Fine resistance adjustment for standard resistors - Page 1
Forum section: electronics / general

author: zlymex
author_rank: Frequent Contributor
timestamp: March 20, 2016, 12:29:47 pm
content: There are circumstances where very accurate resistance are necessary as in the case of standard resistors and step up from 7V to 10V circuits.The main resistor is often of fixed in value, adjustment would be impossible or degrade the performance significantly. Therefore, some peripherals will be added for the adjustment.The adjustment may be performed with trimmers or without, let's look at them in turn.Part one, adjustment with trimmers.Trimmer adjustment often used in places where re-adjustment or regular adjustments are required by the user. Trimmer range is the total relative change in the resistor when trimmer adjust from 0 to the max.. Trimmer range multiply the accuracy should be less than the uncertainty required. For instance, if the trimmer range is 100ppm, and your trimmer is 0.5%, the overall resistor will add 0.5ppm uncertainty because of the trimmer. Therefore, you need either to decrease the trimmer range or increase the trimmer accuracy to get a better result.1. Series connectionThis is the simplest way, but the performance is questionable, because the variation of contact resistance has direct impact on the final value.Example, Fluke 887A differential voltmeter, trimmer range is 400ppm.2. The common wayThis is done by adding two fixed resistors from above circuit and is common used in metrology instruments.The added resistor in series with the main resistor decreases the trimmer range considerably and range is easily changeable . Also, the added resistor in series with the trimmer will decrease the none-linearity of the adjustment.Example one: esi RS925D, this is the precision 4-wire resistor box used in esi 242D bridge, the trimmer range is 100ppm.Example two: Fluke 720A, 0.1ppm KVD, the trimmer range is 43ppmExample three: IET SRX-10k standard resistor, the trimmer range is 66ppm3. The parallel way.Useful when the main resistor is small in value. The trimmer and R2 is parallel connected to the main resistor Rm, which should be made a bit larger than nominal value. The trimmer in this example has 48ppm trim range.Part two, Trimmerless AdjustmentThis kind of adjustment was often performed in the factory when they made, thus not mean user adjustable.When properly designed, this configuration provide the best, as if the adjustment add very little side effect.1. Custom made small value WWA simple, effective, widely used method.The main resistor, when made, should be close to but less than the nominal value.For instance, the main resistor made as 9999.78 Ohm, need a 0.22 Ohm in series to reach the perfect 10kThis 0.22 Ohm, only 22ppm of the total value, a normal WW will be sufficient.The downside, of course, it has to be custom made.Example, SR104, the best 10k ever produced.2. PairingParallel or series connection of two resistors of opposite deviation will cancel the resulting deviation.For instance, a resistor of 4999.3 Ohm in series with 5000.7 Ohm makes 10000 Ohm exact.A resistor of 19998 Ohm parallel with 20002 Ohm gives 9999.9999 Ohm, this in effect is 10k with only 0.01ppm difference.Downside: need a large number of resistors to select from.Example: Guildline 9250-1k Hamon transfer standard.There are 10 arms and each arm consist of four 1k hermetic foils connected in mixed mode. They must be paired for tempco and also for tolerance.3. StatisticalThis means a lot resistors of the same value put together in series or in parallel or mixed.The apparent benefit for doing so is to achieve large power/thermal mass, to achieve low value(when in parallel), and to achieve high value(when in series).This also create a better opportunity for easy resistor adjustment.Example one, Fluke 742A-1, twenty 20.01 Ohm in parallel to get 1.005 Ohm, and parallel another resistor of around 2000 to achieve the final 1 OhmExample two, Fluke 752A, nine 120k and one 119.8k in series, and series connect another small WW of about 200 Ohm to achieve the final 1200k resistor.4. BinaryBinary number such as 0.110100110101..., although long, may represent any figure to any precision.There is a series of resistors in binary value, pre-made, already soldered onto the PCB to be cut open, or soldered to as required.Example one: Fluke 732AExample two: Fluke 732B,They use switches instead of cut/soldering, this is useful for re-adjust but prone for errors because of the contact resistance variations(although very small) and mechanical failure(although very little chance).One thing worth noticing when design is that the errors of the binary resistors mus be considered. Otherwise there exist dead zone(some value cannot be achieved). In order to cover the entire adjustment area with margins, the resistor series should be designed not in the order of 2 for neighbor, but 1.9 instead. To be specific, the resistor series should not be like 10k, 20k, 40k, 80k, 160k, 320k. Rather, it should be 10k, 19k, 36k, 68k, 129k, 245k.Similar way is adopted when they adjust foils resistors at the production line.5. GrindingThis is the photo of one of the twelve resistors in an esi SR1010-1k Hamon transfer standard. I'm speechless.

