-----Original Message-----
From: Mario I. Arguello <MIADsgns@AOL.COM>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Thursday, March 11, 1999 8:50 PM
Subject: Re: [OT] Spot Welder


>Regarding Spot welders... I should have asked before "How is it that  these
>things Work?"  I am trying to build a home brewed spot welder to weld tabs
to
>NiCad batteries.. Can this be done simply by discharging via a large cap
with
>two electrodes on the workpiece..


Well I'm becoming more and more versed in spot welders now that I'm trying
to build one.  Yes, there is a commercial spot welder that uses just this
technique.  Although the standard spot welder is a big transformer and a
timer, with a few thousand turns on the primary and one or two turns on the
secondary, this is not the only way to do it.

It is possible to weld with the arrangement you mention.  The bigest problem
I see is controlling the current.

You see it is best to clamp the work in the spot welder jaws when there is
no voltage applied, get the work positioned and held tightly, and then fire
a burst of cutrrent through it.  Then hold the work for a  second as it
cools.
I don't know a good way to control the very high currents neccesary on the
low voltage side.

I think this can be done on the high voltage side of a transformer, by
suddenly connecting the charged capacitor bank to the primary of a
transformer, and connect the jaws to the secondary.


>
>I guess I am unsure how spot welding is really done with small battery
packs..
>I know I can solder copper braid on the batteries and assemble my packs
that
>way but I am trying to save weight.  You see, I  design and produce 4 ounce
>micro RC controlled helicopters and everything must be ultra light, neat
and
>compact.
>
>Some of you responded to my previous post and I am thankful but I am
looking
>for a simple device, if at all feasible.
>
>Mario
>http://members.aol.com/microchper/miasportt.htm


I'd say you might get by with a real funky setup - a car battery, a car
battery charger, a pair of thick cables, a set of copper jaws that can hold
your work, and a crude switch.  The switch could probably be of the
frankenstein switch variety - an open knife switch.  Build it out of a
coupla copper bars and a piece of wood.  You get the idea.

One of the jaws could be a flat piece of copper, the other could be
handheld.  This gizmo would probably only work on real small welds in thin
material and low duty cycles.

Don't laugh - I watched a guy do arc welding with two batteries, a pair of
jumper cables, and a bicycle spoke for a rod.  I'm not kidding.


But how are you gonna use a PIC in such a setup?