Bench design (static control question)  - Page 1
Forum section: electronics / projects

author: Cliff Matthews
author_rank: Unknown
timestamp: December 02, 2015, 01:17:02 am
content: I have two 6-ft all-wood benches I resurfaced recently and joined with 8-ft plywood sheets. I installed 15 feet of continuous bare metal as a rigid front corner and to control static, but it's not grounded via resistor(s?) yet. Wikipedia info lists ground mats use typical resistances on the order of 10e5 to 10e8 ohms, but that's quite a spread from the normal 10e6.Since this is very conductive, and not a mat, I want to ground just the middle to a water-pipe via resistor, what value is safe for this?

author: KL27x
author_rank: Super Contributor
timestamp: December 02, 2015, 05:25:52 am
content: IMO, you made more problems with the metal. Wood naturally dissipates static, and it can't hold much charge to begin wtih. Large pieces of metal can donate/accept a fairly large amount of charge in a hurry. So even if it's at ground, this means your parts have to be safely brought to ground before coming into contact with the metal strip. As far as a tiny wafer of silicon is concerned, touching the ground pipe directly and touching a large piece of metal is going to not be too much difference. Antistatic mats are for metal, plastic and maybe glass surfaces, IMO. Over wood, my opinion is they're as useful as a tinfoil hat. Adding a strip of metal, now maybe you should put an antistatic mat over the strip.It's not the voltage, alone, that kills parts. There has to be significant current, tooo. That's part of the reason why FETs are the most common thing to go... gate capacitance can sustain the spark on it's end. A big piece of metal can sustain it on the other end. Not a piece of wood. It takes two hands to clap. The component lead might be highly charged. Every molecule in the wood might be highly countercharged. But the resistance of the wood is so high, the net practical effect is about nothing. It takes an object with a high capacitance and high current/low-effective-resistance (like metal), or a surface that generates charge by friction and has very low conductance (like some "static-prone" plastics and synthetic furs/fibers, which I suspect are allowed their properties due to a low ionization threshold on their surface, but very high resistance everywhere else) to provide a significant ESD hazard. And the plastic type of surfaces probably don't do the actual zapping; I would guess they are more of a hazard as far as the potential for charging the component high enough to provide the zap, in the first place. The component still needs a donor/acceptor capable of unleashing the beast.Wood can do neither. It can't effectively charge the component. And it can't turn the other key that launches the electrical storm. Wood is nature's antistatic mat, and it works whether you ground it or not.Unless you place your received components on a grounded mat before you handle them, and wear a ground strap whenever you handle them, and also store your components in a grounded environment, purposefully grounding your wood bench (or more importantly, the metal strip that's on it), is as likely to do harm as any good. If all the above IS the case, and especially if you routinely transfer components/boards from one bench to another, only then is grounding your bench(es) going to be positively beneficial in any way.

author: rdl
author_rank: Super Contributor
timestamp: December 02, 2015, 07:51:10 pm
content: Seems like if you just do the usual thing and ground the metal bar through a 1 meg resistor, then everything should be okay.

author: Cliff Matthews
author_rank: Unknown
timestamp: December 02, 2015, 08:00:24 pm
content: @KL27X - Thanks for all your insights (Sorry for not commenting sooner, but I got tied-up with insurance adjusters who want to right-off my van (some dude tore-of my front bumper on Black Friday.. hope you don't have that mad shopping non-sense where you live..)The benches were originally topped with 2x6 heavy duty lumber, but I kept loosing screws and parts down the slits. The new top is just 1/4" fir plywood and without any durable front edge, it would get torn-up pretty quick. As you can see, I screwed it all down and duct-taped all but the front ridge - if I take it off, it'll look bad and splinter everywhere. For now, I'll just ground it with resistance and fold more duct-tape over it so it won't conduct. If it wears off, I don't worry since Canada is the duct-tape capital of the world.. Beauty eh?

