Russell McMahon wrote:
> I want to be able to make my own custom valued current sense resistors 
> in lowish volumes - 1's to perhaps 100's.
> Requirements are
> 
> - 0.02 ohm to 1 ohm range
> - Temperature stable
> - Up to 1 Watt dissipation - usually far less.
> - Non inductive within reason.
> - Reasonably compact.
> - Easy and cheap to manufacture.
> 
> If nichrome wire was easily soldered then winding nichrome or other 
> resistance wire on a high value resistor is an obvious solution. 
> Nichrome and similar is cheap and readily available and has near-zero 
> temperature coefficient of resistance.
> 
> However, resistance wire types that I am aware of tend to dislike or 
> hate being soldered. Other termination means are usually necessary 
> including welding, spot welding, brazing or crimping. I'll consider 
> all such ideas but thought that someone may have a bright idea on how 
> to achieve this. Crimp contacts seem the next easiest solution but 
> this is not a 'nice' solution.
> 
> I do not want to employ existing commercial resistors, at least at 
> this stage as
> 
> - Values are custom and may need to be adjusted during prototyping.
> 
> - Suitable wattage, temperature invariant sub 1 ohm resistors tend to 
> be specialist and expensive compared to other costs in this 
> application.
> 
> - Temperature invariant resistors are relatively rare.
> 
> - I can control inductance etc as required.
> 
> These are to be current sense resistors handling currents of up to 5 
> amps. Resistor value will vary with current.
> 
> 
> Thoughts ?

Why not just measure the temperature of the resistor and compensate
for any induced resistance change? Presumably it's calibratable and
reproducible over the temp range of operation. Or do you need
the instantaneous resistance to be invariant?

What about using PCB traces as resistors? Air cooled if necessary?
Makes adjustment easy by short circuiting the serpentine.

As for 'adjusting', why not use standard low ohm resistances and
series/parallel them as needed to come with a 'custom' value. 1,2,4,8 sequence
is easy to use in series configurations.
For an existing resistor, it's easy enough to do the math to figure out
what new parallel value(s) are needed to get you to a now lower value by
shunting.

R
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