On Wed, 09 Mar 2011 08:16:00 -0800, mike wrote:
Spot welding is a LOT harder than it sounds.
I agree. I spent about 5 days tinkering with mine trying to get it
right. Every time I added more capacitance, things improved. The
basic trick is to make sure the resistance of the joint to be welded,
is higher than all the other resistances (and ESR of the capacitor).
Mine currently has two 3F 20v caps in parallel. Something like this:
http://ledhacks.com/power/battery_tab_welder.htm
Just to use some round numbers that you can scale for your application...
If you expect to get 10,000 amps out of 10V, you can't have more than
a milliohm of resistance. That's the cap esr + the switch + the wire +
the contacts
+ the contact resistance at the weld. And the only thing that does much
good is the amps x volts x time at the junction between the two parts
being welded plus a little in the weld material itself.
Good point. However, I have no way to measure the peak current or the
current necessary to do the weld. I just kept adding capacitance
until it worked.
An extra couple of milliohms in the system will render the welder impotent.
And the weld will be extremely sensitive to the contact resistance
between the weldments and thus the pressure applied and the contact area
and, and , and.
Well, the idea is to dissipate the power in the connection, not in the
capacitors, SCR, or contacts. As long as the capacitor is huge, the
SCR oversized, wires beefy, and the contacts can handle both the
current and the heat, it should work.
With a low-voltage AC welder made from a microwave oven
transformer, I was getting about 20% good welds. The commercial CD
welder with the same homebrew weld head was closer to 100% good.
Apples and oranges. There's enough resistance in the transformer
winding to probably make the welder useless. If the primary winding
was made from copper buss bars, it might have a chance, but not a
copper wire wound xformer. A big capacitor has an internal
resistance, but that's MUCH less than the resistance of a wired
transformer winding.
You really want something that controls current rather than voltage.
Then, most of the variables end up in one place, the weld.
Using a higher voltage and some intentional, controlled series
resistance can make it much more reliable.
Agreed. Voltage doesn't matter. It's the current that does the
heating. If you shove XX Amps through YY ohms of resistance, it's
going to heat I^2*R (Ohms Law). Whatever voltage it took to deliver
that much current is largely irrelivent.
Many battery tabs are made of zinc. Easier to weld than stainless.
Hobby-Store 5mil brass is very easy to weld, but may be too high
resistance for a dust buster.
I haven't seen any zinc tabs yet. I've seen tin plated steel and
stainless. Brass should work for a Dustbuster as they don't draw all
that much current.
Having said that, you can, with practice, make usable battery tab welds
with a capacitor bank, a pressure controlled contact system and a switch
made out of a nail. Charge the caps, apply weld pressure, slam the nail
across two BIG wires. I was using 4/0 wire. You get one weld per nail,
but nails are cheap.
I used a copper contact knife switch. I welded the contacts a few
times, but was able to break it apart, file it clean, and do it again.
One more idea. Single sided resitance spot welder as used in auto
body shops.
http://www.jcwhitney.com/spot-welder/p2007539.jcwx
However, you'll need an arc welder for this to work.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558