The ink I'm using is Higgins black magic. Says its waterproof after it dries and it IS.
The minimum hole size I currently have are 20mils in diameter. some are at 28mils and others are at 32mils. mounting holes are 100mils.
The charcoal is very fine probably below one mil in diameter. Very fine dust. mixture can be painted on but that's messy. My boards are pretty small so I have only limited number of holes and its just as easy to use a sewing needle. Dip the needle into the ink mixture and get a drop on the needle. insert the tip of the needle into the hole. Capillary action pulls the ink into the hole. I tap the board on a table, covered in throw away paper to catch the excess droplets popped out of the hole. I do this about every ten holes or so. Then I dry it out completely to cure the ink. With only about 20holes or so, the ohmic reading I get from one side to the other side is around 100-200ohms. I think its good enough. Will see.
This idea isn't mine, it came from Thinktink.com. I don't like the idea of agitating the plating solution with aeration. I think that's only bad idea from them. Rest seems pretty sound.
And their lack of good current source.
BTW, you will see a revised schematic for the current source and a thermal shutdown. If people are more inclined in opamp implementation I can do that too.
Thermal shutdown circuit has not been built yet. I will do that tomorrow and check its behavior against my analysis. Past experience says that Vt+ is actually lower than designed value by about 50mV to 100mV. Vt- should be very close to Vbe at onset of turn on so I should be within 10mV or less. Based on crude first pass design. Took me about 5 minutes to do :).
I think I will create a new artwork for this discrete implementation. I use TO-92 transistors and 1206 sized caps and resistors. Anything smaller and my old eyes and my unsteady hand has trouble.
sincerely,
Shawn
--- In Homebrew_PCBs@yahoogroups.com, "Jeff Heiss" <jeff.heiss@...> wrote:
>
> How are you getting the charcoal/ink mix into the holes? What size holes
> are you using?
>
> Jeff
>