Sunday, March 1, 2015

Solving pressure and condensation build up in your Embedded System.

Have you ever had a problem with pressure build up or condensation in one of your embedded devices? Gore-Tex Vents are a good solution today, but I thought you might find The Rest Of The Story interesting. What follows is slightly edited version of a message exchange of mine from the gEDA-User mailing list.

> Dave McGuire wrote:
> > Bob Paddock wrote:
> > I'd put them in a sealed box, with a Gore-Tex Vent so that the
> > enclosure can 'breath' but not pass water.
> This is an interesting idea. Can Gore-Tex be found in small
> squares for this type of application, or would one be stuck
> destroying an expensive jacket to get some?

Salvation Army or Good Will would be a good place to look if you want to go the clothing route, but there is more to the story.

The whole story goes like this: Gore-Tex was invented ~1978, used in clothing as everyone knows, and didn't really find many other uses then. Jump forward to 1982. I was designing a hand held control to run some 50 Ton Coal Mining Equipment. The control was a sealed box with a membrane switch on the front. We very shortly ran into problems. Taking it down into the Mine would cause a pressure reduction that would suck the switches in, activating the switches. Having a stuck switch on a 50 Ton machine with sharp cutting bits is *bad*. Very **bad**. Also found that taking it up in a plane, as non-pressurized luggage, would deform the switch by causing it to balloon out to about four times what it should be. From flat to nice dome. You then had worthless junk, it did not recover.

I had recently read about the properties of Gore-Tex, tracked down one of the engineers in the factory and asked him if he thought it would make a good vent for such an application. He said he had no idea, but he would send me several different types of the stuff to try out. Which he did.

Putting cloth over a whole in mining equipment would last a few minutes, on on optimistic day. So a colleague of mine, Don F., came up with this labyrinth sandwich to put the Gore-Tex between.

Take two flat disks, we used thick fiberglass, each about 1/4" thick. Mill out a pocket in both disks, place them face to face, then drill a hole through both disks at one of the ends of the milled circle. Now rotate a single disk 180 degrees. Put the Gore-Tex between the disks and epoxy. If you try to stick your Sharp Pointy Coal Mining Implement into the hole you hit the fiberglass and not the Gore-Tex. The assembly was then held in the box by a screw in the four corners. Pressure problem was solved.

Went back to the fellow at Gore-Tex to get more material, which he supplied. Thought our idea was a good one and filed for the patent. So he got it and Don and I did not.

Today you can buy Gore-Tex pressure relief vent off-the-shelf. Most a screw in type plug, but they come in all kinds of sizes.

Q: Why does my FET, IGBT, SCR keep going up in smoke? A: Spot Heating

A common problem I see in message boards is that someone's circuit failed, and went up in smoke. People in the more esoteric realm's blame this on things like "Subtle Energy" overload and other such minutia. Here is the far more realistic explanation:

The very old "GE SCR Manual" goes into all of the Gorey details of what is happening inside the part, when the "Magick smoke comes out", as it is unlikely you have the Manual at hand, in a nut shell:

What lets the Magick Smoke out of IGBTS, FETS and SCRs in most cases is turn them on to slowly, causing 'Spot Heating' of the die.

Think of a FET as hundreds of thousands, possibly millions, of very small resistors all in parallel, where each one can be turned on and off individually. The 'resistors' closest to the gate turn on first, and as the gate potential spreads across the die the rest turn on. The ones farthest from the gate turn on last.

With a slow gate turn on, a few of the small resistors nearest the gate are trying to carry all of the load, which they can't do, so they burn up, but the device does not fail quite yet. The next time the device is turned on, which may be only milliseconds away depending on your switching frequency, or days away depending on the application, some more of the resistors further in burn up. When the point is reached that there is simply not enough of the 'resistors' left to carry the load is when the Magick Smoke escapes, and the part dies a catastrophic death.

This is why the parts generally run "for a while" before failing. If it fails as soon as you fire it up the first time, you either had a catastrophic short in the load, possibly shorted caps that take a bit of time to 'wake up' before they hold a charge, generally fixed with 'Soft Start', or the gate drive really sucked big time.

There needs to a be a few *amps* of current pumped in the gate of the larger parts, for short periods of time, to get the gate potential to spread across the entire die as fast as possible.

You also want to get the thing turned off as fast as possible.

If you are not familiar with the concept of Magick Smoke, this is where all electronic parts run on Magick Smoke, because once the smoke comes out of the part, it no longer runs...