Re: Copper Traces as Ballast Resistors

On Mon, 24 Nov 2008 12:04:56 -0600, Vladimir Vassilevsky
<antispam_bogus@xxxxxxxxxxx> wrote:



HarryD wrote:

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.

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.


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.

ftp://66.117.156.8/IR_0026.jpg

ftp://66.117.156.8/IR_0032.jpg

John





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Relevant Pages

  • Re: Copper Traces as Ballast Resistors
    ... His point was that the positive tempco of copper ... traces was beneficial in design over fixed resistors. ... a trace will bring out a flock of H.P. bird dogs. ...
    (sci.electronics.design)
  • Re: breadboarding fast, tiny stuff
    ... The drain is pulled up through a 47 ohm resistor, ... The TDR of the gate (lower trace) indicates that the gate ... The heat softens the epoxy and the copper comes off like a zipper. ...
    (sci.electronics.design)
  • Re: PCB Design
    ... Even with regards to trace width it depends on your allowable trace ... of plated copper compared to your total external trace ... calculator that is no longer offered by the company. ... What is the standard for track thickness for different currents? ...
    (sci.electronics.cad)
  • Re: Copper Traces as Ballast Resistors
    ... 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. ...
    (sci.electronics.design)
  • Re: Copper Traces as Ballast Resistors
    ... 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. ...
    (sci.electronics.design)