author: manganin
author_rank: Regular Contributor
timestamp: March 20, 2016, 01:04:41 pm
content: Nice summary.Quote from: zlymex on March 20, 2016, 12:29:47 pmNeeds a steady hand for sure, but not as difficult as you might think of.

author: BravoV
author_rank: Super Contributor
timestamp: March 20, 2016, 01:16:40 pm
content: Subbed n bookmarked. Thank you !

author: SeanB
author_rank: Super Contributor
timestamp: March 20, 2016, 01:17:59 pm
content: Exactly as done on a ceramic substrate to trim thick or thin film resistors to a closer value. Can be done by hand, but you probably would only do that for a prototype hybrid during development to get the values right.From the look of that wire wound resistor that was done using a small piece of 1200 grit wet and dry paper wrapped around a bamboo skewer, done by a little Japanese lady sitting at a table with the resistor and a precision bridge, adjusting till it balanced, and wiping with a tiny brush after each stroke to get the dust away. She probably had an output of 100 resistors a day at highest precision. When done she probably had another brush to apply a sealing coat of varnish, and a small stamp with her name on it applied to the rear of the resistor.

author: quarks
author_rank: Frequent Contributor
timestamp: March 20, 2016, 02:07:40 pm
content: Very good summary

author: TiN
author_rank: Super Contributor
timestamp: March 20, 2016, 02:30:25 pm
content: Another jewel fromzlymex

author: Vgkid
author_rank: Super Contributor
timestamp: March 20, 2016, 03:26:16 pm
content: Thanks for the information.

author: zlymex
author_rank: Frequent Contributor
timestamp: March 20, 2016, 03:28:08 pm
content: Thanks everyone for the remarks. I actually done nothing except teardown, photo and copy schematics from their manuals.

author: Vgkid
author_rank: Super Contributor
timestamp: March 20, 2016, 04:56:23 pm
content: It is nice having everything in one spot.

author: splin
author_rank: Frequent Contributor
timestamp: March 20, 2016, 05:14:42 pm
content: So what differences are there in thermal EMFs between series and parallel arrangements? Consider the two examples zlymex shows in (3) Statistical arrays; assuming that each soldered joint has a 4uV/K potential, what might be the maximum total thermal EMF be if there is a temperature gradient of 100mK:1) across the 20 x 20.01 Ohm parallel resistors in the Fluke 742A-1 (roughly top to bottom in the photo)?2) across the Fluke 752A's nine 120k, one 119.8k and one 200 ohm resistors in series (say left to right in photo)? Would bottom left to top right be worse?In series there is the potential for all the EMFs to add up but of course thermal symmetries should allow most to cancel out if the temperature gradients are uniform - especially no turbulent air currents, but parallel still looks to be the better option? Of course it's often, but not always, possible to cancel out thermal EMFs when using such instruments by reversing connections but it would still be better to minimize them.This also applies to the series/parallel trimmer arrangements though avoiding having a resistor in series with the main resistor altogether would require unrealistically high trimmer values (with poor stability and TCs) for anything above a few hundred ohms - ie. a 1k resistor would require a 10Mohm parallel trimmer for 100ppm trim range. (The trimmer itself could comprise series fixed and variable resistors without adding significant EMF because of the 1k/10M divider).This also begs the question as to what might be typical temperature gradients encountered within calibration instruments in a lab environment? Would 100mK be much higher than reality for a passive device such as a Hamon divider which doesn't contain a significant heat source? What about when it's placed on or near a powered instrument such as a DVM dissipating 20 or 30W? Thermal EMFs will probably be small at soldered joints between resistors because the temperature difference across the thin solder layer between the two wires will be small. However thermal EMFs will also arise in the connections within the resistors, between the resistance wire and the lead-out wires. Also variations in the composition of lead-out wires and connecting wires subject to temperature gradients will also generate thermal EMFs, thus resistors connected in series ought to come from the same batch to minimize this problem (but even resistors from the same batch may not necessarily have all lead-outs made from the same reel of wire).