I don't claim detailed knowledge of spot welding technology, but I did
build a capacitor-discharge spot welder with transformer-coupled
output for welding weldable strain gauges to railroad rails several
decades ago. About 50 Watt-seconds energy level, as I recall. All
analog control circuitry using a stud-mounted SCR and a PUT
(Programmable Unijunction Transistor) for triggering via a microswitch
on an aluminum channel handle actuated by an aluminum electrode-
holding beam mounted at the rear of the channel handle.
As I recall, there is a "resistance welding" industry group that
offers technical literature and sets standards of welding tip alloys
and shapes. I don't remember the exact name of the group.
Worked great for its purpose. Weldable strain gauges are mounted on
thin sheet SS (I think) - or maybe nickle. They are mounted by making
a string of spot welds to the test object all around the active strain
The whole motive in building my welder was to get portability for
welding remote from line power (and to have fun), so it was operated
off two 67-volt "B" batteries in series. They charged the caps to an
adjustable voltage (we always used the maximum available voltage, as I
recall). This was before the days of readily available, inexpensive,
DC-DC converters, as I would use today with a 12-volt gel cell power
The transformer was a steel-core inductor that had (as many inductors
do) extra space available in the E-I core window for a copper-braid
secondary of very few turns. I think a microwave oven transformer
with secondary removed would work for a capacitor discharge output
transformer. It was chosen somewhat arbitrarily on the basis of
having reasonable looking core and magnet wire size - perhaps by
looking at literature on commercial units. From memory, I'd guess the
core was about 3.5" x 4.5" x 2" stack thickness. I used about 3 feet
of relatively small welding cable between the transformer secondary
and the ground clamp and welding probe.
At the time, very high capacitance, low voltage caps were rare and
expensive, which was the motive for using the output coupling
transformer. I would probably investigate using capacitors directly
today, but worry about sparking and erosion from the relatively
uncontrolled discharge current.
Which reminds me, why are you thinking of using a triac instead of an
SCR in a capacitive discharge welder? I haven't priced them, but
would expect to pay much more for a high current triac than an
equivalent SCR, and very high current SCRs are readily available and
cheap on the surplus market. Check out candhsales.com in Pasadena
(but do it soon, as they are going out of business).
As one raised on tube electronics, I also wonder what the advantage is
of using microprocessor control for a capacitor discharge welder.
Control of both charge voltage and triggering is pretty
straightforward with analog electronics.
Many years later (but still many years ago) I tried using the same
homemade welder for welding nickle tabs on AA, C and D cell Nicads.
It worked, but only marginally. I think about 100 Watt-seconds would
be desirable for welding battery tabs reliably. However, my problem
may have been contact resistance, not welder capacity. I used a two-
pronged copper fork to make ground contact with the battery terminal
to avoid having the welding current pass through the active part of
the battery. I don't know if the welding current would damage the
battery, but I didn't want to risk it. Seems like a bad idea. The
tab to be welded and the welder probe fit inside the fork tines with
just enough clearance to avoid direct contact.
I think it might be a mistake trying to create a welder capable of
both sheet metal work and battery tab attachment, as that covers a
wide range of capacities and the handpiece/clamp and control
requirements are radically different.