Electronics > Projects, Designs, and Technical Stuff
" precision " ballasting resistors.
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mrpackethead:

--- Quote from: T3sl4co1l on December 31, 2018, 05:42:44 pm ---
--- Quote from: rstofer on December 31, 2018, 04:53:14 pm ---In a batch of 1% resistors, all of the 0.1% resistors will be sorted out and sold for more money.  You will likely find a bimodal distribution with a gap where the 0.1% resistors should fall.

I wonder if it wouldn't be easier to use PCB traces given the low value desired.

https://www.allaboutcircuits.com/tools/trace-resistance-calculator/

Or, get close with resistors and tune with trace length.

--- End quote ---

Bimodal distributions were all the rage back in the carbon comp days.

Laser etched resistors (carbon film, metal film, foil..) take longer (= more expensive) to make more precise, there's no money in making billions of super-rough values and sorting them.  The value is measured once, while it's being trimmed, then it's coated and packaged. :)

Tim

--- End quote ---

In this design the exact value of the resistors is not critical,  what is important that the pair are as close as possible.

The Soulman:
Did you take in account the temperature stability of the resistors?
I'd use large resistors (normal tolerance) for minimum temperature rise and find a way of trimming after assembly.
Possibly with trimmer pot ( +couple resistors to define the range) or parallel mounted high value resistors chosen
strategically so that one or more can be clipped away after assembly and measuring.
Either way measuring and manual labor after assembly is mandatory.

Poor design?  :P
mrpackethead:

--- Quote from: The Soulman on December 31, 2018, 08:52:33 pm ---Did you take in account the temperature stability of the resistors?
I'd use large resistors (normal tolerance) for minimum temperature rise and find a way of trimming after assembly.
Possibly with trimmer pot ( +couple resistors to define the range) or parallel mounted high value resistors chosen
strategically so that one or more can be clipped away after assembly and measuring.
Either way measuring and manual labor after assembly is mandatory.

Poor design?  :P

--- End quote ---

Multiple resistors is not really an option.   Each side of the pair will be part of a differential signal path that is carrying a signal clocking at 125Mhz.   I'd hate to think what having 10's of resistors in parrallel/series might do to that signal path.      Provided the resistors in the pair behave in a similar way, having them shift in value a bit over temp is not super critical.

The idea of using PCB traces was innovative. :-) though not practical.
soldar:
At 125 MHz you want super low inductance resistors which rules out wire wound. I have seen carbon resistors drift in value over time so you could install two resistors matched to the max only to have them drift off over time. Make sure to take these two things into account.
mrpackethead:
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