### Author Topic: EEVblog #732 - PCB Sheet Resistance  (Read 12850 times)

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#### RupertGo

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##### Re: EEVblog #732 - PCB Sheet Resistance
« Reply #25 on: April 13, 2015, 10:35:58 pm »
Reminds me of an exam question - take a 1 metre diameter circular piece of a material of 1 ohm/cm2. What is the resistance across two point contacts placed on its surface?

It's one of those questions. No points for trying to hit it head on with mathematics. some points for noticing that there doesn't seem to be enough information in the question for an answer - although you might get unsure if you remember that resistance across a square of a conductor is invariant wrt size.

You win if you spot that theoretical points have no area, so will have infinite resistance no matter what you do with them. (You can also get there with the maths, but it will hurt and you will cry.)

#### firepower

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##### Re: EEVblog #732 - PCB Sheet Resistance
« Reply #26 on: April 14, 2015, 01:29:15 am »
Do larger sheets handle more wattage? 1/4 of a larger sheet 1/4 the watts?

#### rs20

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##### Re: EEVblog #732 - PCB Sheet Resistance
« Reply #27 on: April 14, 2015, 01:46:58 am »
Do larger sheets handle more wattage? 1/4 of a larger sheet 1/4 the watts?

Basically, yes, exactly. More pedantically, if sheet B is twice as wide and twice as long as sheet A, sheet B will have almost four times the current handling capacity of sheet A (for a given temperature rise). Why not exactly four times? Because the edge of the sheet has slightly better ventilation that the center areas, which are surrounded by hot metal on all sides. The larger sheet has a smaller perimeter-to-area ration, so not quite as good at rejecting heat per unit area.

Smf