DIY Resistance spot welder (RSW) – part 1

In these article series I am going to build from scratch a mini spot welder for welding lithium batteries and other small metal objects. Before anything else, we need to define what is resistance spot welding and why it is widely used.

Have you ever shorted a car or lithium  battery? Or the mains power outlet? And what about a charged up capacitor? Have you noticed the sparks? That was almost a spot welding, that what resistance spot welding means. You let electric current (high current) through two metal objects pressed together. You concentrate a lot of energy into a small spot, where the voltage drop increases and the heat is generated.

fig. 1 – basic model of RSW

On the first figure I have tried to represent a basic spot welding scenario, lets see what those red and green spots represent.

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Rewinding a microwave oven transformer (MOT) for general purpose use


In this article I am going to present my struggling about rewinding a microwave oven transformer (MOT) in order to make a low voltage (20-30V) but high current (15-30A) transformer. I have done these experiments in order to make a cheap but a very handy transformer for a bench supply. These types of transformers 500-100 VA transformers cost hundreds of USD where I live, but these microwave oven transformers can be purchased for a few dollars used, because they can be only used for special applications (like where it comes from, microwaves). They usually get shorted or interrupted on the secondary winding due to the high voltage, poor insulation, thin aluminium cross section wire. Many times the primary winding remains intact.

Before starting with the practical work, we have to settle a few theoretical consideration, and proving them in small scale.

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Home made bench power supply

A bench supply always comes handy when you play with electronics on a daily basis. As anything else – if something comes handy you have to pay it`s price.
These supplies are quite expensive compared to the price of what is made of – basically you pay the engineering not the parts that are made of.

Main parts

Because I was in the need of one bench supply I made my own, out of a few cheap components so I have spent 10 bucks(USD).

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Cheap but usefull PCB drill from broken electronics

In my opinion our society is built upon wasting. Buy, use than throw away. Every year million tons of harmful trash is thrown away or in better cases recycled. But even when recycling the raw materials are reached out of them, the finished product is destroyed. Of course in industrial sizes this is the most efficient and only way of doing it.

As a student I can`t afford buying expensive tools for my hobby projects. For example, what about this post will tell, a small PCB drill which cost up to 15-25$ if bought from a store. So if it is possible I make my own tools. In this post I am going to talk about a small, inexpensive PCB drill with changeable drill bits. Building this thing cost me about $1,5 and half an hour. It can run from a 12V and 2 amps power supply.

Here is the parts list:

  1. 1 x small 12v dc motor taken from a broken hairdryer
  2. 1 x small switch from a dead microwave owen
  3. pair of cables left from an old ATX – yellow and black
  4. some solder
  5. zip ties
  6. insulating tape
  7. DC jack
  8. old battery charger that can output from x volts to 12v and around 2 amps

This is how it looks in action:

The momentary switch was taped around the motor and the wires soldered to the terminals, at the other and of the cable there is a jack which helps connecting the drill to the power supply. On the rod of the motor there is a mount in which are placed the drill bits. The only things I had to buy were the drill bit mounting and the small drill bits.

When the drill was finished the testing phase was the next. I made small holes in things laying around my table, including two of my fingers. I had 3 types of drill bits for wood, it worked best with the smallest which is around 1 mm wide. I tried it on several surfaces like wood, plastic and of course on a PCB. Of course, it went through on anything even on thin copper.

The drill draws around 0.3 amps without load at 12 v, and around 1.5-2 amps when drilling depending on the hardness of the surface.