CENCO 83412 Potentiometer - silk purse from a sow's ear - almost. - Page 1
Forum section: electronics / general

author: trobbins
author_rank: Frequent Contributor
timestamp: September 18, 2025, 06:47:57 am
content: Central Scientific Co (CENCO) appears to have made instrumentation mainly for school/uni labs, and this vintage 83412 model potentiometer fits that application although no catalog info has been identified so far. As a cost-conscious potentiometer it needs an external battery supply and rheostat to provide 1.600V, and has no galvanometer. The earlier 83411 model was effectively the same and had +/-0.04% step tolerance, although my 83412 presented step tolerances of up to 1%.The base 1x ratio multiplier provides 1mV dial discrimination over a 0 to 1.6V span. The lowest ratio of 0.01x provides 10uV discrimination up to 16mVdc. The potentiometer comprises 10R switched steps, and a 10R slide-wire, in a robust steel chassis along with (I think) brass 4mm terminals. The ww resistors are on ceramic tubes using (I think) Constantin or Eureka wire.The 10R steps all have convenient terminations on to the Volts switch that allow a parallel resistor to tweak down each step resistance, so that allowed the base divider steps to be initially matched to <0.03% using a CC and HP 34401A voltage monitoring comparisons.I retrofitted a 1.6Vdc reference internally, powered via a new socket and external 12Vdc regulated plugpack. I have over a dozen spare pcbs to call on, each with a LM3999Z reference, an 8x 1k DIP array, 2x 10T trimpots and LM324, that required only minor reconfiguration to provide the required stable and buffered 1.6V output. I fitted an open shroud to the 3999Z to curtail local air movement, and curtailed the trimpot spans to give coarse and fine adjustment.The main voltage steps are now within +/- 50ppm except for lowest end, based on logging with my uncal 34401A. I have some confidence that 1V and 10V FS accuracy of my 34401A is within spec, but 100mV is just out of spec, based on transfer spot measurements with a Fluke 8588A last year. There is a 14uV offset related to the slidewire 0mV end terminal, which is current dependent (10mA infers a 1.4 millohm interface), as wiring and other interconnections are at low uV.The slidewire also shows a 100mVFS display resistance 2.4% lower than nominal 10 ohm, so can't be trimmed to directly display accurate mV values, so measurements will need a 2.4% fudge factor applied.The x0.1 and x0.01 ratios were easily trimmed to within <5 ppm, and the lower operating current from the ratio dividers curtails the slidewire end offset uV.I'll check for total tempco and stability over a few weeks, and perhaps further trim down the step accuracy, but it was nice to see that such mundane student gear can perform with a little TLC.Ciao, Tim

author: trobbins
author_rank: Frequent Contributor
timestamp: October 06, 2025, 03:37:15 am
content: I used a simple thermal chamber to do 2C step increases of the CENCO enclosure from about 18C to 26C. The 1.5V reference tempco was suppressed using a 50R NTC//200R trimpot in upper divider arm, from about -15ppm/C to <-1ppm/C, assuming the 34401A tempco was not an influence.Given the Potentiometer can switch the fixed output to 1.5V in 0.1V steps, I checked the tempco for a divider 1V setting and a 0.1V setting. Measured tempco was circa +1ppm/C for 1V setting, and circa -5 to -10ppm/C for 0.1V setting.I then changed to the x0.1 multiplier for 1V setting and the tempco was circa +18ppm/C. With the x0.01 multiplier the measured voltage was down at 10mV, with sample dither of circa 1uV, although tempco appeared to be less than 50ppm/C.This testing appears to indicate that the wirewound resistors within the unit were providing fairly low tempco performance. Certainly not a 'sow's ear' now, and may well come in useful on the bench.

author: donlisms
author_rank: Frequent Contributor
timestamp: October 10, 2025, 04:42:41 am
content: I am impressed by the performance of this device, given its humble nature, and the effort you've put into it.I wonder if some of what you saw as tempco in the lower ranges was thermal EMF.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 10, 2025, 11:03:45 am
content: Very likely to be part of the mix of error contributions. I tried to avoid thermal EMF at the cable interfaces to the 34401A and the CENCO from contributing to the tempco comparison measurements, where the CENCO and its cable interfaces were at stable local air temperature steps within a chamber, and the 34401A's cable interfaces were thermally covered and in general the 34401A was not experiencing significant hourly temperature variations (although the retrofitted internal temp sensor was being noted and sometimes showed a few degC change over many hours).I just used simple 4mm cable/interfacing between the CENCO and the 34401A, rather than copper termination interfaced cable, although I do add thermal shielding to the 34401A socket/plugs. The 34401A 10mV level monitoring had significant fluctuations even with 100NPLC, so tempco comparisons were definitely suffering and I couldn't be confident that they were actually better than 50ppm/C. I try not to do measurements where thermal EMFs could be a significant contributor - but that said, as I still have the CENCO on the bench, I will try and do a comparison with pure copper interfaces to see if I can determine a difference in that 10mV environment.PS. the 34401A measurement has too much fluctuation to resolve any tempco of the CENCO at x0.01 multiplier. I reckon I would have to repeat the CENCO testing in a temperature chamber and use a null voltage comparison to a stable low voltage reference that is held at constant temp. I have a discarded L&N 8686 two-decade (10mV and 1mV step) control in with a REF102 stable supply that is in a temperature controlled enclosure, but that would be for another day

author: donlisms
author_rank: Frequent Contributor
timestamp: October 11, 2025, 12:04:17 am
content: I guess maybe also within the CENCO -- terminals, contacts, junctions. I'm no expert, but my impression is that thermal EMF is pretty much always there, if you look closely enough! The old standard "average of forward and reversed supply voltage" technique might be interesting.

author: trobbins
author_rank: Frequent Contributor
timestamp: October 11, 2025, 12:26:49 am
content: I'd anticipate that although various metal junctions within the CENCO contribute thermal EMF offsets, that the tempco of those offsets are a benign contribution compared to the tempco of the ww resistors. That would be due to any step temperature change having sufficient time to equate temperature across any thermal EMF junction caused by internal metal differences.I added a later PS to my last post, with my view of a likely better way to approach measuring the tempco of the low mV levels generated by the multiplier resistors.