CD Spot Welder Design

What I am wondering is do you think I can find a relay that is rated at a high enough DC current to do the job of delivering the high amperage needed to do some tab welding instead of a thyristor? Or is a thyristor the best thing for the job?

No, you need an electronic switch otherwise all you will be doing is transferring the spot welding to your relay or contactor contacts instead. Your only other option is to not use a switch, but as mentioned in the article you won't get a repeatable weld.

Large SCRs are expensive new but shouldn't be that hard to find at a second hand electronics store or online (either sold by themselves or incorporated in a piece of electronic equipment) for a fraction of the price.

What are some common devices that would employ a thyristor, do you know?

spot welders use suitable thyristors ( sorry- joke)

Motor controllers would be a good bet. Although your application uses a lot of current, the voltage requirement is rather low - so a suitable one shouldn't be hard to find. I think that the surplus ones won't be hard to find and will have a better chance of working than a salvaged one.

I had made a thermocouple CD welding circuit almost thirty years back using an ordinary limit switch that worked for decades. In my opinion it is the breaking DC that causes arcing problems more than the making contact. Any bounce free contact should work though 1.6 F is large. Do you really need that much? Since capacitance discharge is controlled by the R-C value is timing of any use?

The SCR is the way it is done in capacitive discharge spot welders. So long as your repetition rate is not to high small heat sinks are ok design depending what style of SCRs you decide to use.

You need a multi tap transformer to charge your capacitor bank for different voltage levels also with multiple capacitors which can be tapped you will attain control over the amount of energy delivered to the weld.

A 400Amp rms, 200Volt peak phase control SCR should be OK as a starting point.You need a control circuit to recharge and control inrush current to the capacitors which should cut out when full state of charge is attained this must be interlocked with the output switching circuit so that your setting is consistent and isolate the charging circuit when welding.

If you do some searching on Ebay you will find SCRs for sale. Also look up GE handbook on SCRs control.

This link will give you some pointers on pulse control.

http://www.abiscus.com/HV/PulseGenerator/PulseGenerator.htm

Do yourself a favor and stay away from relays and contactors. You will wind up welding the contacts together and the welder will stop working. The large capacitance, low voltage caps you will be playing with are electrolytic capacitors. This means they have + and - terminals marked on them. Make sure you observe this polarity when wiring or the life of the capacitor will be greatly reduced. The biggest problem with this whole design is that these large electrolytic capacitors are not designed for this type of service and have limited life. Much better to use a lower value capacitor rated at a higher voltage (remember that the energy stored goes up as the square of the voltage). So if you double the voltage, the energy quadruples. The bad news is that then you need a step-down pulse transformer, but you will have a much better welder. See http://hackaday.com/2009/06/23/how-to-build-your-own-spot-welder/

Try adapting a starter solenoid. Separate the coil winding from the contact circuit if they are connected.

Good Luck!