might want to run your output in parallel rather than series. If you used steel screws/bolts then this is likely causing some of your current to be burned up in heating the screws. The difference in resistance is important when welding. I suggest you try finding something made out of mostly copper to make tips. You might want to try a single transformer and then move up to two if you still think it's needed. Good luck and stay safe.
I got the Amperage over Voltage thing sorry that was badly said. The circuitry is all fine and me and my dad came to the same conclusion about the tips.
Thanks for all the help anyway I'll try and comment a picture when its working
You might have the secondaries  (output) of your transformers connected out of phase.  You may need to swap the wires on one side of one of your transformers.

The phase of a winding depends on which direction it is wound, clockwise or anticlockwise, in relation with the other windings.  Your  transformers should be setup like the diagram below.  The dots on the terminals represent the phase polarity, like positive on a battery.  If the primary is wired the opposite way the phase flips on the secondary

If one of the secondaries is wired the other way or it's phase is flipped then you will get little or no voltage from your welder and the secondaries will get hot. The 2 secondaries are fighting each other instead of helping (adding) each other.

If you connect one of your secondaries the other way your welder should work. But as jds said more current is better than more voltage.  Only time you may need more voltage is if you have a high resistance metal or joint.

Phase is also important for parallel connection as well.  Even more so beacuse as soon as you apply power the secondaries are "fighting" and you can blow a fuse or a winding.
transformer_series.pngTransformer_parallel.png
Mine is set up as your first diagram shows. I really have to talk to my electrotechnology teacher about finding the phase better.

The way I found it was connect it one way, see how powerful the spark was, then switch and compare them. Connected one way I only got a small spark, the other way there was a bright glow, akin to a arc from an arc welder. I assumed this is because the power was adding on as opposed to cancelling each other out. Would I be right in assuming this?
I would say bigger spark would mean that you have the secondaries wired in phase.

The easiest way to check phase is using an oscilliscope so you can see both AC waveforms at the same time.  Wire the mains to the primaries in parallel and then use a dual trace scope to connect to the secondary windings.  The windings should not be connected together.  If the waveforms are in phase, the peaks and troughs will happen at the same time.  If out of phase the peaks will line up with the troughs.  Once in phase the positive of each probe will be the dot.

But not everyone has a scope.

The other way would be to measure wire up the secondaries in series.  Then measure the voltages across each secondary, V1 and V2.  Then measure the total voltage VT.  If VT= V1 plus V2 then in phase.  If not equal, probably close to 0VAC, they are out of phase, flip the connections on one secondary.  Use an AC meter for the voltage measurement.  You also might need a resistor across the secondaries, say 100 ohm at 5 Watt for this situation, the resistor may get hot.

Note: this only checks the phase relationship between the secondaries.  It will not check the phase relationship between primary and secondary (dot convention) of each transformer.  To do that you need a different type of setup for safety.
Transformer_in_phase.pngtransformer_out_of_phase.pngTransformer_phase_test.png
Thanks for your help, I'll try get back to you once school starts as i may have access to an oscilloscope. Fingers crossed =).
Muy bueno gracias...
the.rollie3 years ago
those are the only two reasonings i ever needed to build something.... wanted one, and needed one for some miniscule task/problem
Where can I get some 4 Gauge wire? Will this do? http://dicksmith.co.nz/product/W1230/4-gauge-power-wire-transparent-red
jds1969 (author)  electroboy13373 years ago
That's very similar to what I used but it was 25 stands of 14 gauge rather than 1700 of 48 gauge. In the end, it's about current carrying capacity. Welding supply shops have this sort of wire if you are looking for another source.
ustech3 years ago
HELLO AL
CAN WE USE IT AS BATTERY TAB WELDER(BATTREY REBUILDING FOR LI ION, NICAD ETC
harlyquin3 years ago
I'm sure I'm restating general knowledge here; if you can get wire for the secondary new, it will make winding much easier. Copper work hardens and becomes ridged and brittle if any deformation occurs. If you find 10 g wire difficult to wind, it will only get harder if you try to redo it!
Udon4 years ago
Hey, great instructable.
I follow how it works, but I got a question.
I'm not sure if this has been asked and answered yet, but, what is the shunt in the MOT?
What does it do, and what affect does it have on the transformer?

Thanks
jds1969 (author)  Udon4 years ago
Hey sorry. I just noticed that no one replied to you. I will try to answer but again I'm just a transformer theory novice.

The shunt is two pieces of iron that short out the flux between the primary and secondary coil. In the end it limits the amount of current that can flow through the transformer.

Removing the shunt could in fact cause a blown fuse if the total current exceeds 110V/15A. MOTs are sometimes referred to as unregulated transformers and there are lots of warnings because of it. Hope that helps.
maxpush994 years ago
It would help if you try to have the 2 windings in resonance. after you remove the secondaru windings you should weight it you use the same amount of copper by weight in your new winding,\ Just calculate lenght by thickness or weight the new windings good luck
But just think of all that fine guage wire that was wasted..... I have like 50 spools of wire from transformers, I just cannot let go of those little guys....
 *electromagnets*
I use it for electromagnets, I wind my own guitar pickups, use it on pcbs, etc... Great stuff!
It would take FOREVER to unwind all that wire. It would be just easier to cut it off like he did. Maybe you could brng all that copper to a recycling center for some money.
rimar20004 years ago
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!
jds1969 (author)  rimar20004 years ago
Love your instructable:

http://www.instructables.com/id/Hacking-a-welder-pirateando-una-soldadora/
Thanks for your kindness, jds1969.
jerkey4 years ago
Keeping the shunts in the transformer was a good idea.  They serve the function of limiting the total current, in my understanding.  Without the shunts, you would have a very hard time keeping from blowing the circuit breaker.

You should mention something about getting a bit of soldering flux, and brushing that onto the work before spot welding.  It can make the difference between a great weld and a poor one.

Great instructable!
rimar20004 years ago
Very, very interesting.

I don't have a Microwave Oven Transformer, but one of 220 to 110 V, 500 mA.

¿Do you think that transformer can be useful? Consider I live in Argentina, the line is 220 V.

Here is not easy to obtain a Microwave Oven in the trash...
jds1969 (author)  rimar20004 years ago
I'm inclined to say not but since you're left with few options you might give it a try. My transformer is "rated" for 1000W and it seems like yours is not. It may limit the current or it may increase the heat in the core. It would be wise to watch for overheating. Since the duty cycle can be reduced (i.e. 10 seconds on and minutes off) maybe you can get away with it.
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