Gound plane resistance calculator

What, you don't want to solve Poisson's equation for an arbitrarily shaped piece of copper with a point current source and a point current sink?

Oh Joerg. Tsk tsk.

No, I want to do two mouse clicks and read a nice little ohms value :-)

Come to think of it -- there is! Or at least there will be as soon as I post this:

The resistance between two arbitrarily chosen points on a ground plane is (drumroll) infinite!!

Because points have zero circumference, and you need some finite circumference (well, area) to make a contact. That's assuming classical physics, of course.

Wouldn't that resistance be infinite?

John

Yes, I realized that and supplied Joerg with an online calculator. It was easy -- didn't even require programming.

Reality is different :-)

Last time an alligator clip slipped off fell onto a ground plane there was a flash, a loud bang, some yelling and hollering, and the wire from that alligator lead had vaporized.

He could just physically model it with a sheet of copperclad, and scale the sheet resistance.

I think Sonnet Lite may do this. I downloaded it once and played with it, but the learning curve exceeded my immediate need. In other words, I couldn't get it to do anything.

It would be nice to have a sheet resistance calculator, one that let you define the sheet outline, force regions to be voltages, and measure the currents. It would be handy for thermal situations, too. It's algorithmically trivial, but the programming problem is mostly the graphical entry stuff. I guess you could use Paint, like ATLC does.

John

The resistance between two "points" will depend on the contact area and shape.

John

Teledeltos paper. ;)

Cheers

Phil Hobbs

AFAIK Sonnet is not the right tool for that. Its more for simulating strip-line designs. Off topic: Sonnet is a bit tedious to setup and it doesn't work wel for antennas. If I have the time I want to see how well the simulation matches the real world.

I've dinked with it for magnetics problems, but never verified results.

Ahh, but that was neither zero area, nor was it classical physics.

On second thought, I'm actually not sure what happens to the math when the contact area gets down to zero -- but if it does say infinite resistance, then we have a pretty good idea that the math is wrong.

The resistance is more or less constant, irrespective of where you contact on the ground plane. If you look at the effectiveness of RFI shielding coatings, they specify it in "ohms per square". Square what? It doesn't matter (inch, metre) - something to do with topology.

So it will probably depend only on the copper thickness. And as John Larkin said, the nature of your contact points. Sorry, I don't know what the actual value would be, but maybe you could measure it on an actual one ounce or two ounce copper clad board before it is etched?

Nemo

Make a Teledeltos paper model ;-) ...Jim Thompson

Ok then, lets assume a needle stuck into the metal. The local resistance is usually negligible.

Thanks, Tim, that one looks very interesting. Got to try that out.

That would be the best method, like what Robert Widlar would have done. Can you still buy that stuff?

As the contact area goes down, the resistance trends to infinite. So the exact contact area of the needle, and how hard you push it, matters a lot.

John

Thing is, the plane won't be square, possibly not even perfectly rectangular.

Can be done but iffy, has to be a four-point measurement because of the low ohms. Or a controlled current drive and then voltage drop measurement. But I'd have to cut a new board every time we change the shape of the little circuit during the idea phase. And I am down to three scroll saw blades :-)