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)

I cant remember much transformer theory - does it have to do with
efficiency ?

Tim

A spot welder depends on current between the tongs to get the job done.
Even a small hand held unit needs several thousand amps to do a couple
sheets of .049" steel using .062" diameter tips. To do weld nuts,
thicker stock, or larger diameter spots just ups the amperage.

To get to those currents with reasonable KVA ratings, the closed circuit
voltage is around 1/2 volt. There is a balance between current, voltage,
transformer rating, and losses in the arms and windings.

Open circuit voltage is not really relevant for the actual weld but a
higher OCV makes it easier to punch through any residual oil film or
other contaminants.

TMN wrote:
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)

I cant remember much transformer theory - does it have to do with
efficiency ?

Tim

TMN wrote:
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)

I cant remember much transformer theory - does it have to do with
efficiency ?

Tim
NO it has to do with I squared R .
...lew...

What matters is power produced at the point that needs to be
melted. It is a product of current and resistance of the joint (which
changes as the metal melts). Therefore, all that matters, given a
particular joint, is the current produced.

i

Ignoramus4770 wrote:
What matters is power produced at the point that needs to be
melted. It is a product of current and resistance of the joint (which
changes as the metal melts). Therefore, all that matters, given a
particular joint, is the current produced.

Right, and in order to do that, most of the resistance needs to be in
the joint. That is why the secondary winding & the tongs need to be
such heavy guage - to keep their resistance down and avoid dissipating
power in them.

Bob

On 2007-11-21, Bob Engelhardt wrote:
Ignoramus4770 wrote:
What matters is power produced at the point that needs to be
melted. It is a product of current and resistance of the joint (which
changes as the metal melts). Therefore, all that matters, given a
particular joint, is the current produced.

Right, and in order to do that, most of the resistance needs to be in
the joint. That is why the secondary winding & the tongs need to be
such heavy guage - to keep their resistance down and avoid dissipating
power in them.

Yes. I will have a spot welder shortly (5 kVa Peer industrial
spotwelder), and will play with it to see what it can do.

i

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.

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

TMN wrote:
....
But why do most plans on the net say to aim for about 4-5V OC ??...

I'd say it's because their secondary winding and tongs are "designed"
such that their resistance is too high and too much voltage is dropped
there. For instance,
http://www.5bears.com/welder.htm
uses #4 wire for the winding and brass for the tongs!! #4 is much too
small and brass is much too resistive. I'm impressed that it works at all.

Bob