On 15/11/2016 17:56, mike wrote:
On 11/14/2016 8:54 PM, Clifford Heath wrote:
On 15/11/16 12:45, whit3rd wrote:
On Monday, November 14, 2016 at 2:44:55 PM UTC-8,
wrote:
I found 220 mfd 450 caps at JustRadio.com. I'm not sure if these will
discharge fast enough.
For a welding purpose, 100l milliseconds is very fast (no functioning
capacitor you are likely to find would have too high ESR for this job).

I agree that the speed doesn't matter, but low ESR caps have wiring that
will survive higher currents. No sense having a whopping cap
with internally-fused connections.

I also have a bench-top spot welder, from my father who was an
orthodontist. It has a 110V input (so we needed a transformer
from 240V), and that feeds via a small Variac into a selenium
rectifier. I suspect the caps need replacing (again - they were
last done 25 years ago) and I have a bunch of 300V photoflash
caps from disposable cameras that I hope will suffice.

The welding contacts are an anvil and an upper contact connected
to the foot pedal, via an adjustable pressure-operated switch
that fires the contacter. That leaves both hands free to hold and
position the work. When you press down hard enough, the thing
fires, very cute.

What I don't know is how to determine the maximum energy I can
dump through the output transformer without saturating it.
Anyone know how I can tell (other than just keeping the
capacitance below the original value)?

Clifford Heath.
That's not a simple question.
The first order approximation is that CURRENT is what
creates the magnetic field that saturates the transformer.
Be careful, remember there is a secondary current and its magnetic field
will largely cancel that of the primary, so looking at the primary
current is a red herring unless the secondary is open-circuit.

The rate of change of flux in the core will be proportional to the
primary voltage. What you need to look at is actually the integral of
primary voltage with respect to time, as this will give you the change
in the flux in the core. The longer the pulse lasts, the less voltage it
will take to cause saturation. (For similar reasons, 60Hz mains
transformers will saturate at a lower voltage if used on 50Hz mains.)
You could get more energy into the weld if you increse the voltage of
the primary pulse and decrease the duration, because the dissipation in
the weld is proportional to the integral of v sqared with respect to
time, whereas the change in the flux in the core is proportional to the
integral of v (not squared) with respect to time. Also, make sure that
the core starts out with maximum flux in the opposite direction. This
might be achieved with a reset pulse as others have pointed out,
provided the core material is one that will retain the magnetisation. If
not, you could perhaps feed a reverse current through the primary using
a low voltage power supply, to build up reversed flux just before the
main welding pulse. That might make the switching a bit more complicated.