Author Topic: Esd wrist strap, is it working?  (Read 271 times)

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Offline Moriambar

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Esd wrist strap, is it working?
« on: May 20, 2021, 06:34:03 pm »
I "recently" redid my lab with an esd mat, a ESD plug connected to the mains socket and a wrist strap bounded to it through the mats.
All bought through a very respectable source in the EU.
But how can I check whether I'm properly esd-ok? should I shove a dmm probe into the earth terminal and measure the resistance to my hand? Or the voltage? Or neither?

Perhaps I should learn a bit more.

Offline madires

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Re: Esd wrist strap, is it working?
« Reply #1 on: May 20, 2021, 07:10:31 pm »
A resistance check should be fine. Expect 1 MOhm between mat and PE, and also 1 MOhm between wrist strap and mat. That resistance is meant to protect you from mains voltage (it limits the current) in case of some mishap.
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Offline Feynman

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Re: Esd wrist strap, is it working?
« Reply #2 on: May 31, 2021, 09:01:06 pm »
The resistance between wrist strap connection point and earth should be between 1M and 5M, which can be measured with a DMM easily.

Offline Doctorandus_P

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Re: Esd wrist strap, is it working?
« Reply #3 on: June 01, 2021, 08:11:32 am »
ESD mats may very well only start leaking current at higher voltages, and it may be "open" to a DMM even if it can measure to 100MOhm.

Testing ESD mats is usually done with "Isolation testers" which generate 1000V or more.
In the "ugly outside world" lots of things that seem simple are not well modelled by lineair things such as resistors. Take for example a "spark gap". At low voltages, it's just an open connection, but above some threshold you get a spark and during a brief moment resistance is very low.

As far as I know, the plastic polymers in ESD mats are a bit like varistors. Below their threshold voltage there is virtually no current.

Calibrated ESD mat testers are overly complex and expensive. There are test procedures wich specify test contact area with the mat, and test weights or pressure on the mat to get a calibrated measurement.
But if you make a (somewhat calibrated) 1000V generator, add a few series resistors to keep the current low (and make it safe), then you can easily measure the voltage over one of shunt resistors (Resistors are usually rated for 200V or so) to get an estimation of the mat resistance.

One of the "dangers" with ESD mats is the wrong kind of cleaning substance. Some may leave a very thin but isolating film over the mat.

For the wristwrap it's easier. Your DMM indeed probably measures some resistance between your hand and mans earth (if your wrist wrap is properly connected).
« Last Edit: June 01, 2021, 08:23:12 am by Doctorandus_P »

Offline Moriambar

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Re: Esd wrist strap, is it working?
« Reply #4 on: June 01, 2021, 08:25:11 am »
Thanks all, I was able to measure my resistance at about 3 Megs. I was just poking the wrong thing before!


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