Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Org's MOT Spot Welder
Hi all,

I finally registered. I do not want to make the post too long, so I will add my progress over a few posts.

I bought a second-hand MOT from an appliance repair shop. For those in Centurion, you can buy it from Ryk's electrical (just google them) for R150. New ones is from R400 upwards.

I first cleaned the transformer, by using a hacksaw to cut through the secondary windings on the transformer, as close to the metal as possible.

Then I used a punch (or a bolt in my case) to punch the windings out of the metal.

The final result:

Next up: Choose the secondary wire size!

I first tried normal 2.5mm2 household wire.

Then I tried my inverter welder's earth cable which is 16mm2.

Next I tried a piece of cable from my old Oil-bath-welder. This is a bit thicker than the 16mm2 wire I tried previously, but I do not know the exact spec.

All of these wires were only 2 windings.

As can be seen, the thickness of the wire makes a big difference.
Welcome Org, thanks for your contributing, I've "split" out your post to your very own thread.
Im still a little confused as to how the secondary wire size actually limits the current draw. but at close to 800A you looking quite good. well done.

Im really looking forward to see your "controller" implementation.
Next up: The first electronics!

So I tried the route of an Arduino uno, coupled to a LCD screen with some buttons, connected to a Fotek SSR25DA solid state relay. Unfortunately this only lasted for 3 attemps before the SSR stop working. (This is the second SSR I lost in a small period of time. The Fotek SSRs is a hit-and-miss, with most of them being bad chinese copies!)

Then I decided to rather build the Triac control circuit I originally planned. Here is the main circuit. It was done in Fritzing and some component values is wrong:
4N35 must be H11AA1
MOC3031 must be MOC3021 or MOC305x

Here is the Fritzing vero board version:

Here is a picture of the prototype HighVoltage board:

One thing I would do different ifI build another, is to add the 240V to 5V PSU also to this board.

Then I added this to the Arduino Uno with the LCD shield and button:
Next up: Testing!

I had no copper for electrodes, so I used some copper water pipe fittings that were in my way in the garage Smile

Next I tried to weld 2 nails together. This worked relatively well, but it was a bit inconsistent. So a friend bought us a piece of copper rod 6mm in diameter.

But we still had inconsistent results. Then someone (I think it was Gordon) mentioned that you must not really press down on it. So we tried to just let it make contact. This worked a lot better. See here for a piece of stainless plate welded to a stainless knife blade. This tells me we have hope!

Before I forget: the zero crossing circuit is not efficient at all. It will waste more than 1W in heat. I wanted to relook at it, and add a capacitor, but for now this will do.

Here is the current part list:

2 x 33K 1W Resistors
1 x 100R 1W Resistor
3 x 470R 1/4w Resistors
1 x 390R 1/4w Resistor
6 x 10K 1/4w Resistor
10 nF 400V
1 x LED (optional)

MOC302x Optocoupler (I used the MOC3021)
STF20A60 Triac (I used the STF16A60 without problems, but this is on its limit)

2 x 2 entry screw terminals (
1 x 40 pin 2.54" male header (
1 x 40 pin 2.54" female header for Arduino pro mini(
2 x Veroboard(strip board) (

1 x i2c LCD (You can also use a non-i2c one, but minor code changes will be needed) ( This one is very expensive)
1 x rotary encoder (
1 x Foot switch (Make your own. I used a normal microswitch with a roller lever like this:
1 x Arduino pro mini (
1 x 240V AC to 5V DC power supply (

All the below can be replaced witrh soldered wires, but I prefer a more modular system (
1 x 4 way Female to female wire for LCD (if i2C)
1 x 5 way Female to female wire for Rotary Encoder
1 x 4 way Female to female wire for connection between LV and HV boards

1 x 2 core wire for connection to Footswitch

Jumper wires (Any thin wire to use to jump between tracks on the veroboard)

1 x Mains power switch (Check for a 16A one)
So I finished my Welder (except for an opposing weld electrode fixture and Current control).

See some pictures.

Show and Tell and Demo tonight at H4H.

(2016-10-04, 08:57 AM)Org Wrote: So I finished my Welder (except for an opposing weld electrode fixture and Current control).

See some pictures.

Show and Tell and Demo tonight at H4H.

Nice Work Org !!!!

Forum Jump:

Users browsing this thread: 1 Guest(s)