In this two-part tutorial, we will show you how to make a spot welder for battery contacts. Its main component is a rewound MOT (Microwave Oven Transformer) from an old microwave oven. In the first part, we will show how to correctly rewind the MOT and make a control unit that will switch the primary winding of the transformer.
The contacts of the batteries must not be soldered, if the cell overheats, the battery may be destroyed or even explode. For correct joining, so-called spotting is used, it is a type of welding in which, due to the transient resistance between the tip and the nickel strip, the metal is melted and then „welded“ to the battery contact.
Even before starting the „gutting“ of the secondary winding, it is necessary to take the transformer to the workshop or anywhere where it is possible to make a big mess, because it was really a lot of mess. There are a total of 3 windings on the transformer. Since this is a high-voltage transformer, the secondary winding will have a larger number of turns with a weaker wire, so we need to get it out. There is also a winding on the transformer that has 2 turns, it is called the „heating winding“. In reality, however, this winding is connected directly to the magnetron.
By this time I had already rewound the MOT twice, each time I tried a different method. First, I cut the winding at the same time as the side of the transformer sheets using a hacksaw, then drilled out the inside and pulled out the remaining windings from the other side. This method took about 15 minutes, but there was a lot of mess left on and under the work table, both the remains of the winding and the dust from the paint that was used to paint the winding. For the second attempt, I took a sharp chisel with which I wanted to „cut“ the winding, although the work was overall cleaner, but also 4 times slower. At the end, I cut the winding with a saw like in the first case.
After removing all the residues, we proceed to winding a new secondary winding. I used a CYA 1x35mm² wire (cable), which comes out on 3 turns. It would be ideal to use a thicker wire and only 2 turns, but I didn’t have access to another option at the moment and for my purposes it will still do. I used 60 cm of wires and crimped cable lugs on the ends, which will be attached to the copper busbar.
In order to achieve the same properties for each weld, it is necessary to somehow control the switching of the primary winding of the MOT. There is a module directly for this application, or a simple timer with NE555, but neither solution is suitable for more „professional“ use. The time during which the MOT supplies the current is in the order of tens of milliseconds, so each switch must come at the same time. This means that the connection of the primary winding to the network must occur at a time when the amplitude is always the same value. This ensures that each quarrel is „one like the other“.
I decided to build the control unit on the Arduino platform. For many users, this way of „programming“ is closer and thus the production is more open to a larger range of people. The heart of the connection will be the ATmega328 on the Arduino Pro Mini development board. A simple alphanumeric display with 16×2 characters (16 characters, 2 lines), 2 buttons, an external trigger input and zero-amplitude detection in the network will be connected to the processor. The primary winding will be switched using a triac. It can also be replaced with a ready-made SSR relay, but I wanted to get the entire circuit on one printed circuit board.
The parameters that can be set include the pulse duration (5-95ms) and the number of pulses (1-4). If the user presses both buttons at the same time for 2 seconds, the values will be stored in the EEPROM memory and the parameters will be retrieved from the memory at the next start. The external input can be connected to a pedal switch or a manual switch on the „lever“ of the scoring machine.