At beginning of this project I setup two goals - make spot welder for thin battery tabs and power it from CR2032 cell. Looks like I was able to fulfill both the points, as seen on previous project log.
Needless to say, this project doesn't end up with particularly useful welder. Having to wait for day and half to make a few welds isn't terribly efficient way of doing this, but on the way I learned a few new things
- Coin cells do contain surprising amount of energy
- The energy isn't trivial to get out, especially when you are in hurry
Those two facts were main limits between which the project oscillated. At one side I had to extract energy as slow as possible, to utilize the CR2032 cell as good as I can, on the other hand I had to charge fast, in order to not lose energy via capacitor self discharge. The charging circuit had to be of unusual type, drawing constant power from cell.
Apart from that, this design was little exercise in simple analog design, with no microcontroller involved. 555 timer in its CMOS variant performed very well and with voltage reference and comparator (all of them being low-power parts) I achieved also goal of automatic charge termination and relatively low consumption (~30uA) after that. Current consumption is not the key factor of this project, as the coin cell here is to be depleted and its charge transformed into tiny bits of liquid metal in blink of an eye.