| Author |
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Nick Huckaby
Guest
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 Posted:
Wed Mar 02, 2005 9:16 pm Post subject:
Re: Converting stick welder to tack welder. |
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"mike" <spamme0@netscape.net> wrote in message news:4224B217.6070909@netscape.net...
| Quote: | 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. - Steve
This CD welder (below) requires 680,000 mirco F.
http://www.powerstream.com/spot-welder.htm
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?
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billh
Guest
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| Posted:
Wed Mar 02, 2005 11:07 pm Post subject:
Re: Converting stick welder to tack welder. |
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"Nick Huckaby" <simoung@gmail.com> wrote in message
news:1109775854.1769515ae74296d1006e4d74363bebb0@bubbanews...
| Quote: | "mike" <spamme0@netscape.net> wrote in message
news:4224B217.6070909@netscape.net...
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. - Steve
This CD welder (below) requires 680,000 mirco F.
http://www.powerstream.com/spot-welder.htm
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?
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 |
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mike
Guest
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| Posted:
Thu Mar 03, 2005 2:02 am Post subject:
Re: Converting stick welder to tack welder. |
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Nick Huckaby wrote:
| Quote: | "mike" <spamme0@netscape.net> wrote in message news:4224B217.6070909@netscape.net...
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. - Steve
This CD welder (below) requires 680,000 mirco F.
http://www.powerstream.com/spot-welder.htm
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?
|
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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