Ron D'Eau Claire wrote: >Greg K2UM wrote: >One touch and no static sparks to my equipment during operation, especially >during the winter, and a whole desk mat to work on my K3s and other >equipment. > >------------------------------------- > >If you want to discharge the static with a "touch" you need to touch a real, >bare-metal ground. Those mats do not provide a "ground" but rather merely >dissipate static charges as they accumulate.
Sorry, that isn't correct. ESD protection is NOT simply about connecting things to "ground". Grounding has some part to play, but it's missing the main point. Remember, it isn't the static charge that does the damage - it's the DIScharge! A sudden direct discharge creates a large pulse of current which can be very damaging, and can also induce damaging voltages and currents in surprisingly distant locations. That is why ESD mats are deliberately designed to PREVENT a sudden discharge to ground! By having a large built-in resistance they force static charges to dissipate quite slowly, over timescales of seconds. >Sure, if you hold your hand on >one and don't move for several minutes, you should be discharged too, but a >"touch" won't do it as evidenced by a simple check with an ohmmeter. Almost >no DMM will indicate anything but an "open circuit" when the probes are both >touched to the mat. > That much is correct - Greg is being optimistic that literally "one touch" to the ESD mat will be enough to remove the static charge on his whole body. That's what the wrist strap is for. >Optionally you can discharge yourself with a wrist strap that has a >metal-to-you contact through a 1 Megohm resistor to ground. The 1 Megohm is >adequate to rapidly discharge your body while limiting any accidental >currents from touching circuit to a safe level. The main advantage of the >wrist strap is that you don't need to remember to keep touching a ground. >It'll take care of any static charge you accumulate as it builds, and your >mat will keep dissipating any static charges on the equipment you lay on it. > The wrist strap is not "optional". Your own body is the largest charge carrier around, and it needs to be kept at the same potential as the equipment sitting on the ESD mat. When we're wearing a wrist strap, it is working for us all the time - not just when we remember to touch something. Also the 1M resistors are not optional - they allow static charges to be dissipated over timescales of a few seconds, but 1M is high enough to avoid the risk of electric shock from the equipment we're working on. NEVER ground yourself with a strap that can't be pulled off and doesn't contain a safety resistor. Those are the reasons why all ESD mats and wrist straps are made precisely the way they are (including the pop studs) and not any other way. > >When I sit down at the bench to work, I first touch a bare metal ground, don >the wrist strap and set to work with the parts and boards on the static >dissipating mat. > The main point is that the ESD mat and the wrist strap are bonded together, so that everything in your ESD-safe zone is at the same potential. Having done that, then you can start to think about "grounding". To make sure that the any external connections you bring into the ESD-safe zone, such as the soldering iron and ground-connected test probes, are also at the same potential, the whole setup needs to be bonded to the electrical supply ground. That is: wrist strap (including 1M resistor) connected to the stud at the corner of the ESD mat. That stud is then directly connected to the mains ground bus for the whole workbench. So the principles are - in the following order: 1. Keep everything in your ESD-safe zone at the same potential. 2. Use a purpose-made ESD mat and ESD wrist strap (with the 1M resistor) to make sure that any static charges dissipate GENTLY. 3. Having done 1 and 2, also bond your ESD-safe zone to the electrical ground. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
