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Sometimes I need to disconnect my ESD wrist strap, but forget to reconnect it again. Apart from improving my discipline to work more structured and more carefully, I wondered if there was any device/circuit that could warn me for not having reconnected the ESD wrist strap.
Searching on internet, I found an ESD wrist strap with build in ground alarm

How does such a device work, how does it detect a disconnection of the wrist wrap?

I couldn't find any schematic on internet, so thought of a schematic how I think it may have been implemented.

schematic

simulate this circuit – Schematic created using CircuitLab

A (modulated?) current source causes a defined voltage drop across R_device when SW1 is open.
As soon as the ESD wrist wrap is connected (pictured by SW1), the series resistance of R_connection_wire plus R_wrist_wrap plus R_human_body plus R_shoes/floor will be in parallel with R_device, which will lower the defined voltage drop.

With a multimeter, I measured the resistance of my own wrist wrap (bad measurement, but not relevant anyway) and the resistance of the connection wire to the wrist wrap. I made assumptions w.r.t. to the resistance of the human body and other resistances in the path (shoes, floor).

Next to the question:

How does the ESD wrist strap with build in ground alarm detect a disconnection of the wrist wrap?

I'd like also to ask:

Do you have other ideas/suggestions to detect a disconnection of the wrist wrap?

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  • \$\begingroup\$ There are simple test meters where you touch a exposed terminal and it indicates if your finger has a suitable path to ground. you could make touching this test meter part of the workflow or have disconnect the bench supplies if it has not been touched since you unplugged the strap connector, lifted your bottom from the chair or floor sensor or some such. \$\endgroup\$
    – KalleMP
    Apr 25, 2019 at 6:53

1 Answer 1

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The automatic monitoring straps that I have used have two electrical connections in the strap. The strap has a mini plug (3.5 mm?) on the end of the cable so both connections can be made to the sensor. The strap is electrically built in two isolated pieces, the metal ones had two 180 degree halves. If the monitoring device didn't detect some resistance in the person's skin, it would sound an alarm. We often had to use a special cream on our wrists to get a good enough connection. Each signal had a 1M resistor in the plug for safety.

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  • \$\begingroup\$ But how does it detect the disconnection from earth (like SW1 in my schematic?) \$\endgroup\$
    – Huisman
    Apr 24, 2019 at 22:25
  • \$\begingroup\$ No, this system did not detect a disconnection from earth. But, this connection is much less likely to fail than the wrist strap wires that are flexed constantly. \$\endgroup\$
    – Mattman944
    Apr 24, 2019 at 23:00
  • \$\begingroup\$ I agree with your remark that the strap likely fails to make a good connection to the human wrist. (I can't check the link somehow, but) I think the wrist wrap I linked to does also check the resistance of the human skin. However, this does not answer my first question. \$\endgroup\$
    – Huisman
    Apr 25, 2019 at 7:44

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