Jeff Wisnia wrote:

Jon Elson wrote:




snipped



These welders take close to 15 A from the
wall socket during the weld cycle. (Some take even more, or need 220 V)
So, a slodering gun is hopeless, they get around 40 A output.



40 Amps sounded a bit low to me. I couldn't see that amount of current heating up
something roughly the size of a piece of number 12 copper wire to soldering
temperatures in a few seconds.

It only took a moment to stick the jaws of my Amprobe through the soldering tip
on my ancient 250 watt Weller gun.....the one with the brown Bakelite housing.

I pulled the trigger and (as Claude Rains put it) I was shocked, just shocked, to
see it displaying 460 amps.


I have some doubts about that, as the transformers of these guns radiate
HUGE
fields, and may have affected the clamp-on pickup coil. You might hold
the clamp-on
next to the soldering gun, but without the probe clamped around the tip
wire, and
see if you still get a reading. My guess is you will get a substantial
reading from it.

I think that was a valid reading, but my electric field theory is quite rusty, so
somebody ping me if I used an incorrect measuring technique.

But, I still doubt that a soldering gun transformer would be ballsy enough to
resistance weld blades.


I'd be willing to believe currents up to maybe 100 A for a 250 W gun,
but not much above
that. The problem with the blade welder is the steel is not such a
great electrical conductor,
especially when heated red hot. So, the blade welder needs to deliver a
lot more voltage
across the blade to work. The soldering gun is heating a very short
length of copper
bar to a much lower temperature, so the voltage drop must be quite a bit
less, a fraction
of a volt. The blade welder probably needs several volts open circuit,
and at least a
Volt or two when doing the weld. Two volts at 460 A would require
almost a KW.
460 A at half a volt would only require 250 W, so maybe your numbers are
OK, if
the complete circuit of the gun, including the one turn coil in the
transformer
secondary only drops half a volt. But, then it would need a resistance
of 1 milli-ohm,
which is AWFULLY low. I can't believe the resistance of the tip, with
two pressure
joints, is that low.

Jon