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DIY Arc Welder Project



First, I must admit this project is mainly inspired from Dansworkshop Project and numerous others similar projects found on the web. 

Dan's has done a wonderful job of documenting his project, and his 8$ PDF worth buying if you need to understand the very basics of this project construction that I won't cover here.  I will however share with you complementary information I haven't found on the web and learned from my experiences. 

I'm not a professional welder, I've learn on my machine, and from many other ones, and I'm pretty convince you can build a decent machine , better than the Canadian tire ones, from scavenged parts. and if you find all the nececearry part, build yourself a DC Welder.

This won't replace a professional welding machine, but may give better result that the average AC farmer Stick Welding machine .

         First Version (PVC wire coil)


        Final version (magnet wire Coiling)

Scavenge transformers tricks. 
  1. The bigger the transformer, the powerful and the reliable the welder 
  2. The bigger/heavier  the Oven, the bigger the transformers 
  3. The brownier(older) the Oven, the bigger the transformers 
  4. Inverter-Type Oven = No-Go , they are transformer-less. Newer panasonic ones are often this type, They use a switching type power supply to build-up there High-voltage. These oven are un-usually light.  
  5. Keep and eye open free items in local classified add or trash. All the 6 ovens I've found in the trash had working Transformers inside.
  6. One of my transformers actually come's from an UPS. I've loose trak of wich exact model it was, but it was a 1000VA. The transformer is  120v in and 10v out. It may not seem has powerfull as the other one, but its my best one since it always stay cooler than the others. The secondary coil is probably better winded than my personel ones.  Be open minded!  

How many transformer ? 

1 MOT   From my experience you won't do much with only one modified MOT. except having fun at melting stuff. 
2 MOT   Will get you going in max 2-4 mm Steel (1/8in)
3 MOT   Will get you going in max 4-6 mm Steel (3/16in)
4 MOT   Will get you going in max 6-7 mm Steel (1/4in)
              Regulator will get you the range to weld from 0 to Max 

Rewire the transformers.

Typical Microwave Oven Transformers  are in the range of 700-1000VA and are designed to get normal 120V ac voltage and step-it up to 2000Vac. This is exactly the opposite of what we need.  But because of the high voltage isolation required in there design , the 2000Vac secondary coil is usually easy to remove and replace. This is why MOT are so interesting

If you use MOT, You basically need  to target around 8-12Vac output from each transformers to sum up at 40-50V for 4 of them in series.  Around 20 turns in each transformer of 6-8 AWG will do the job. This guy give good start and equation for rewinding Transformers.

DO-NOT cut the core, From my experience it will lower the efficiency of the transformer. 20turns of wire is easy to do without cutting the core. Removal of the original high voltage secondary coil  is easy with a manual metal saw and a a little wood stick + rubber hammer to smash it out.  The only MOT i've open-up to remove the coil is my less efficient transformer now (it heat more). 

    Do :                     Don't :   

I was using 8AWG NMD-90 left-overs for my first machine like this guy and many other one. This was a left-over from my cloth dryer installation. A setup like this kept me going for a while . but actually the wire melted, letting my with a bloob of melted plastic and fused copper. 

Even if this kind of wire is easy to obtains, the plastic insulation is using a lot of precious space in the core, and it is only sustaining 90oC. You want to stuff the core whit the biggest quantity of copper, plastic is lost space, therefore lost power.  90oC limit you to a lower current than magnetic wire that may let you go up to 150oC, thus letting you weld for longer period of time. 


After the Plastic Melt-down , I've rewired my transformer with magnet-wire. My machine never broke since. 
More specificly i've used 10 x AWG-18 paralleled. It sum up to approximatly AWG 8. 

To do it i've estimate the needed length, laid-down 10 strand of AWG-18 , attach them-up to the floor at one end and to my hand drill on the other end to twist them-up lightly together. The result custom 8AWG equivalent was really easy to maneuver and to coil-up on the cores. Wire length as to be pretty equal between wires, and wire connection will take more time to achieve as you will have to manually remove the varnish on each wire. 
Trick : Screw-down the iron core to a study surface to do the manual coiling.  

The iron core edges can easily scratch the magnet wire varnish/enamel . Try to use the old plastic/or paper isulation that was there before, or you can use 5min epoxy to glue 2-3 layer of plain paper as a protection layer. Electric tape an do it , but won't be a resistant.

The core and the coil will vibrate a lot when a lot of current will be drawn. This is why i'v tye-wrap the coil thingly. Not only it damp the busing sound a lithle bit , the copper wire will last longer. 



I've try to build my first prototype without the SCR TRIAC regulator described by Dan and 2 MOT. It was possible to weld , but my welding was inconsistent, hard to achieve, and running the MOT at full current got them really hot, really fast . 

        First welding examples without regulator To fast , current to high 

The regulator helped them to stay a bit cooler because the transformers do not run a 100% and share the load. Dan's regulator design may seem a little weird for many reasons, but trust me, it is working. The hardest part is to find a big enough SCR. I found mine in a commercial 600v 100A 3phases motor controller in a junkyard. 

You DO want to oversize this part. Dan's design is working great, but doesn't take into account the transient current spike, switching harmonics, short circuit currents , parasitic inductance/ capacitances. and many other hard to mitigate and predict electric phenomena. Even if it is the theorical minimum requirement, a 240V 30A SCR will not last long.

My Triac is a normal Triac scavenge from a normal light dimmer. 

Semi-conductors (Diode Scr Triac Transistors Mosfet)  generally fail instantaneously from over-voltage transient (you man not event knew happened) or big and fast over-currents that blew them up instantly. They can manage slight over current for short amouth of time, but not overvoltage. My SRC is rated at 1200V 330A , It could dim AC voltage of couple of houses, but hey, it was free. A 1000V 80A  could have been way enough.


You may have to fine tune resistors and capacitors value to give a good working range from your welder. As you'll notice my component value differ a bit from dan's ones because of many factors, but mainly SCR and TRIAC characteristics. Trial is the easiest method for the resistor value tuning.

The Schematic

 R1 : 250k Pot
 Q1 : Scr  1200V 330A , T 330 N 12 
 Q2 : Triacs 200V 15A , Q2015LT (From 120v light dimmer, might fail since only 200V) Q4015LT would be better
 D1 : Part of Q2
 C1 : 500 pF 1000V (haven't found really usefull) 
 C2 : 0.1 uF 250V
 C3 : 100V 2200uF
 L1 : Custom coiled see pic. 
 BRIDGE : 4 diode 200V 160A,  1N3264 (scavenge in abandoned factory) 

Other Regulator Type :

 I did'nt build or try the easy start circuit described by Dan. DidNt feel it was really usefull

Dc rectifier 
There is nothing special to mention about the DC rectifier. The Cap really help to get a little current boost when striking and arc without your stick stay "glued". The coil help to easily keep you're arc. I found it really easyer to keep the arc in DC !  


Getting your machine to stay at decent temperature mean's you'll be able to weld for longer period of time before stopping or getting the machine to break. That being said, installing temperature cut-off switch and fans is a must. 
Do not under-estimate you're design on this mater.     


You'll notice I've install some CPU heat-sink on my transformers. It is only helping a little. 


Breaker Sizing 

If i'm a little bit to hard on my machine I can easily trip my 20A Breaker. 
240V-25A for a 4 MOT Welder is a minimum 
240V-30A for a 4 MOT Welder is optimal 
But if you use an SCR regulator , and only use your machine at 0-60%  capacity , 20Amp can be sufficient (as I do) .

A nice Soldering Example !