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ESD mat seems potentially dangerous?

Discussion in 'General Electronics Discussion' started by NuLED, Jul 5, 2013.

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  1. NuLED

    NuLED

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    Jan 7, 2012
    I searched the forum but did not find anything relevant, which is strange. But anyway, I am going to start studying some of the more ESD sensitive things (ICs and uCs, etc.) and from what I read and also view on YouTube, it looks like an ESD mat should be connected to ground from the power outlet. Then you have the wrist strap on you, to the mat. So basically you are connected to ground, literally.

    Isn't this potentially dangerous? You are now the "common"?

    Along these lines, if we are trying to prevent ESD discharge from damaging components, why do we need to have the mat grounded to the wall outlet? If the mat itself is connected to us (via the wrist strap), and the mat is also semi-conductive, and will be touching the PCB/breadboard and the component, then we should be voltage-neutral with respect to the mat and the component, no?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    Both the mat and the earth strap have very high resistances in series with them (effectively)

    The mat itself is not actually conductive.

    EEVBlog has an excellent video on this topic...

    This isn't the one, but maybe there are some links from it. (Here -- about 6 min mark) Sadly, it doesn't really address your question.

    You should ground your mat just so that you don't charge it up. In theory, other stuff (e.g. stored components) will tend toward ground potential.
     
  3. NuLED

    NuLED

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    Jan 7, 2012
    I bought this mat (and strap).

    There is no plug to the wall socket.

    Should I get a 3 pronged cable, strip it, carefully seal up the live leads, and then attach the ground lead to this cable from the mat? And plug in?
     

    Attached Files:

  4. NuLED

    NuLED

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    Jan 7, 2012
    I will check continuity (and resistance) from the yellow plug to the mat (top and bottom) later also.
     
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    Well, both of those alligator clips should be connected to an earthed connection.

    One of the mats I have has the wrist strap connected to the mat and another lead from the mat that is supposed to be earthed.

    The wrist strap should have a high value resistor (in series) incorporated in it somewhere.
     
  6. NuLED

    NuLED

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    Jan 7, 2012
    I was thinking to strip a power cable and then use that as the earth connection. I don't see any connection from my surge-protector power strip but it has a green light that says GROUND turned on, so the ground is active. And that is the power strip I will be using for other equipment later, so I will be sure everything has the same common.
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    Jan 21, 2010
    Connect it to the metal case of some already earthed equipment. If you have a bench power supply it probably has an earth socket.

    I realise it's probably difficult to mis-wire what you're suggesting, but I still wouldn't do it.

    If you can find a plug with plastic pins for live and neutral and only a metal earth pin, then it might be safer.

    I really have a problem with connecting a conductive strap to my arm then plugging the other end into the mains. This is especially true where you don't know if the mains has (say) live and neutral reversed, coupled with an extension cord with earth and neutral reversed...

    In a fixed installation, it's a little different because you can check it once and be sure it hasn't changed...
     
  8. NuLED

    NuLED

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    Jan 7, 2012
    Ok I understand what you mean. I hope to stay alive enough to annoy you guys with more questions. :-D

    Is there any way I can check if any chassis is grounded? My laptop chassis does not seem to expose grounding. Sometimes I can "feel" an electric buzz if I run my fingers over the external chassis. My printer does not seem to have any metal parts exposed that I can clip to. It is all plastic outside.
     
  9. NuLED

    NuLED

    294
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    Jan 7, 2012
    (This might be simpler later when I make the investment to buy a power supply; I was thinking to DIY one myself to save money but maybe that is not the best idea considering I am a newbie). The power supplies that I see are around $150 USD (?)
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    Jan 21, 2010
    Yes, we try to keep even the most annoying people alive ;-)

    The easiest and safest way is to measure the resistance from the metal case to the ground pin on the power plug (obviously whilst it is unplugged). The resistance should be close to zero (less than 1 ohm).

    It is also worth having a power point tester that will indicate if your power points are correctly earthed etc. In a professionally wired house this *should* be the case.

    Neither may be earthed. The SMPS powering the laptop *mat* be earthed, or it may have either a high resistance or capacitive connection to ground. Feeling a timgling or a buzz from the power supply connection or exposed metal parts is generally indicative of them not being earthed. The dead giveaway is the use of a non-earthed mains plug though.

    Building a power supply is a cool thing to do, but if it's mains powered then there are risks.

    I recently got a small bench power supply (actually two) for less than $100 new (each). There are lots of options for low power devices at reasonable prices. You probably don't need high currents or voltages. Even 1A is way more than many circuits need.

    This is similar to what I purchased, and this (or this) is a cheap alternative. See also this.

