On Nov 21, 9:00 pm, Joe Pfeiffer wrote:
TMN writes:
Hi

Why do small spot welders have an OCV in 4-6 Volt - at first glance
one would think that using a higher voltage would make it easier as
you could use thinner tongs,leads etc - 600 amps at 6V is the same
as 300 amps at 12V (P=VI)

OCV is irrelevant to a spot welder -- all that counts is how much
current you can pump through the spot and the spot's resistance.
While it's mathematically equivalent, you're much better off thinking
in terms of I^2R than VI. By the time you could see that 12V, the
current would be much, much higher than 300A because the resistance
very low.


1)Clearly there needs to be a large resistance differential between
the tongs and the joint otherwise
everything gets hot.

2)Athough p=vi = v^2/r =I^2R I^2R is the key as the heat is
proportional to the square of the current. (for the same reason power
lines are high voltage)

Thanks you guys have jogged my memory !

But why do most plans on the net say to aim for about 4-5V OC ??
Miller at http://www.millerwelds.com/pdf/Resistance.pdf have nice
spotwelding primer and show OCV for their units from 1.6 - 3.5V

Thanks
Tim