Wednesday, September 22, 2021

Resistive tap built with sharpie PCB etch

I'm keen not to burn out the input to the spectrum analyser. It's 50 ohms in with a maximum of +30dBm. The solution is to transmit into a dummy load and sample the signal with a resistive tap.

A good candidate for a simple PCB and I used Paul Taylor, VK3HN's, technique of drawing resist on the board with a sharpie. Components are all soldered on to the top of the board so laying them out to figure out where the tracks go is easy.

Here's the circuit and the components for the board:

I cleaned the board to remove fingerprints with Isopropyl Alcohol - is that the best thing to use? Using components to lay out the board, tracks were drawn with a standard sharpie. I didn't go to much trouble and could probably have filled in more of the board with earth plane to minimise the amount of copper to be etched.

In to a small bath of Ferric Chloride. While gently rocking we had a magnitude 6 earthquake which was fun. It took about 20 minutes to clear the copper.

After washing under water the etch looks great.

Steel wool was used to rub off the sharpie ink. 

Here's the completed board. 

I've got some more of the nice board edge mounting SMA sockets coming so had to make do with a panel mount for the tap output.

What should I use to protect the copper? I think Paul just tins the whole board but maybe there's a spray lacquer that would still allow solder modifications but prevent oxidation.


caulktel said...

Hi Peter,

I bought one of these for that very purpose,
Works great. Thanks for tour great Blog.


Peter Marks said...

Thanks Joel, the Powersnitch looks great. Hadn't seen that before.


Unknown said...

Good to see you like this PCB method, Peter. If you put the plastic etching container into a larger one with hot water and rock the etch tub back and forth, a small board like this one should etch in under 5 minutes with fresh FeCl3.

Also, the Sharpie ink comes off easily with methylated spirits. You can also use metho on a small piece of paper towel as an eraser of markup errors.

To stop the etched board copper tarnishing, either spray with clear enamel, or tin it with your iron and regular resin cored solder, cleaning up with steel wool.

73 Paul VK3HN.

Peter Marks said...

Thanks Paul,

Incidentally, I found the blue Staedtler permanent Lumocolor pens you recommended at OfficeWorks for AU$4.


Nev said...

Nice Blog. I've recently tried this to "protect" the copper... Works really really well.