On Mon, 14 Nov 2016 22:56:14 -0800, 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.
If you put a scope current transformer on the primary,
load the secondary with the resistance and inductance of
your welding setup, crank up the voltage until you see
a sharp rise in the slope of the current pulse. That's
the too-much point. What happens when you add more caps
is dependent on the transformer characteristics.
Energy is linear in caps but quadratic in volts.

25 years ago, I was tasked with fixing an OEM forward
converter that would randomly self-destruct.
I built a fixture to synchronize the load transient with
the switching frequency. I could watch the primary current
head for the sky on the scope as the load transient crossed the
switching point.
Took it to the vendor site and proceeded
to blow up power supplies until they conceded that they had
a board layout problem to fix.

After the weld, the field will be sitting somewhere on the
B-H curve of the core. The next weld might be very dependent
on where you left it last time.

This is a page from my Unitek CD spot welder. Shows how they
reset the core.

http://imgur.com/ZeZerGx

I experimented with a microwave oven transformer battery tab
welder. I was hitting it with a timed pulse. Repeatability
was horrible.
When I synchronized the pulse with the line and gave it an
integral number of full cycles, the starting point on the
B-H curve was consistent and the welds got much more repeatable.

There was considerable discussion on exactly when you should terminate
the pulse, but mine was constrained to somewhere near zero current by
the triac.
Greetings Mike,
The machine I am working with is a Unitek model 1-048-03. Is this the
same as yours? If not do you know if it is close enough to use for
trouble shooting and operating the machine I am trying to get working?
If it would be useful is there some way you could get a copy to me?
Maybe scan and email? I don't mind paying you for your trouble. I
wouldn't even ask but so far I have not been able to find a manual of
any type for the model my son has.
Thanks,
Eric