resistance standard - which Vishay resistor series to use ? - Page 1
Forum section: electronics / general

author: rob77
author_rank: Super Contributor
timestamp: August 12, 2016, 10:16:04 am
content: Hi Guys,i decided to build myself a resistance standard box a very simple one - just a single 10K resistor in a hermetically sealed box with binding posts. it's supposed to be used as a reference for a hobbyist.the main question is which resisor series to use from Vishay ? one would say Z series with 0.005% accuracy but there are S series and VAR series which are half the price of the Z series but still has excellent 0.01% accuracy.tempco is the same 0.05 ppm/C for Z and VAR series and a bit higher of 2ppm/C for S. the load life stability of all 3 series is the same 0.005% / 2000hrs.so considering the price, accuracy and tempco one would choose the VAR series, but that's an "audio resistor" as per datasheet. but is the VAR series suitable for a resistance standard box (hobby grade) ? as per datasheet the VAR series is a "naked" Z foil resistor , but if i put it into a hermetically sealed box, it shouldn't be a problem.any suggestion or idea is very welcome from you Guys.Rob.

author: TiN
author_rank: Super Contributor
timestamp: August 12, 2016, 11:24:18 am
content: I don't think that you will be able to find hermetic connectors for your hermetic box (they exist, but cost is outside of hobbyist realm), so I'd go for already hermetic VPG VHP202Z or VHP203Z resistor. They cost 20-30$/pc but since you just need one, should be ok. If you have access to, or know a friend with precision DMM, you could take your resistor box and get measurement of it's value to use as reference. Then you can just have 0.1% tolerance without paying extra.

author: rob77
author_rank: Super Contributor
timestamp: August 12, 2016, 11:46:45 am
content: thanks for your reply, i don't have anyone with a precision DMM, that's why aiming for the higher accuracy vishay resistors.the price difference is kind of significant between the mentioned Z series and VAR series... Z series 10K is 37eur + VAT = 44.4Eur for a single resistor while VAR series 10K resistor is 18Eur + VAT = 21.6Eur (prices @ farnell).regarding the hermetical seal, that won't be a big deal. there will be no pressure difference between inside and outside of the box (apart from standard atmospheric pressure deviations) and it won't be exposed to big temperature differences, therefore sealing ordinary binding posts is pretty easy (or i can even use a box inside a box to improve the seal but i think that won't be necessary).

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 12, 2016, 12:11:42 pm
content: Quote from: rob77 on August 12, 2016, 10:16:04 amHello Rob,buy VHP202Z, or better VHP101, which has a better guaranteed (!) T.C.Please, forget about this 0.05ppm/K fairy tale, that is typical data, or wishful thinking only.No, that's Vishay advertising only, but not real.(We had a long discussion about that in a long, long precision resistor thread)VHP101 has a guaranteed max. T.C. of <10ppm over 15..45°C, so essentially < 0.3ppm/KALL other similar types have a max. spread of their TCs of 2ppm/K.. I once bought 5 EA VHP202Z, in reality I measured between 0.3 to 1.0ppm/K.Buy an oil filled type (VHP...) for best long term stability of < 1ppm/year.Anyhow, these resistors are extremely stable and precise, beyond of amateur use, so they deserve a good mounting.Buy a small Hammond alu box, and 4 Pomona 3770 jacks, to connect in 4W configuration.Do not cut the legs of the resistors, and avoid any heating of the resistor element.Then you might want to add a precision thermistor or PT100, to determine the resistors temperature. Craft an alu block as heat reservoir and coupling between resistor and thermometer.That way, you can also measure its R(T) curve, for precision resistor calculation. This also let you determine its real T.C.I recommend also to let some volt-nuts or cal lab determine the absolute value, at a defined temperature, and then you may buy a cheaper 0.1% type.Please search for the precision resistor thread, there you will find a lot of additional information.Frank

author: rob77
author_rank: Super Contributor
timestamp: August 12, 2016, 12:57:26 pm
content: thank you very much for the reply.but it yields one question.. where to buy those ? as per Vishay datasheet the VHP100 10k 0.01% part number is Y007810K0000T9L - when i search it on google it throws only digikey at me and they have 500pieces minimal order quantity and 0 on stock - and it's the same for VHP101 part number Y407810K0000T9L and the very same for VHP202Z part number Y607110K0000T9L.where can one buy those in single piece quantities ? i mean from distributors, not individuals on e-bay.

author: zlymex
author_rank: Frequent Contributor
timestamp: August 12, 2016, 01:29:00 pm
content: The most important factor of a standard resistor is aging rate or longtime stability. So look for those with 2ppm/6 years spec.Better to compensate for value if building your own standard resistor. Otherwise the deviation may be very large in the standard resistor point of view.Better still to compensate for alpha T.C. but this takes skills and proper measurement instruments.

