Copper Traces as Ballast Resistors

Tom Bruhns had an interesting article published in the 11.17.08 edition of Electronic Design. His point was that the positive tempco of copper traces was beneficial in design over fixed resistors. Not trying to reduce his fine article but it is interesting to note that the 10mV drop that he depicted is approaching the limit in trace voltage drop before heating of the trace's center hotspot will be greater than 5 degrees centigrade and the PC boys will do their Henny Penny renditions. Copper traces fail at 1000C and we allow resistors to reach 150C but 5C rise in a trace will bring out a flock of H.P. bird dogs. Most designers of PC traces, less than 2 inches, worry about max current but max voltage drop is more indicative of trace heating. Keeping all PC traces less than 12mV guarantees all trace hotspots will have less than a 5C rise. Cheers, Harry

Reply to
HarryD
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Once I did a mistake of using a PCB trace as a resistor for overcurrent protection sensing. The problem is that the copper thickness can vary dramatically from board to board. The copper weight spec guarantees that there is no less copper then X ounces, but the upper limit is very inexact.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

I sometimes measure trace resistances, and we seldom get actual "1 oz" (about 525 uohms per square, I think) copper even though we specify it. Numbers like 700 uohms are not unusual.

The 12 mV number is a neat ROT. The 5 deg C limit seems a bit conservative, though. Lately, it seems like the parts are heating the traces; the thermal images of our boards are mostly glowing opamps and dacs and voltage regs and such.

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John

Reply to
John Larkin

Yes, and it can also vary within the board. I've got some horror pictures about circuit trace burnouts here but not at liberty to share. Suffice it to say that even though the burn-outs didn't happen here but days before the boards to be investigated arrived it still took me on average a whole day to vent the stench out of the lab. It was bad.

--
Regards, Joerg

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Reply to
Joerg

It's the temperature limit of the base material that is of concern, regardless of the heat source, as normal operating limits for commercial printed wiring are listed in the pertinent safety docs.

Relating hotspot rise to mV trace drops is asking for trouble. Stick to I^2.R and cm^2, like the good book says.

As the same surface area is required to dissipate the same power for the same rise, regardless of the source, real estate advantages are only achieved if unpopulatable board surfaces are employed in this fashion.

Process variations in copper thickness may not be relevant if limiting is only intended under fault conditions, where more extended temperatures are permitted. You'd have to be aware of the circuit's behaviour under the continuous worst tolerance condition and be satisfied with the results before adopting the component cost reduction method.

Henny Penny? These things are not always optional - look in the book, if in doubt.

RL

Reply to
legg

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