How do ESD monitors work ?

[zibilytou]

Hi everyone.
I want to know how do ESD monitors work, those who detect if someone is wearing the ESD wrist band or not. The problem is that there is only one wire feedback from the wrist band so i don't know how can you measure anything from just one wire.
Thank you.

 

[BradtheRad]

Strange but true... Static charge can be detected by a bare input terminal of:
* transistors
* fets
* mosfets
* logic gates
* op amps
etc.

The key is to have a high impedance run of wire. Just a few inches of wire is influenced by a positive or negative charge, which in turn causes movement of electrons in the bias junction of a sensitive amplifying component.

Such as this simple inexpensive static charge detector. I wouldn't be surprised if a similar circuit is in the ESD monitor.

amasci.com/emotor/chargdet.html

 

[zibilytou]

Thanks for your reply but i think you did not understand what i am really asking, what i want to understand is not how to detect a static charge but to detect whether a person is wearing the ESD wrist band or not.

 

[BradtheRad]

There is the capacitive method to sense proximity. Hardware stores carry handheld instruments which detect things behind plasterboard such as pipes, wiring, wood, etc. One description said it measures the dielectric constant (which has to do with capacitance). It's not quite the same as a metal detector.

I imagine a similar capacitive proximity detector is built into the wristband.

 

[ads-ee]

It is interesting that the website the picture post is from is a website that has a description of the methods used to detect a person in the "loop"

https://transforming-technologies.com/esd-fyi/how-does-an-esd-constant-monitor-work/

Here is something that describes how the detection is done for the capacitive type of wrist strap detection:

 

[zibilytou]

Hi @ads-ee , i've already read similar description to what is written in that website and i know that a human body acts similar to a capacitor but still i can't figure out how to use that information to concept a circuit able to detect a human body cause am beginner in electronics, i seek detailed description to understand the idea.

 

[FvM]

The device shown in post #1 is a dual wire (resistance measuring) monitor according to manufacturer information. Hence your assumptions are wrong.

It's of course possible to design both types of wrist strap monitors, a certain level of electronics knowledge is obviously required to do so.

 

[zibilytou]

Oh apparently the picture shows a dual wire ESD monitor, my bad then. But there are plenty that work only with one wire and that is what i seek to understand how.

 

[BradtheRad]

Memory got jogged... temperature sensing may be the easiest way to detect someone's wrist.

 

[d123]

Hi,

I thought that at first, but what if the lab is very cold, what temperature counts as 'living being'?

 

[zibilytou]

Temperature sensing is a good idea but i don't think that is how they do it, cause I've read elsewhere they use a capacitance detecting method taking into account that the human body behaves just like a capacitor in contact with the ground.

 

[FvM]

Capacitive monitoring is described in the link provided by ads-ee, along with typical capacitance threshold values. The measurement circuit must be implemented in a way that 1. doesn't interrupt the discharge path, 2. works with larger series resistance of the wrist band and 3. isn't disturbed by picked-up mains frequent voltage.