[sdiy] [TekScopes] Re: OT: Where to get cheap ESD safe electronics parts drawers?
cheater00 at gmail.com
Mon Mar 13 06:52:00 CET 2017
thanks for your email. Your questions are interesting. Let me try to
clear up some confusion.
On Sun, Mar 12, 2017 at 9:40 PM, Bruce Griffiths
bruce.griffiths at xtra.co.nz [TekScopes] <TekScopes at yahoogroups.com>
> A snake oil vendor strikes again.
> Rewriting the laws of physics to suit his own ends.
> Earthed metal surfaces don't accumulate charge.
No one said that. But, hey, now that you mention it - actually they
do. What does "earthed" mean? Well, it means that the piece of
conductor is at a similar potential as the earthing rod you ultimately
connect to. That's all it means. This potential could be lower,
higher, or the same as the potential your electronic part was
connected to before. So what happens when you make a circuit between
both, e.g. by moving charge (physically, stored on the part) from one
of those potentials to another? Current happens. And because this
current is not limited, it can create a lot of power dissipation on
the part, and damage it.
But in what you're referring to, it doesn't matter whether the
conductor is earthed or not. What matters is that it is conductive.
That is: if you stuff your part into a sheet of aluminium foil, it
will act as a conductor *between the pins of the part*. That is all
that is necessary. Charge across a semiconductor junction will
dissipate through that newly created circuit and it will create
current which is not limited and that will create excessive heating
and damage the part. That's why it's important that whatever circuit
is created contains a large resistance in it, that is, the material
you stuff your part into must be *dissipative*, and not *conductive*.
This is science. It's basic electronics that you should know if you
frequent this forum... there's no snake oil.
> By the snake oil salesman's reasoning one shouldn't use metal enclosures for electronics due to ESD issues.
> Do you really want to use an ESD treated plastic to shield sensitive electronics from ESD?
> What about metal traces and planes in PCB's, should these be replaced with ESD safe plastic as well?
> As an extreme example would you rather be inside a substantial earthed metal Faraday cage or inside a large ESD safe plastic box during a severe lightning storm?
The Faraday cage has a different role to dissipative, ESD-safe material.
A Faraday cage's purpose is to divert existing current in a lightning
strike through a conductor which means it will not reach anything on
the other side of the cage. So the idea is "lightning carrying strong
current happens and we need to prevent it from damaging stuff".
The point of ESD-safe dissipative material is to prevent strong
current from happening *in the first place*. In your situation, air
between the clouds and the ground is insulative, meaning it's an
insulator, so it allows no current, and the charge can build up. At
some point the air ionizes and ionized air is conductive, allowing
very strong current. It goes from one extreme to the other. However,
if there were at all times a connection between the clouds and the
earth, which were dissipative, like ESD-safe material, charge could
not build up, and lightning strikes *would not happen at all*. That's
how in this case ESD-safe material would prevent lightning damage:
prevent lightning in the first place. Obviously you will be quick to
point out that you could instead connect the clouds and earth with a
conductor, and that would be fine as well. But in our case, a part
could have charge built up, and I talk about this in the previous
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