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.

The red spots represents the low resistance contact where the smallest possible voltage drop needs to be achieved, between the welding electrodes and the nickel strap. If the contact is not good enough you weld the electrodes to the strap, and not to the battery.

The green spots represents the high resistance contact, where we want the welding to be happen. But, keep in mind when I say high and low resistance, the values varies around 0.1-0.001 ohms.

A welding process involves two high current impulses (hundred of amps). In the first current pulse the metal is softened to reduce contact resistance on the red spots, than it is let to cool down. The second pulse is longer and the actual welding happens here. Weld times vary from 0.01 to around 1 second.

Now lets see a basic block diagram of our welder.

fig. 2 – block diagram of the spot welder

The working principle is quite simple. I charge up 4 x 10.000 uF capacitors to a voltage between 12-50V and after reaching the desired voltage shorting them on the electrodes.  For power switches I am using 20 pcs of 80V 100A CSD19501KCS MOSFETs from Texas Instruments. They are absolutely great transistors with small Rdson (5.5mOhm at 10V), high current rating(305A pulsed drain current), relatively small gate charge(38nC typical), fast rise and fall time (15nS and 5nS). The only drawback of these FETs that you have to use drivers to turn them on and off fast, so the circuit gets more complicated.

For charging up the capacitor bank we use a simple Arduino controlled boost converter, where we can set the desired voltage. The desired voltage and welding pulse duration will be displayed and can be modified on a 16×2 LCD screen.

The hardest part is where you have to deal with high current, short pulses, aka the power switch, so in my opinion I have to start with this. If I fail building it, than the whole project is failed.  The simplified schematic is shown below (fig. 3):

fig. 3 – power electronics schematic

And here we have the transistors already mounted on a heat sink and the capacitors soldered together on small pieces of double sided PCBs to reduce the wire resistance as low as possible (figure 4).

fig. 4 – finished capacitor and FET module

Before anything else, I have to build a prototype module for testing the drivers. Here we got the setup:

fig. 5 – FET driver experimental setup

I got a best of 180 ns rise time, and a slightly shorter fall time (at around 150 ns).  I am not totally satisfied with the results, because the datasheet specifies a maximum rise time of 40 ns with a 1.8nF capacitive load.

The biggest question is, can I go further with these “horrible” results? The answer is yes. Why? Because 1 nano second is the 1/100.000.000 of a second or in other words one nano second is  one billionth of a second. There is enough time to turn on and off the FETs at least two times in half a second, the time required for welding.

In the next part I am going to test both the FETs and the capacitors, and trying to figure out the maximum power that can be delivered through the welding electrodes.