author: sarepairman2
author_rank: Frequent Contributor
timestamp: December 02, 2015, 11:27:32 pm
content: lol that looks mad ghetto, like something in my work place. At least you don't have cardboard...What I did with my home bench is use a heavily stained piece of plywood that is covered with 2-3 coats of floor clear polyurethane. It looks great so long you don't spill acetone on it, so long you sand it well.I used vertical 2by4's as joists. I recommend a depth of 4.5 feet if you plan on getting any larger equipment (i.e. microwave sweepers, gigantic dc power supplies, etc). On top of this I got the heavy duty RUBBER esd mating material. I would also recommend a secondary small ESD mat for doing soldering on. Don't clean stuff on the electronics workbench because it will get fouled up, I have separate areas for various operations.My 3x8 bench is getting small, although I built custom wood shelving. and I have a ridiculous amount of test equipment ready to go (my less used stuff like thermal meters, pulse generators, various amplifiers, counters,etc sits unconnected line up like books on a book shelf. My main bench has an area above a spectrum analyzer cleared with easy to access extension cords, so if I want to do something I can bring down up to four other pieces of reasonable sized equipment and fit it into my "wall". Having this ability to be modular is useful when you start doing very specific experiments, so you can choose the gear appropriate for your experiment.and please don't be that guy with cardboard boxes in your work area!!! just spend the extra dollars on a decent plastic tub and throw the cardboard out, it just looks so cheap! women like nice boxes btw

author: KL27x
author_rank: Super Contributor
timestamp: December 03, 2015, 12:30:21 am
content: QuoteMy plywood bench is finished with linseed oil for that reason.(It looks like crap, even if you don't spill acetone on it.)QuoteWell, I'm mostly talking out of my rear. So take that post for what it's worth.I wouldn't bother putting duct tape over it. A sheet of steel on the wood like that, isn't going to do anything to hurt - as long as it's not directly grounded to earth. I was more making the point that it wasn't going to expressly help, IMO.Again, take that for what it's worth.I have never killed a component, and I don't take any particular anti-static measures, other than storing FETS in original containers and/or other antistatic materials.*** I was thinking on this some more, and my first post is probably all kinds of wrong. I don't think steel/copper/alum can even be an acceptor/donor, at all. Only the surface of a nonconductive object can generate any significant charge. It would take such an object, like the aforementioned sythetics/furs to accept/donate the electrons. The metal is only dangerous because it can transfer the surface charge of any object that is in contact with it, such as if you are leaning against the steel wearing a fuzzy sweater. I forgot too many things I used to know, but this sounds a lot more correct!

author: sarepairman2
author_rank: Frequent Contributor
timestamp: December 03, 2015, 09:17:22 am
content: if you accidentally short something to a metal table...

author: Cliff Matthews
author_rank: Unknown
timestamp: December 03, 2015, 12:28:30 pm
content: Quote from: sarepairman2 on December 03, 2015, 09:17:22 amYeah, I had a dream about working on a hot TV chassis the other night..BTW, are there any cheap sources for that heavy duty RUBBER esd mating you might suggest? The only real mat I have got damaged, so I cut it down to 12" x 18"

author: Marco
author_rank: Super Contributor
timestamp: December 03, 2015, 02:35:12 pm
content: Quote from: sarepairman2 on December 02, 2015, 11:27:32 pmCould use two component system as well if you want to be able to spill acetone, decided not to myself though because I didn't want to buy 50$ worth of lacquer. I'll try to just not spill acetone instead.

author: Cliff Matthews
author_rank: Unknown
timestamp: December 04, 2015, 01:10:00 am
content: @KL27x - I don't wear fuzzy sweaters (at least I haven't since those itchy Travolta turtlenecks died-out long ago), so I guess I'm safeMy biggest concern now, is buying another van (my wife wants a car so I won't haul surplus electronics), but I drive, so I get to choose.