0
\$\begingroup\$

As suggested in an answer to an earlier question of mine, bonding to a laptop can be done by connecting an anti-static wrist strap to the ground of an USB-port. (After disconnecting the AC-to-DC adapter.)

At which locations should there be a 1E6 ohm resistor? My quess: at A and B.

enter image description here

\$\endgroup\$
5
  • 2
    \$\begingroup\$ I'll repeat the first sentence in Richards answer: you're overthinking this. The 1M resistor is not a fine tuned value that has to be respected. Just add one on every lead and have peace of mind. \$\endgroup\$
    – Wesley Lee
    Nov 29, 2016 at 21:04
  • 1
    \$\begingroup\$ I read your other post and I like your original plan better. Because when you are going to connect the grounding cable to the laptop, if there is a potential difference, the circuitry around the USB port is going to be much more susceptible to damage than the DC power input. \$\endgroup\$
    – rioraxe
    Nov 29, 2016 at 21:06
  • \$\begingroup\$ @rioraxe, when following the original plan (connecting the GND of the DC-jack to the common ground point) then should there also be a resistor in between? (Between DC-jack and CGP.) \$\endgroup\$
    – user127725
    Nov 29, 2016 at 21:09
  • \$\begingroup\$ @rioraxe, although... the USB_GND should be connected directly to the motherboard's ground-plane AFAIK. So, little circuitry to damage. Or not? \$\endgroup\$
    – user127725
    Nov 29, 2016 at 21:15
  • \$\begingroup\$ @marty Sorry for late reply. If at the time of of connecting a cable to the laptop, a 2kV ESD occurs, would you want that to go in through the USB port, which is probably on the motherboard, hoping that most of the current would go through the ground plane (although the USB port should be hardened already), or through the dc power input instead. I would put a resistor between DC-jack and common ground. Also, the common ground should be low impedance to earth ground. \$\endgroup\$
    – rioraxe
    Dec 2, 2016 at 0:46

1 Answer 1

2
\$\begingroup\$

The purpose of the 1M resistor is to protect you from being wired to a solid earth, in the event of another piece of equipment having a live to chassis fault.

Point A will protect you, if you are grounded nowhere else.

Point C would be good as well, as it protects you even if you are touching the (fairly conductive) mat.

The laptop ought to be isolated by its PSU, but do you trust those cheap small hot running things? Put another one at B, they're only a penny each.

Basically one in every lead you own or use will offer you the best protection against shock, however you configure the leads, and will not degrade the protection against ESD.

\$\endgroup\$
3
  • \$\begingroup\$ With "PSU" you mean: the AC-to-DC adapter? Before bonding, i disconnect the AC-to-DC adapter. \$\endgroup\$
    – user127725
    Nov 29, 2016 at 21:05
  • \$\begingroup\$ The anti-static mat should have a resistance well above 1E6 ohm. \$\endgroup\$
    – user127725
    Nov 29, 2016 at 21:10
  • \$\begingroup\$ Having dismantled and inspected an Ebay style laptop PSU, I now have a policy of never leaving one plugged in unattended. \$\endgroup\$
    – Ian Bland
    Nov 29, 2016 at 21:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.