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Re: Unreliable Hardware

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  • thinkpast
    ... I d have to inspect the physical geometry, but it sure SOUNDS like you ve created a capacitor and are charging it up with the towel! Charges build up on
    Message 1 of 11 , Jun 2, 2010
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      > --- "Agustus Finknottle IV \"Gussy\"" <john_crane_59@> wrote:
      >
      >> Good point. I'll start replacing with CMOS B. No cats or
      >> sunlight, but it does have the nice metal case. This
      >> comes with a clear plexiglas box that's a dust cover
      >> for the expansion cards. And I do keep a towel over
      >> the unit to keep dust off when it's in storage.

      >> I'm wondering now if the towel moving across the
      >> plexiglas may have generated some voltage.
      >> Maybe I should store everything disassembled and
      >> wrapped in foil?

      "awasson2001" <awasson@...> wrote:
      >
      > Hmmm,... I'm not so sure if cloth and plexi are such a
      > good combination around static sensitive chips. It
      > could make a very effective Van de Graaff static
      > electricity generator.
      >
      > Andrew

      I'd have to inspect the physical geometry, but it sure SOUNDS like you've created a capacitor and are charging it up with the towel! Charges build up on the inside of the plexiglass, and "induce" a charge on the PC board underneath by electrostatic attraction. If the plex is charged "positive" underneath, it would attract "negative" charge from the circuit board.

      The charge has to find a discharge path, and it may be through your chips to some kind of "ground" path if the board and its power supply is not plugged in. If there are cables attached, it may discharge through the cables to some point of attachment. Or discharge at the sharpest corner or wire or point into the air. Sharp objects discharge quickly, flat surfaces slowly, has to do with electric fields and such.

      Static electricity is a funny world - it's not about "current flow" but about charges moving around, very high voltages but almost no "current". CMOS is also funny stuff, especially the early chips which were pretty sensitive to it, before they added feature to limit static discharge. (I'm an old Electrical Engineer so I remember this stuff from when it first was done. CMOS that is, not static electricity!)

      If you want to experiment, create a similar physical layout, but put a PC board in place of the cards. Most any board, populated or not will do - just be sure you never want to use it again if it has components.

      You'll need a neon lamp of some sort, from Radio Shack or salvaged from a 110 Volt "self illuminated" switch, etc. etc. (An LED needs too much current.) Attach it to the PC board on it's "ground" or "power" lines (or just the blank board copper) and ground the lamp through some resistor of a few hundred Kohms (or none it just limits current). A "ground" would be, oh, say any convenient pipe or metal object, chair or table, etc. I would not suggest a wall socket ground, don't want to create something at risk.

      Then stroke the plastic in darkness with the towel, and watch the neon lamp!

      Clever experiments aside, I imagine you could cover the plexiglass with some translucent "antistatic" sheet cut from an antistatic bag, and attach a clip to that bag to the chassis the "window" is mounted on. I'm not sure if you can put the sheet on the INSIDE surface and accomplish the same thing.

      If your plexiglass is an open box acting as cover, you may try replacing it with a wood cover, or a metal cover, or just cover the unit with an antistatic bag (open or closed) and see if the problem goes away. If transparency is important, you might make a wood or metal cover with a "window" of plex or antistatic sheet.

      I probably don't know for sure how your cards are boxed, but I hope from the descriptions above, you have some ideas as to what to consider.

      Herb Johnson
      retrotechnology.com
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