    Making a bench power supply from an old ATX power supply yourself is also a cool thing to do. You *know* the metal case is earthed if you use a 3 pin power cord, and the power supply is quite well protected against overloads etc. However they can supply massive current (without limit other than to protect the power supply) so you can easily toast your circuits.
     
  11. NuLED

    NuLED

    294
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    Jan 7, 2012
    OK thanks Steve. Let me UNPLUG the laptop and check the chassis.

    I also checked the prong and it is 3-pronged to the wall.

    (No buzz today on the chassis; maybe I was using another power strip with no ground another time).
     
  12. eKretz

    eKretz

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    Apr 8, 2013
    It may be 3-pronged where it plugs into the wall, but in almost every case the earth (ground) connection does not go through to the laptop computer, it terminates in the power brick or in some cases isn't even connected to anything.
     
  13. NuLED

    NuLED

    294
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    Jan 7, 2012
    OK, no dice on the laptop.

    Yup, even though it is 3-pronged, OL on the meter when testing with the chassis. I checked the cable ITSELF (unplugged that even from the power brick) and while the cable itself has continuity with the 3rd prong and the 3rd pin to ground (to the brick), the brick ITSELF has no linkage between its earth pin and the laptop chassis.

    BUT I discovered that there is a LITTLE BIT OF METAL exposed at the back of my laser printer (also on my desk) and I tested THAT and it was 0.7 Ohms to the earth prong of its power plug. So I reckon I can clip my mat to that little bit of exposed metal in the back (with the ports for USB etc).

    That is OK right? Cuz I am not sure if the USB thing is going to be bad, having a connection back to the laptop (although the chassis that I will clip onto should be insulated from any of the USB connections).
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    Yep, that exposed metal on the printer would be good.

    You might be able to use some earthed part of the USB connector, but that's probably connected to your computer.
     
  15. NuLED

    NuLED

    294
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    Jan 7, 2012
    Hey, new findings. I took some photos.

    I discovered that there is 2 kOhms between my USB KEYBOARD chassis and the COMPUTER EARTH.

    Is this good or bad?

    a) should I use the KEYBOARD CHASSIS to clip the ESD mat instead?

    b) OR if I clip onto the laser printer chassis, is there going to be some kind of short circuit danger or something when my keyboard is sitting on the mat, and is also linked to the laptop (ostensibly via its USB) to the laptop power supply ground? (Seems OK right? Since in fact both of them - the printer and the laptop - are on the same common ground, on the same power strip).

    c) I checked and the ESD mat has infinite resistance and no continuity between itself (top surface or bottom surface) and the little dongly alligator clip. Is this right?
     

    Attached Files:

  16. NuLED

    NuLED

    294
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    Jan 7, 2012
    I checked the little dongly cable on the ESD mat, and it is fine. It has 1 Megaohm resistance (as labelled). But the mat itself I can't measure. There is Over Limit from the snap button on the mat (where the dongle snaps onto) and the mat itself, both surfaces. I assume this is correct? (Then how the heck does it discharge my static? Super slowly and dependent on very high voltage static buildup? Am I thinking this correctly?)

    PS: Because, y'know, ya gotta have a lot of energy per unit CHARGE for the high voltage :-D
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,838
    Jan 21, 2010
    Antistatic mats are of one of three types:

    1) slightly conductive
    2) static dissipative
    3) static dissipative over slightly conductive.

    1 & 3 should be earthed. 2 is designed not to build up a charge in the first place and to dissipate any charge that appears.
     
  18. NuLED

    NuLED

    294
    0
    Jan 7, 2012
    I am going to solder an alligator clip to a solid core wire (to clip to the back of my printer).

    Then the (insulated) wire will go across my desk and be clipped onto, by the alligator clip of the ESD mat.

    However, I want to have something at the end of that wire so that it is easy for my ESD mat alligator clip AND the clip on my wrist strap.

    I was thinking a coin. So, I would solder the end of that wire to a coin.

    Any suggestions on this point, regarding solder properly bonding to particular coinage? US pennies have copper plating right? If I scrub it clean and flux it, should work? (Assuming sufficient wattage on the iron, but since it is not an electronic component I am just going to heat it up big time).

    Or any other suggestion?
     
  19. eKretz

    eKretz

    251
    27
    Apr 8, 2013
    Why not just clip it to the wire? Strip it so there's enough room for both clips. Does your mat not have two button attachment points? Soldering to a penny would work, but no sure how it would be better than just clipping right onto the wire.
     
  20. NuLED

    NuLED

    294
    0
    Jan 7, 2012
    Yeah I could clip it like that but I have found that wires can work themselves out sideways eventually on clips and I might not realize it if it gets hidden after a few days of stuff on my desk.

    Only 1 button on the mat for this Elenco product. They had to save a few cents profit on another button. :-D
     
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