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PICList Thread
'[OT] Spot Welder'
1999\03\09@122440 by Mike Keitz

picon face
On Tue, 9 Mar 1999 08:47:13 -0600 Lawrence Lile <>

>On the HV side, a relay would be simpler than Triacs, though.  Just
>how fast
>can a 50 amp contactor operate?  Probably pretty slow.  I'd be
>surprised if
>such a contactor could close and open in less than 0.2 seconds.
>That's WAY
>too long for welding thin sheet
>metal.   Besides, I can probably lean on somebody for samples of a

I think the one in our shop pre-charges a capacitor to a certain voltage
(adjustable by the operator).  There is a meter on the front calibrated
in watt-seconds, though obviously it is just measuring voltage squared.
When the button is pressed, it discharges the capacitor into the weld
through a contactor.  Thus the speed of the contactor is not important,
as the amount of charge in the capacitor approximately determines the
energy delivered to the workpiece.  An inductor in series with the
contactor would probably be a good idea to keep the current from rising
too fast.  I've never seen the inside of the machine, which is a
testimonial to its reliability.  It's for small work.  The maximum energy
is 100 W-s.

For bigger welds, a step-down transformer-based machine like you describe
seems practical.  Definitely use a triac to switch the primary on and
off.  If you want to optimize it, don't switch at the zero-crossing
point, it actually works better to fire the triac about 40-60 degrees
into the cycle.  This keeps the transformer from saturating on the first

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1999\03\10@015435 by Mario I. Arguello

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HI all you spot welder do-it-yourselfers!

I just caught a section of this thread and was wondering if anyone has any
plans for a spot welder to weld metal tabs to small NiCad batteries for
assembling into small packs of 6-8 cells.

Not to change the subject but if you are interested in what I do with these
Nicad batteries see my Web page at:


Mario I. Arguello

1999\03\11@100427 by Lawrence Lile

Hmmm   to summarize previous posts, I might be able to make my converted
buzz box/spot welder with just a contactor.  I'll turn the amps all the way
up (200 amps on the output side of the transformer), hit the contactor on
the 240v side, and have a SHORT, HEAVY cable on the low voltage side to a
thick copper jaw.  It will only produce an AC pulse, not a DC pulse with a
nice bell curve as some have recommended.

Aw shucks, I still want to make it out of triacs just to have done it.  A 70
amp 600 volt triac is only $8, a couple of those would do the trick just
fine.  I'll probably use optoisolators to communicate to it (I've got to
look up that driver circuit...) and still probably set the timing with a
PIC.   I won't water cool it, because my duty cycle will not be very high.
This is only a ptototype shop, after all.   The PIC may even be programmed
to limit the duty cycle to less than 20% or somelthing.

250 milliseconds is a fine pulse for 26 gauge steel and do-able with a
contactor, but I may have to weld down to very fine gauges.  The thickest
gauge I will weld is about 26 gauge.  Sometimes I will need to weld .010"
steel - very thin.

A 1500 watt 120 vac spot welder only costs $505 new, so I'd better not spend
an awful lot of money cooking one up.   The specs on the new one say it has
a 0 to 5 second timer.   That's the range I'll put into my prpogram.

1999\03\11@104128 by RCGipson

my 0.02 worth...

i work with embedded spot weld controls everyday.  all of the controls i
use have not only a timer on the current pulse but they also time the
gun closing.  here is the sequence of operations:

1. the gun is closed for a small period of time to allow the metal to be
squeezed together before the weld process.  sometimes enough current is
allowed to flow to distort the metal so that it can be squeezed tighter.

2. the actual weld current is allowed to flow via a pair of scr's.

3. important!  the metal is held until the weld nugget cools.  if this
is not done, the metal can shift while it is still hot and a bad weld is
the result.  the cooling time is very short and on the order of 0.25

ther are also about another six or eight variables that can be set such
as preheat, gun squeeze pressure, constant voltage or current control,
number of half cycles of weld current, etc..  but for a simple control,
i would only be concerned with the weld current and the amount of time
the metal is held before the and after the weld.


1999\03\11@165839 by Lawrence Lile


I was looking at a spot welder today - the specs on the side claim that they
have a secondary current of 6000 amps! this is a 120 volt 2.5 KVA unit,
probably with a one turn secondary and a 6000 turn primary.  Can this be

and does my 200 amp buzz box have any chance of making a spot weld in thin
sheet metal? (with the correct timer/controller of course..)

