I'm not native English speaker. I hope I can be clear enough. Sorry.

My scenario is me trying to convert a 3D printer into a CNC PCB mill and as the bed and I need to use the drill as a probe to work as Z axis endstop. So the printer circuit has a pin with a pullup resistor (12v) and it being driven to ground marks the z=0 height. So I connect the PCB to ground and the drill to that pin and that's it.

But I observed electrical arcs between the drill and the PCB and I think that reduces the precission. And I need that precission. I think the board is "sensed" before actual contact is made. 0.05 mm is enough error to reduce que quality of the result as I am milling 0.16 mm with a cone drill.

So, Is there a simple way to prevent that arc? My first thought was to put a resistor but there is already one there, the pullup, so I think it might be useless.

circuit representation

  • \$\begingroup\$ What's the diode doing, exactly? \$\endgroup\$
    – Hearth
    Nov 9, 2019 at 17:30
  • \$\begingroup\$ Nothing in this case \$\endgroup\$
    – aalku
    Nov 9, 2019 at 17:31
  • \$\begingroup\$ When working as 3d printer I use an active capacitive probe and I don't fully understand it so I wanted to make sure it could not damage the board supplying current. \$\endgroup\$
    – aalku
    Nov 9, 2019 at 17:35
  • \$\begingroup\$ what value is the pullup resistor? ... i sounds like it allows too much current through the contact \$\endgroup\$
    – jsotola
    Nov 9, 2019 at 18:03
  • \$\begingroup\$ I don't know. It's the printer board and it works like this as far as I know. I'll try to add a resistor in series and watch if it detects the plate at the same height. \$\endgroup\$
    – aalku
    Nov 9, 2019 at 18:09

1 Answer 1


Sounds like the current is rather high- it is good to use tens of mA to detect a normal switch closure because of minimum wetting current requirements.

If it isn’t damaging the PCB you can use it if you detect the closure rather than opening. 12V won’t jump a significant distance (you can look it up, probably tens of microns), however when opening you can get ionization effects that allow the arc to be sustained over a relatively large distances.

Edit: Dielectric strength of air (at STP) is 3E6 V/m so 12V will jump 4um. It might be a bit less if the parts are pointy.

  • \$\begingroup\$ It's barely visible. I don't know the distance. If it's so small then I don't care. \$\endgroup\$
    – aalku
    Nov 9, 2019 at 18:45
  • \$\begingroup\$ I didn't find the distance for 12v but it seems it's 0.03mm for 400v. So it won't affect the precision. Thank you. \$\endgroup\$
    – aalku
    Nov 9, 2019 at 18:50
  • \$\begingroup\$ Keep in mind that applies for initiating the contact. If the current were very high after closure it might be able to sustain an arc over 10mm as the contacts were opened, at least until the metal burned away. That's how a Jacob's Ladder works. \$\endgroup\$ Nov 9, 2019 at 18:52
  • \$\begingroup\$ The machines do a rebound on purpose and there is not an obvious arc. LOL. Maybe there is a small vibration in the moment of contact and it's not sustained so after the first touch there is a small arc, but it would be tiny and after the touch. \$\endgroup\$
    – aalku
    Nov 9, 2019 at 19:10

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