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Converting stick welder to tack welder.

M

mike

Jan 1, 1970
0
Steve said:
mike wrote:




Hi Mike,

We needed to weld some exotic metals, that required CD welding. Our
welder was built in a glove box, The electrode construction was similar
to your, we modified a toggle clamp to do the job with 1/16" diameter tips.

The cap- bank was around 2200uF (10 x 220uF 400V reservoir caps) Energy
supply was a large variable O/P PSU, large because thats what we have
around. Drive was 0-400V. Welding occured at around 40V.

Discharge was effected by a very large old automobile relay , with
contacts bigger than US pennies (around 1" - like the old UK pennies)

Thanks for the info. I was never successfull trying to switch the weld
current. Welded a lot of contacts, but no battery tabs ;-)
Typical CD system discharges more volts into a step-down transformer.
mike
Job was pinched in the jaws of the spotter, then the hands had to
operate two buttons simultaneously to activate the spot.

Yes, I'd have preferred to use a huge ignitron, or a hockey-puck
thyristor, but we didn't have time - this was a two day
oh-god-we-have-to-do-this-yesterday kind of thing.

We just about managed to weld molybdenum foil ~0.2mm thick, with it.

Steve



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S

Steve Taylor

Jan 1, 1970
0
mike said:
Thanks for the info. I was never successfull trying to switch the weld
current. Welded a lot of contacts, but no battery tabs ;-)
Typical CD system discharges more volts into a step-down transformer.
mike

It would need to be a very good transformer, to handle high speed, high
current transients like this - I doubt I could design one off the top of
my head.

Steve
 
M

Martin H. Eastburn

Jan 1, 1970
0
mike said:
You're being too picky.
The manufacturer publishes a specified waveform for their device
under controlled conditions.
Gives you some idea of what you're up against welding battery tabs.
mike
When I was a Senior Scientist for Schlumberger ATE, to picky isn't it. It saved
the company and customers a number of times.

Tr is rise time of a waveform - 10-90 or 20-80. Some waveforms measured from 0 to 100
would never make the two levels - unless a static condition exists.

Math is based on the two levels.

Martin
 
M

mike

Jan 1, 1970
0
Martin said:
When I was a Senior Scientist for Schlumberger ATE, to picky isn't it.
It saved
the company and customers a number of times.

Tr is rise time of a waveform - 10-90 or 20-80. Some waveforms
measured from 0 to 100
would never make the two levels - unless a static condition exists.

Math is based on the two levels.

Martin

I don't understand you at all...
The vendor published a picture of a waveform. On it, they clearly
defined a parameter Tr and quoted a number for a time associated with
that graphical representation.
I can't think of a more unambiguous way to define the parameter.

You have in your head a definition for a thing called Tr. That's where
the ambiguity resides. And it's unrelated to the picture and number
provided by the vendor. Unfortunate choice of parameter name?
Seems so in your case.
But it's still UNAMBIGUOUS!!
Picture >= 1,000 words. Math is not required.
Feel free to relabel the vendor data to suit your needs.

mike
Me do loves a good pissing contest...

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B

billh

Jan 1, 1970
0
I don't understand you at all...
The vendor published a picture of a waveform. On it, they clearly defined
a parameter Tr and quoted a number for a time associated with that
graphical representation.
I can't think of a more unambiguous way to define the parameter.

You have in your head a definition for a thing called Tr. That's where
the ambiguity resides. And it's unrelated to the picture and number
provided by the vendor. Unfortunate choice of parameter name?
Seems so in your case.
But it's still UNAMBIGUOUS!!
Picture >= 1,000 words. Math is not required.
Feel free to relabel the vendor data to suit your needs.

mike
Me do loves a good pissing contest...

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While the risetime typically is not the 0 -100% rise the picture is what the
manufacturer indeed says this is what the waveform should look like. Not
surprising since the waveform is presented for setup or checking purposes
and it is much, much easier to verify with respect to the indicated points
rather than finding 10% and 90% points. Unless you have a storage scope or a
modern scope with memory it is a real PITA to locate intermediate points
with the one-shot nature of the device. Also, the rise-time in terms of the
edge of the waveform may not be the real issue but rather the the energy in
the pulse which is related to the area under the curve. The 100% point as
indicated does give an partial measurement of this and the voltage drop from
the peak is specified.

Billh
 
N

Nick Huckaby

Jan 1, 1970
0
Steve Taylor said:
Discharge was effected by a very large old automobile relay , with
contacts bigger than US pennies (around 1" - like the old UK pennies)

You mean a starter solenoid?
Job was pinched in the jaws of the spotter, then the hands had to
operate two buttons simultaneously to activate the spot.

Are these just safety buttons to prevent electrocution?
The cap- bank was around 2200uF (10 x 220uF 400V reservoir caps) Energy
supply was a large variable O/P PSU, large because thats what we have
around. Drive was 0-400V. Welding occured at around 40V.

