Homebrew PCBs

On Sat, 02 Jul 2005 10:22:38 +0200, Les Newell <lesnewell@...>  
wrote:

> I don't think the power supply has to be that complicated for this
> application. Many moons ago I used to own a Sincleir ZX81 computer. The
> Sinclair printer used special paper that was basically a thin coating of
> aliminium over a black background. The printer used a toothed belt to
> drag a metal stylus over the paper and wherever it needed to print a dot
> it applied a voltage to the stylus and vapourised the coating. Being a
> Sinclair product you can be sure the circuitry was very basic. Our needs
> are very similar, the only real difference is that the coating is  
> thicker.
> Normal EDM needs a carefully designed power supply because each spark
> removes a very small amount of material. We don't need that kind of
> accuracy so we can use a much higher energy density. I think that if we
> use a large capacitor and keep it charged through a simple linear power
> supply the sparking will be pretty much self regulating. All we need is
> a big beefy transistor to turn on the power when we want to remove
> material. The transistor does not need to switch for every spark, it is
> simply held on for as long as we need to remove copper. As a kid I used
> to cut patterns in aluminium foil using just a 12V car battery as the
> power source and it worked quite well.
> Les


It would probably work with a simpler PSU, yes.
However, you need to somehow limit the current at the electrode, or that  
big beefy transistor will get damaged at some point. If you use a resistor  
to do that the power wasted would be great (the guy who designed that  
complicated supply writes). Now if we use less voltage/current anyway it  
might well be acceptable.

The charge current could be pretty much self-regulating if you use a  
transformer supply, i agree. Remember that guy used a rectifer right off  
the mains.
I have two 50V transformers here that i could put in series, they can  
almost continuously supply the current i expect is needed for cutting PCBs.


I'll wait for Curt's operation parameters (what voltage and currents he  
uses for PCBs, and if he uses the boost cct at all), then i will think  
about how to build that supply, and which simplifications to make, if any.

I expect if you go really slow that "light duty" supply might even suffice  
for initial experiments.

ST