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" precision " ballasting resistors.
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mrpackethead:
Im considering a design idea at the moment, which requires a pair of closely matched resistors.   The pair could fall in the range 1.45 - 1.55 Ohms, will need to be rated at 0.5W,  Ideally there will be less then 0.1% difference in them.

I've had a hunt for such resistors and while they do exisit on a datasheet they have very long lead times, and are basically product unobtainable.

If i bought a reel of 1% parts, would it be insanity to consider measuring them, sorting them into 'range' bins, and then getting closer pairs?      They would still need to be placed on the pcb as pairs, and that is quite a lot of work, since they wont' be on a tape any longer ( though they coudl be loaded into custom trays and picked up, its still quite a lot of work.   And yes, i potentially have to place thousands and thousands of them.
Miyuki:
If you buy reel they will be very similar
Basically in same batch they are almost same, but wary batch to batch most times
T3sl4co1l:
You can get 0.1%'s (and .2 and .5) in larger values, and connect them in parallel.

Since you're doing that anyway, it seems the more effective (better guaranteed) method? :)

The metrologists call it a "statistical array" -- if the resistors are independent variables, then resistors in series add arithmetically while their errors add vectorially (i.e., RMS).  The expected tolerance of the resulting chain is T / sqrt(N), so you'd need 100 resistors to get the next-decade improvement from 1% resistors.

Obviously, such a strategy is susceptible to systematic errors, like if reels/batches have a bias.

The relationship for resistors in parallel is slightly different, because smaller values dominate.  This is obvious when you have 1 ohm in parallel with 100, but less pronounced when you have 1.01 in parallel with 0.99.  You should get comparable results this way, in any case (i.e., a hundred 100R 1%'s in parallel should be close to 1R 0.1%).

Tim
mrpackethead:
100 Resistors in parrallel might be a bit problematic for other more pracgtical reasons.
soldar:

--- Quote from: mrpackethead on December 31, 2018, 11:54:42 am --- If i bought a reel of 1% parts, would it be insanity to consider measuring them, sorting them into 'range' bins, and then getting closer pairs?
--- End quote ---
I have done this in the past. The question is how many you need to sort and whether it is time/cost efficient for you.
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