On Sat, 11 May 2002, Olin Lathrop wrote:

>> The new question then is how to figure out the resistance
>> of a PCB trace.  From AWG/resistance tables for copper wire,
>> I can calculate the resistance of the copper layer with the
>> known thickness/width, but the solder coating adds an unknown
>> factor, as I haven't seen any PCB house specs on that.
>
>The board house should be able to spec the copper thickness, usually in
>"ounces", which can be translated to ohm/square after some figuring.
>However, they will usually spec only the minimum thickness, and I would
>worry about differences between boards and production batches.  This could
>be overcome by doing calibration at production time and storing values in
>EEPROM or whatever.  It's going to be tough to get a PCB trace to handle
>100A though.  I still think that measuring the drop accross the existing
>wire or inserting a calibrated wire is a better solution.

I had PCB traces handle 50A pulsing current in a SMPSU. They were less
than 1" wide and very short. I made sure the board gets havily tinned.

Also if my advice will be followed (soldering a piece of suitable
conductor on top of the shunt trace) then the resistance will depend more
on the soldered wire than on the trace and tin, on account of the cross
section ratios. The board only serves as support, the conduction is done
by the soldered wire shunt.

As Bob Ammerman has said there are problems with attaching connectors and
wires to the board at this current. Directly soldering a 0.3" stranded
cable onto a large pad where the extra wire also starts will work.

Just remember that these things run hot (shunts, wires, boards,
everything). Add to this ambient heat in a car and you get a very short
liived project. Use automotive rated parts (125C etc).

Peter

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