Print PDF
Favorite
Email
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.
Remove these ads by
Signing UpStep 1The transformer (core)
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.
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.
| « Previous Step | Download PDFView All Steps | Next Step » |
My multimeter is saying is that my primary resistance of 3.5Ohm
I think it strange
Dry air and floor are vital as electricity resistant gloves ...
Electricity is no game ...
Be sure you don't bet your life !
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!
Any help would be appreciated
Thanks for all the help anyway I'll try and comment a picture when its working
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.
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?
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.
CAN WE USE IT AS BATTERY TAB WELDER(BATTREY REBUILDING FOR LI ION, NICAD ETC
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
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.
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!
http://www.instructables.com/id/Hacking-a-welder-pirateando-una-soldadora/
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!
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...