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Gound plane resistance calculator

U

Uwe Hercksen

Jan 1, 1970
0
Joerg said:
No, I want to do two mouse clicks and read a nice little ohms value :)
Hello,

if you want to get a nice and precise ohms value, you need to know not
only the exact shape of the ground plane and the location of the two
points, but you also should know the thickness of the ground plane.
Unfortunately, there are large tolerances for the copper thickness, a 35
µm layer may be anything between 45 and 30 µm, for galvanic copper even
between 60 and 25 µm. The thickness is even not constant over the whole
plane.

Bye
 
J

Joerg

Jan 1, 1970
0
Don said:
Um, won't the resistance always be the same?

Ohms per square and all.

Doesn't matter per square what.

Well, it ain't a perfect square :)

In fact, it isn't even rectangular.
 
J

Joerg

Jan 1, 1970
0
Uwe said:
Hello,

if you want to get a nice and precise ohms value, you need to know not
only the exact shape of the ground plane and the location of the two
points, but you also should know the thickness of the ground plane.
Unfortunately, there are large tolerances for the copper thickness, a 35
µm layer may be anything between 45 and 30 µm, for galvanic copper even
between 60 and 25 µm. The thickness is even not constant over the whole
plane.

Yes, sure, and I have that data. Well, sort of. I was pressing for that
information from the manufacturers and they didn't have it (!). Could
not believe it.
 
J

Joerg

Jan 1, 1970
0
Tim said:
I should refuse, on the grounds that I don't know what the heck I'm doing.

But I sent it anyway.

Thanks, Tim, that was very kind.
 
J

Joerg

Jan 1, 1970
0
Spehro said:
I like John's idea of using regular old copper clad and scaling it.
You can even solder to it. Put a few amps through it and the voltages
become quite reasonable. If you've ever searched for a short between
power planes on a multilayer board..

There'd be the chance of a wee problem with the TSA folks. "What's this
large copper thang?" ... "It's to shield from evil rays from outer
space" ;-)
 
J

Joerg

Jan 1, 1970
0
John said:
You could do it in Spice. Make a grid of resistors... lots of
resistors. Some clever cascaded cutting and pasting might work. Then
short out the resistors that represent contacts.

There used to be a thermal analysis program called SAUNA that worked
that way. Structures were modeled as grids of resistors, and a circuit
solver was sic'd on it.

Good idea. Just like I (so far) did all my mechanical drawings using my
schematic editor because for the life of me I can't get used to the
icons and hieropglyphs of my ME CAD.
 
R

Rich Grise

Jan 1, 1970
0
Joerg wrote:

<snip>

What's a "Gound plane?"

;-)
 
J

John S

Jan 1, 1970
0
He meant Gourd Plane. That's a cargo aircraft that's used to ship
vegetables.

John

No, he meant Gorund Plane. That's a plane that missed the runway and has
to go rund again.

John S
 
R

Rich Grise

Jan 1, 1970
0
Joerg said:
Folks,

Is there an online tool, calculator or cheat sheet that lets me
calculate the resistance between two arbitrarily chosen points in a
ground plane?

All I found was heavily math-laden scholarly articles and instructions
how to program it in Excel. Reason behind this is that I am dealing with
a ground plane right now that can't be thicker than a few micrometers.
Frankly, it sounds like a variation on this one:
http://www.mathpages.com/home/kmath668/kmath668.htm

Cheers!
Rich
 
C

Charlie E.

Jan 1, 1970
0
Can't you model it in pspice by modelling the ground plane as a grid
with resistors:


+--R--+--R--+--R--+
| | | |
R R R R
| | | |
+--R--+--R--+--R--+
| | | |
R R R R
| | | |
+--R--+--R--+--R--+
| | | |
R R R R
| | | |
+--R--+--R--+--R--+

Dunno how many nodes Pspice can cope with tough.

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
 
J

John S

Jan 1, 1970
0
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
 
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...

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).
 
J

Joerg

Jan 1, 1970
0
John said:
No, he meant Gorund Plane. That's a plane that missed the runway and has
to go rund again.

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

Joerg

Jan 1, 1970
0
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.


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".
 
R

Rich Grise

Jan 1, 1970
0
Joerg said:
No, it's a grind plane when it ground itself into the ground in the
potato field at the end of the runway :)
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
 
J

Joerg

Jan 1, 1970
0
Rich said:
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! :)


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.
 
J

Joerg

Jan 1, 1970
0
Jim said:
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.

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.
...Jim Thompson

[On the Road, in New York]


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

Joerg

Jan 1, 1970
0
Jim said:
The _infinite_ sheet is trivial to solve (in the limit :) for resistance
between two points. Bounded is impossible by hand, easy with a simulator ;-)

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

Spehro Pefhany

Jan 1, 1970
0

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.

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
 
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