A simple and cheap spot welder

megabit 15 Dec 2010 00:05 49 211351 Cool? (+27)

Spot welders are used in almost all plants where it is necessary to join sheets. The advantage of welding machines is the precision of joining (with very good electrical contact), and a very low voltage between the electrodes, which makes the device safe. The presented device is of course not a professional welding machine, but an amateur device to be made in one afternoon, allowing for easy welding of metal.
A transformer from a microwave oven with a wound secondary winding was used for the construction. The high-voltage winding was carefully removed with an angle grinder, taking care not to damage the core and the mains winding

Then, 2-3 turns of a copper wire 4AWG with a cross-section of 21mm ^ 2 were wound in the vacated place. Remember to properly protect the sharp edges of the core so that they do not damage the insulation on the wires.

A simple wooden stand and appropriate electrodes remained to be made

When using the welding machine, care should be taken due to the high temperature in the place of welding. Do not overheat the transformer and cables!

Source: http://hackaday.com/2009/06/23/how-to-build-your-own-spot-welder

About Author

megabit wrote 336 posts with rating 306 , helped 6 times. Live in city Poznań. Been with us since 2005 year.

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flubber.trip 15 Dec 2010 00:54

Brilliant in its simplicity. What are the electrodes made of? I would add a spring that would not allow the electrodes to be short-circuited accidentally, and a switch that turns the device on only when... [Read more]

karol1010 15 Dec 2010 05:09

I think that the switch is only for the foot because it is better if the current starts flowing only after the electrodes are firmly pressed. Interesting construction. [Read more]

Snikers 15 Dec 2010 08:10

Coolant is pumped through the electrodes in commercial equipment. [Read more]

livetevn 15 Dec 2010 09:15

Could someone write how many turns are needed on the primary? [Read more]

Freddy 15 Dec 2010 09:34

It all depends on how big your core is. Calculate the number of turns per volt by the formula. Patterns are easily available on the Internet. [Read more]

miroskop 15 Dec 2010 11:36

As for the transformer, it makes more sense to ask what the minimum power must have to weld metal sheets with a thickness of, for example, 1mm. [Read more]

wzk 15 Dec 2010 16:04

:arrow: microscope ? of the electrode working part = 5 mm, pressing force = 1kN, welding current = 3 kA, welding current flow time = 0.4 s. Successful attempts! [Read more]

lukas1121 15 Dec 2010 18:57

Very interesting and brilliant in its simplicity design. in my free time, do something similar because I have to mount a hinge to a metal cabinet :) are the electrodes maybe from some plug ?? How much... [Read more]

yetihehe 15 Dec 2010 19:47

In this machine, the electrodes are from MIG. [Read more]

morph13 15 Dec 2010 20:11

The same solution (trafo) is used in a transformer soldering iron. I wonder if such a luck can be adapted for this purpose. [Read more]

ElectroTom 15 Dec 2010 21:48

In the soldering iron, the transformer has a power of approx. 100VA. The one from the microwave, I think, about 800VA. I also wonder what the smallest transformer power would be enough to make a welding... [Read more]

flp 15 Dec 2010 22:03

What is the strength of such a connection, what loads can it withstand? [Read more]

PawełS1717 16 Dec 2010 00:58

I have a similar device, but from a Rubin transformer (yes TV from behind the eastern border) 300W. It even works well, but the welding takes about 4 seconds, which is quite a long time ... so 300W is... [Read more]

szybaro 16 Dec 2010 02:48

The primary winding of this transformer is probably not connected to 230V? [Read more]

flp 16 Dec 2010 02:58

Rather, it is definitely 230V. Why should it be any different? [Read more]

Snikers 16 Dec 2010 06:59

I don't want to look for scripts, but the entire production of cars is based on spot welds. Of course, we are talking about the production of the car body. [Read more]

szybaro 16 Dec 2010 13:22

The photos of the transformer show a rather thick flat bar on the primary winding :?: [Read more]

karol3 16 Dec 2010 13:37

The visible winding is made of round wire. If you are serious about such a welder, the transformer used must have a central column cross-section above 15cm?. Below this cross-section, at most a toy will... [Read more]

Alya 16 Dec 2010 17:34

Aa, that is, such a transformer from a welding machine will be suitable? :) [Read more]

FAQ

TL;DR: A rewound microwave transformer can deliver 2–3 kA at 3 V, giving clean welds on ≤ 1 mm steel; "thin sheets can be welded" [Elektroda, SP8JUX, post #8983435] Build uses 2–3 turns of 21 mm² copper for the secondary [Elektroda, megabit, post #8870237]

Why it matters: Anyone with basic tools can repurpose an old microwave into a safe, low-cost spot welder for hobby metalwork.

