Gound plane resistance calculator

This crops up in laser trim. One way to do the calculation is with spice. You create a mesh of elements by using a program to generate the spice netlist. Then you connect where appropriate.

You make the element with resistors set up as a square. That is, top, right, bottom, left connected in series, with the edges being point in the subcircuit. Then you build the array and tap.

For laser trim, it's a bit more complicated. Generally you connect hard to the end of the array and then eliminate elements in the array to do the trim.

Frankly, it sounds like a variation on this one:

Cheers! Rich

I can tell you that back in '98 a certain defence contractor was using PSpice to model temperatures along a wing surface. Used a netlist that was all LRCs between about 10K+ nodes...

Charlie

Spice can do decent thermal simulations, given the limit that thermal systems are distributed/diffusive and electrical systems are usually lumped elements; hence the huge resistor/capacitor grids.

The only real diffusive Spice element that I know of is the lossy transmission line model.

John

And, isn't that one-dimensional?

John S

About the same as a chip ;-) ...Jim Thompson

That's how IBM heat transfer people modeled mainframes, as far back as the early '70s. ...except it wasn't Spice. ;-) IC/package power distribution is still done the same way (well it was as recently as '06, when I retired).

No, it's a grind plane when it ground itself into the ground in the potato field at the end of the runway :-)

In laser trim we never did that calc. We did meander cuts with empirical determination of how fast and how far, or shadow cuts. After one empirical hand-programming session we cut it loose and it produced zillions of hybrids until someone yelled "enough".

Yup. And the effect is the same as with some professors at our university. A seemingly simple problem grows like a mushroom and at the end of the session you have a humongous formula that has little practical usefulness :-)

Trimming thick film resistors, you cut across the resistor until you get close, then cut along the length for fine trim... easily automated... did it at Dickson Electronics, 1970-73. ...Jim Thompson

The _infinite_ sheet is trivial to solve (in the limit :-) for resistance between two points. Bounded is impossible by hand, easy with a simulator ;-) ...Jim Thompson

A few years ago, one of the bargain airlines had a nose gear malfunction, so decided to burn off a few thousand pounds of fuel and do an emergency landing at LAX. The nose gear had somehow got turned 90 degrees, so the wheels wouldn't roll. They carried the whole thing on live TeeVee, and that nose gear strut made a WHOLE BUNCH of sparks, but nobody was hurt, and the only damage to the plane was a ground-down nose strut.

But it was WAY FUN to watch the drama on live TeeVee! :-)

Cheers! Rich

A friend had such a white-knuckle landing happen on a huge military transport. He was the pilot.

JetBlue is not a bargain airline. It's a very fine airline and I like flying with them.

There's many ways to do that. Meander cut, shadow cut, L-cut. We usually didn't do L-cuts if avoidable because every second of laser time counted. At tens of thousands of hybrids per year, 50+ resistors per, and hard cost limits sometimes you have no choice. Luckily I always had someone from the factory who knew how to minimize laser time. It's almost like Fedex route planning.

Nowadays I guess direction change is easier but hardly anyone does hybrids anymore. I can't even remember when I did the last one, definitely 20+ years by now.

Still? What are you doing there? Trying to unseat the state governor? :-)

Everything in life is bounded. Some of the bounds are found out the hard way :-)

JetBlue is indeed classed as a low-cost or "no frills" carrier. As is Southwest. A bit ironic these days, particularly for those travelling in coach class.

"If you travel as much as we do, you appreciate how much more comfortable aircraft have become. Unless you travel in something called economy class, which sounds ghastly."

-- Prince Philip

Best regards, Spehro Pefhany

Scooping up $$$$$ ;-)

Actually a lot of our thick film trims were abrasive jet. ...Jim Thompson

Hope you like paying the NYS income tax! ;-)