Homebrew PCBs

Re "wire in water being cooled". Actually the contrary is what happens.  
The speed with which the pulse happens is much faster than the 
conduction of heat so water or no water is not much different, but 
because the arc is constrained by the surrounding water, the arc channel 
pressurizes and is small with a corresponding increase in resistance 
which aids the concentration of energy deposited in the arc rather than 
losses in the wires and capacitor foils connecting to the exploding 
wire.  Arc voltage drops in the air are typically only a few ten's of 
volts dominated by the voltage of nitrogen arcs, and if seeded by metal 
ions, the voltage is even lower.  So the energy deposited in an 
exploding wire will be very small fraction of the available energy that 
was stored in the capacitor bank.

Re: Low voltage, high current.  The general trick of the exploding wire 
is that the copper vaporizes and is "driven" by copper vapor pressure to 
fly onto the wall of the hole and condense.  Low voltage tends to just 
melt the wire whereupon it balls up, but doesn't vaporize, a necessary 
condition for "plating" the wall of the hole.  I'm not saying you can't 
vaporize wire with low voltage, high amperage, but rather it may not be 
fast enough.  The low voltage arc deposits heat by the contact of the 
arc with the wire, causing it to melt and a small amount of metal to 
vaporize at the arc contact point -- think about the common arc welding 
process.  The welding rod doesn't explode or even get very hot, but the 
arc contacting the rod melts it very efficiently as well as the thing 
being welded.

RE: Danger of explosives.  Absolutely agree with your whole paragraph.  
But the discussion has already crossed the boundary of general safety 
with multi-KV capacitance storage.  Lethal possibilities start when it 
got above about 50 V.  Many deaths have occurred at 110V.  There are 
also dangers at low voltage. Mention was made of an aircraft battery.  
One of the stories I'm aware of is of a mechanic working on a 28V 
aircraft battery while still wearing his wedding ring.  He got the ring 
across the buss. Ring almost instantly melted off of his finger -- I 
don't remember if he lost the finger or not, but I'm absolutely sure it 
wasn't healthy for the finger. (Just looked on the web for the story and 
found this similar story involving a car battery:
http://www.popularmechanics.com/cars/how-to/repair-questions/4213127
My take is that we're discussing interesting possibilities.  Why not 
open up the discussion to other possible techniques just to stimulate 
the discussion?

RE: It is also probably not needed to explode the wire in PCB...arc 
eroding a tube.   This is essentially the description of a plasma 
sprayer.  Generally they're bigger, but why not a miniature version for 
thru hole plating?  Interesting thought. (This is what I mean by 
stimulating discussions.)

While we're on safety and hi-voltage another thought about it.  At 
multi-KV numbers, keeping the spark/arc where it belongs is 
non-trivial.  It may just as easily transfer to the PCB on its way to 
the circuit return.  Putting the arc through the hole and carbonizing 
the wall will guarantee a very poor connection to the copper layers 
involved.  Keeping the exploding wire supply feeding to the hole 
requires a lot of **GOOD** insulation including lots of air gap.

Regards,
Charles R. Patton

On 1/2/2013 12:14 PM, Bob Macklin wrote:
>
> If you want to melt a wire use a 28V 600A aircraft battery!
>
> Ask me how I know!
>
> Bob Macklin
> K5MYJ
> Seattle, Wa.
> "Real Radios Glow In The Dark"
> ----- Original Message -----
> From: brane212
> To: Homebrew_PCBs@yahoogroups.com 
> <mailto:Homebrew_PCBs%40yahoogroups.com>
> Sent: Wednesday, January 02, 2013 12:09 PM
> Subject: [Homebrew_PCBs] Re: plating holes with an exploding wire
>
> --- In Homebrew_PCBs@yahoogroups.com 
> <mailto:Homebrew_PCBs%40yahoogroups.com>, Charles R Patton 
> <charles.r.patton@...> wrote:
> >
> > A few other comments:
> >
> > First: 200uF @ 2KVusing Watt-secs or Joules = 1/2 (CV^2) gives 200
> > Watt-secs.That seems like a lot of heat in a 10 mill dia 62 mill long
> > hole.
>
> Not neccessarily. Your example of wire exploding inside water needs 
> much more energy due to water acting as coolant
> Also exploding might be done in many different ways and stages.
>
> Fo example, you could do:
>
> 1. Using high voltage source ( say 1kV, fev milliamps) to make initial 
> spark when moving the wire trhough the PCB to background copper.
>
> 2. When that spark jumps, activate lower voltage but higher current 
> wire to heat the spark plasma which would "eat" remainder fo the wire.
>
> Doing things in this way would minimize requirements on the power source.
>
> It is also probably not needed to explode the wire in PCB . One could 
> have small tubeinto which he could gude the thin wire and explode or 
> plasma erode it there and let the resultant mix of hot gasses flow 
> through PCB hole so copper could get deposited.
>
> Lightflash application you mention is interesting, but it has 
> different requirements. Here it is not that important if flash lasts 
> fo nanosecond or microsecond and there it was not important what 
> happens to the copper afterwards.
> .
> WRT to playing with explosives I think it is stupid. You had 
> opportunity to watch posts of the people who got wrong simple 
> capacitance of serial configuration of equal capacitors. What do you 
> think is going to happen when those talents start playing with 
> explosives - or maybe you counted on natural selection to do its job 
> and lift quality of posts ? ;o))
>



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