This CD welders (below) require 680,000 mirco F.
What's the different about yours?

http://www.powerstream.com/spot-welder.htm
 
M

mike

Jan 1, 1970
0
Nick said:
You mean a starter solenoid?




Are these just safety buttons to prevent electrocution?




This CD welders (below) require 680,000 mirco F.
What's the different about yours?

http://www.powerstream.com/spot-welder.htm
Energy proportional to the square of the voltage on the caps.
Wonder how they get 1000A through their output connector?
Wonder how they get 1000A through the small wires to their
welding tweezers?
My Unitek 125WS welder has half inch diameter output terminals
and runs #2 wire to the head.
Wonder what they use to switch the caps to the load?
Would be interesting to see a graph of weld voltage and current vs time.
mike




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M

mike

Jan 1, 1970
0
Nick said:
Could an automotive relay switch them on/off quickly to acheive
the wave form you posted?

"Automotive Relay" is a pretty broad term.
In general, a relay can turn on arbitrarily quickly, in terms of
risetime. Turning off can be a problem if the contacts weld together.
Even if they don't weld, there are mechanical and magnetic flux time
constants that limit the minimum pulse width.
The second pulse is problematic if the contacts are all "burnt" from the
first pulse.
I expect there are relays that could do the job, but not likely they'll
be found in an automobile.

So you're saying that capacitors are less likely to explode?

Again, we'd have to be more precise about the definition of explode.
Take a new fully charged car batery. Shine up the connections.
Slam a 1" square copper bar across the contacts. Have somebody
videotape the experiment. Report back your results. It's likely that
the result can be used for my definition of explosion.
Do NOT try this at home.

Now pick a capacitor and voltage that gives you 100 Watt-Seconds.
Short it with the copper bar. Compare the results to the battery
experiment.
mike



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N

Nick Huckaby

Jan 1, 1970
0
<snip>
Would be interesting to see a graph of weld voltage and current vs time.
mike

I found a graph for a dual pulse CD welder here.
http://religion.p5.org.uk/cdwelder.htm

I assume that most CD welders require about 680,000 mirco F.
How's it possible that Steve can weld with only 2200uF? How does
the power supply provide the necessary current to the capacitors?
 
B

billh

Jan 1, 1970
0
Nick Huckaby said:
I found a graph for a dual pulse CD welder here.
http://religion.p5.org.uk/cdwelder.htm

I assume that most CD welders require about 680,000 mirco F.
How's it possible that Steve can weld with only 2200uF? How does
the power supply provide the necessary current to the capacitors?
The size of the job would presumably determine how much current is required
and thus set the minimum size of the capacitors. You can also vary the
amount of charge held in a capacitor by changing the voltage. The Charge is
Q and the Total Charge in a Capacitor is Q=CxV. Charge is also related to
Current (I) by the formula Q=IxT so IxT=CxV and then I (current)= CxV/T.
This means you can increase the current capability by increasing the size of
the capacitors C and/or the Voltage V or shortening the discharge time T.
Note that these are ideal equations and the resistance of the leads and
workpiece also factors into what happens in real life. The voltage level
also has to be within safety standards. However, you can't just increase the
current by shortening the time it is applied and expect it to work. The high
current has to be present long enough to heat and fuse the workpieces.

The above is also related to charging the capacitors. By increasing the time
to charge the capacitors you can do it with relatively small amounts of
current. In this type of welding done manually that is not hard to do
because repostioning for the next weld etc gives seconds to charge the
capacitor so the power supply does not have to deliver high currents.

Billh
 
M

mike

Jan 1, 1970
0
Nick said:
I found a graph for a dual pulse CD welder here.
http://religion.p5.org.uk/cdwelder.htm

I assume that most CD welders require about 680,000 mirco F.
How's it possible that Steve can weld with only 2200uF? How does
the power supply provide the necessary current to the capacitors?

You snipped the answer from my previous post. Doubling the voltage
requires 1/4 the capacitance. 18V...400V, do the math.

You have at least two major problems.
You have to store the energy somewhere. The voltage and the capacitance
are the easy parts.
Second, you have to deliver all that current to the load.
You need extremely low resistance in the switch and all the path to the
weld. The simple "fix" is to just up the energy to account for the
relatively huge losses. Looks good on paper, but makes the process
extremely sensitive to the weld resistance which depends on force,
contamination, material, luck...
Tweaking it to get a good weld ain't all that hard. Getting a dozen
good welds in a row is not so easy.

My Unitek 125 switches 400 odd volts with an scr into a transformer.
400V in >> 7V out into .001 Ohm. It's not your ordinary transformer.
I was surprised how small it is. IIRC about 2" on a side.

mike



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