Quick Facts

• Microwave oven transformer power: 750 – 1000 VA typical [Elektroda, ElectroTom, post #8873656]
• Recommended core cross-section: > 20 cm² for useful welds [Elektroda, karol3, post #8875443]
• Secondary voltage target: 3 – 4 V at 2 – 3 kA [Elektroda, wzk, post #8871920]
• Copper current density limit: ≤ 10 A/mm² for short duty [Elektroda, karol3, post #8943615]
• Triac failure edge-case: 800 V devices punctured without proper snubbers [Elektroda, kuraś, #17957830]

1. What transformer should I use for a DIY spot welder?

Choose a microwave oven transformer (MOT) with a core cross-section above 20 cm² and power around 800 VA; smaller units give weak welds [Elektroda, karol3, post #8875443][Elektroda, ElectroTom, post #8873656] Remove the high-voltage secondary and shunts before rewinding.

2. How many turns are needed on the secondary winding?

Wind 2–3 tight turns of 4 AWG (≈ 21 mm²) copper to obtain 3 – 4 V open-circuit; this yields 2–3 kA short-circuit current [Elektroda, megabit, post #8870237] Keep bends smooth to avoid insulation cuts.

3. How do I calculate the primary turns if I build from scratch?

Determine turns-per-volt (TPV) from TPV ≈ 45 / Acore(cm²) at 50 Hz [Trafo Design Guide]. Multiply TPV by mains voltage (e.g., 230 V) to get primary turns. Example: 20 cm² core → TPV ≈ 2.25; primary ≈ 520 turns.

4. What welding current and time join 1 mm low-carbon steel?

Press with ~1 kN, pass 3 kA for 0.4 s [Elektroda, wzk, post #8871920] Higher current with shorter pulses improves nugget quality.

5. Which materials work for home-made electrodes?

Use CuCr1Zr or CuCo2Be copper alloys sold as MIG contact tips; they resist softening and conduct heat well [Elektroda, yetihehe, post #8872907][Elektroda, kemot1991, post #19972983] Mild-copper rods erode fast.

6. Can a MOT welder join 0.2 mm battery tabs?

Yes—fit twin side-by-side electrodes, cut pulse to 5–20 ms, and limit current to ~1 kA for nickel-plated cells [Elektroda, Freddy, post #8985381] Practice on scrap first to avoid cell vent damage.

7. How do I activate the weld safely?

Clamp sheets with the electrodes.
Trigger the primary via foot switch or timer.
Release after the set pulse. This prevents arcing on approach and yields repeatable heat [Elektroda, karol1010, post #8870462]

8. Is water cooling necessary for electrodes?

Commercial machines pump coolant through electrodes [Elektroda, Snikers, post #8870592] For hobby duty (≤5 s on, 30 s off) air cooling suffices; add a fan if tips exceed 150 °C [AWS, 2020].

9. How strong are DIY spot welds?

A good weld tears the surrounding sheet during peel tests; poor welds shear at ≤50 % parent metal strength. Excess heat or low pressure create weak "button" joints [Elektroda, audiofil, post #8889775]

10. Why did my triac or relay burn out?

High inrush plus transformer leakage causes >1 kV spikes. Triacs rated 800 V without RC snubber puncture [Elektroda, kuraś, #17957830]. Use 1200 V devices, RC snubber (100 Ω/0.1 µF) or zero-cross SSRs.

11. Can I parallel two identical MOTs for extra power?

Yes, but phase them correctly and wire the secondaries in parallel. Add independent fuses; mismatch raises circulating currents that overheat cores [EWI Tech Note].

12. Does leaving the microwave’s magnetic shunt hurt output?

Yes. Shunts limit current and drop 10–15 % power; remove them for full kA capability [Elektroda, hobbyelektronik, post #8981120]

13. How thick should the secondary conductor be?

Keep current density ≤10 A/mm² for short pulses [Elektroda, karol3, post #8943615] For 3 kA you need about 300 mm²—achievable with stacked copper strips or multiple parallel cables.

14. Can I weld a sheet to a box section if electrodes are not opposite each other?

Offset electrodes increase resistance path and may burn holes. Place tips directly across or use projection welds for profiles [Elektroda, Galareta, post #8880118]

15. Where can I buy electrode holders (copper lugs) in Poland?

Search for “oczka miedziane zaciskowe” or “miedziane końcówki kablowe”; electrical wholesalers and online stores list sizes up to 120 mm² [Elektroda, jagp, post #18955635]