I'll probablyt be closing the jaws manualy - but I'll need to remember your
sequence so I don't make bad welds...

{Original Message removed}

1999\03\11@215031 by Mario I. Arguello

picon face
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..

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

Some of you responded to my previous post and I am thankful but I am looking
for a simple device, if at all feasible.


1999\03\12@101813 by Lawrence Lile

-----Original Message-----
From: Mario I. Arguello <.....MIADsgnsKILLspamspam@spam@AOL.COM>
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
>NiCad batteries.. Can this be done simply by discharging via a large cap
>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
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
>I know I can solder copper braid on the batteries and assemble my packs
>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
>Some of you responded to my previous post and I am thankful but I am
>for a simple device, if at all feasible.

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?

1999\03\29@235400 by John Griessen

Did you get my comments on welding with capacitive setup for thermocouples
and battery tabs, and mention of the way Miller makes
a 10KVA water cooled setup?  I can send you some of the Miller
docs if that would help decide how to use you cracker box transformer in the
current loop.

You WILL want a big turns ratio, and huge, brief amperage.  The capacitors
give you huge amperage without a transformer.  Their ESR (equ. series res.)
is the only limiting thing to the pulse speed.  Energy is easy to control by
chargin amount, and/or banks of caps switched in/out.  Maybe a PIC and a LCD
could be the user interface for that function?

You won't be happy if you spend effort hooking up your cracker box only to
get a weld time of 3 seconds for two sheets of 16 ga steel.
The Miller does that energy jolt in .1 sec for a 1/8" dia spot weld, or .2
second for a fully dimpled in melted weld the same outline as the 5/16 round
weld tips.  It does that by drawing 40 amps through each of two AC
transformer lines (240 V at 40A ~= 10 KVA)

>From stick welding experience, a 200 A welder probably will generate that
kind of energy in about 3/4 second, but with the heat flowing out of the
weld clamp tips, you would need more energy, and so, more time.

The integral one turn 5000 Amps big copper clamp with removeable tips is the
main feature of a spot welder above a certain energy, and I'm not thinking
of a product you could salvage such from except other spot welders so far...

I just had an idea... you need coils that have a big opening to get the huge
copper weld electrode/clamp through.  How about solenoids that come in
washers to work water valves. They're 120V, with a big hole for the valve
body and moveable slug of iron.  The coils come off easy, and are plastic
lined and humidity resistant.  Find some iron cores or make out of 1/2"or
whatever steel bar bent in a U with a bar welded on the end after the coil
is on--then dunk and cool quick. String bunches ( 6 or so) of those
assemblies along a (3/4 or whatever you want) inch diameter copper rod, and
you may be on to a good current pulser to drive by caps or house power.  The
magnetic circuits would add independently, so you can drive each 120V coil
with 240V house power--temporarily, of course...)

John Griessen
Austin TX

> {Original Message removed}

1999\03\30@092327 by Lawrence Lile

Yes I did see your comments.  From other posts, I've determined that a 200A
pulse for 3-5 seconds might meet my needs very well.  That will reduce the
size of the copper jaws to a much more manageable diameter (maybe I can use
a copper pipe with a short copper bar clamped in the end of it.. )  The
complexity of controlling something for a few seconds vs. a few hundred
milliseconds is more manageable, and it uses stuff I have anyway.  Remember,
this isn't a production welder - for me 5 seconds and 500 milliseconds are
no different, as I might only make 10 welds a day.

I may see if I can try a couple of experiments by using the on-off switch
manually and a few pieces of scrap copper.  If a funky manual no-cost setup
works once, then the whole shebang might be worth putting some effort in.

{Original Message removed}

1999\03\30@133955 by John Griessen


Correction!  See
for why!

I got my magnetic circuit wrong.  Where I said run the
secondary conductor through a bunch of coils connected
in parallel as primaries, that would not make a transformer, just some kind
of heater element.

So, to make a transformer, the amp turns are equal in all
windings around any one core. The other equation is ohms law
for the amps in the primary, which can be as many turns
as you can get wound through the output iron/ferrite/sendust
cores with single or two turn output windings.  solenoids won't
be good for that--too many turns.  The modules from flat transformer
have center tapped windings and another inductor which would not be needed
for a spot welder.  You could just use a bunch of cores with a big enough
opening to get 100 loops of 20 ga teflon wire through, plus a single turn
output winding for each, out of 16 ga wire.  All the 16 ga. wires would
hookup in parallel.  Know a source of cheap toroids out of ferrite?

> {Original Message removed}

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