# Ques about spot welder OCV

• posted

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

• posted

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.

• posted

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

• posted

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
• posted

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

• posted

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.

• posted

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

have nice spotwelding primer and show OCV for their units from 1.6 - 3.5V

Thanks Tim

• posted

TMN wrote: ...

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,