author: Gyro
author_rank: Super Contributor
timestamp: August 12, 2016, 01:46:58 pm
content: Quote from: TiN on August 12, 2016, 11:24:18 amThis question of hermetic enclosures seems to keep coming up. Wouldn't constructing a tin plate box with soldered seams and solder-in feedthrough capacitors or insulators as interconnects count as hermetic? The feedthrough caps I've seen all had soldered ends which would presumably seal the centre conductor.Granted, there may be some flux fume contamination to (hopefully) condense out on the walls, but wouldn't it still count as hermetic?

author: TiN
author_rank: Super Contributor
timestamp: August 12, 2016, 01:59:27 pm
content: rob77VPG sells single qty resistor on their website, so you can try shoot email to them and see what happens. I did bought like that before without any issues (except 30week L/T, but I had few special SKUs there).

author: rob77
author_rank: Super Contributor
timestamp: August 12, 2016, 02:29:43 pm
content: Quote from: Gyro on August 12, 2016, 01:46:58 pmexactly that would be my box in a box setup - resistor in a tin box with glass feedthrough caps as you described , enclosed in a plastic sealed box with binding posts sealed with silicone (silicone in the contact area with the box/panel) and the back of the binding post sealed with epoxy once the wires are in place (or again silicone) . but i think the plastic box with the sealed binding posts alone would be enough.

author: dacman
author_rank: Frequent Contributor
timestamp: August 12, 2016, 02:34:05 pm
content: Try a Y1453 or Z201.

author: rob77
author_rank: Super Contributor
timestamp: August 12, 2016, 02:34:46 pm
content: Quote from: TiN on August 12, 2016, 01:59:27 pmthanks for the suggestion, i already checked their web - but they have only the "POP9" in stock... 9 series of resistors but neither VHP101 nor VHP202Z is there....in the meantime i ordered the cheaper VAR series 0.01% 10k resistor from farnell (17 eur + VAT) and will keep trying to obtain one of those apparently "unobtanium" resistors from the VHP series

author: rob77
author_rank: Super Contributor
timestamp: August 12, 2016, 02:39:05 pm
content: Quote from: dacman on August 12, 2016, 02:34:05 pmthanks for the suggestion. basically the Z201 is available but double the price compared to the VAR series (and seems like same resistor molded into plastic case) . for that price it should be possible to get the oil filled metal can VHP series (but apparently no-one has them in stock and no-one sells them in single piece).

author: Andreas
author_rank: Super Contributor
timestamp: August 12, 2016, 05:57:32 pm
content: Quote from: rob77 on August 12, 2016, 02:34:46 pmHello,I had only average results on my 2 Vishay VAR samples:https://www.eevblog.com/forum/metrology/t-c-measurements-on-precision-resistors/msg879467/#msg879467With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 12, 2016, 06:33:36 pm
content: Hello,since the high end resistors are made in Israel and depend on many factors like $ exchange rate it makes few sense to list prices.The VHP is around factor 2-3 from a Z201.With best regardsAndreas

author: Andreas
author_rank: Super Contributor
timestamp: August 13, 2016, 05:05:06 am
content: Hello,that is not the point.of course you are right: Powertron is the most serious source for small quantities (at least in Germany).The point is that those prices are only valid for around 4 weeks (you forgot to mention the expiration date).And shurely these prices are without VAT (which is different in Europe). (that may explain the +25%)And Powertron also charges a small fee for shipping.With best regardsAndreas

author: rob77
author_rank: Super Contributor
timestamp: August 18, 2016, 12:43:17 pm
content: someone might find these interesting - the Vishar VAR resistor under microscope..not the best quality, took them with hobby/school grade microscope with VGA sensor (bresser duolux), but still details visible

author: zlymex
author_rank: Frequent Contributor
timestamp: August 18, 2016, 11:38:01 pm
content: Five similar photos of VHP202Z, 9k9995 0.01%, if anyone interested.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 19, 2016, 04:54:26 am
content: The trimming is done by LASER.Frank

author: zlymex
author_rank: Frequent Contributor
timestamp: August 19, 2016, 04:01:42 pm
content: The first Vishay foil resistor I teardown was this photo 7.5 years ago. At that time, I believed the trim was done Not by Laser.

author: Gyro
author_rank: Super Contributor
timestamp: August 19, 2016, 09:53:08 pm
content: Looks more like trained moths to me!

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: August 20, 2016, 04:50:22 pm
content: Vishay, et al, have used lasers for many years to do the trimming, they were using lasers when I was still at Ultronix and Sfer Nice installed a metal film department back in the early 80s. Those little round spots visible are the laser 'burns' of the trimming traces, I've seen/know the manufacture of these resistors from beginning to end. Yes, those laser burns have effects on the trace's characteristics of the metal elements, local but still measurable. Films/foils cannot be trimmed in the same manner as precision wire wounds, they are trimmed in discrete steps mandated by the shorted elements in the 'trimming' area of the circuit. The intrinsic value of each discrete step is determined by the circuit path's resistance, these resistors are etched just like any PCB and the etching also leaves an significant effect on the resistive element, one of the reasons why films/foils have 1/f noise and wire wounds do not.

author: Dr. Frank
author_rank: Super Contributor
timestamp: August 22, 2016, 07:01:14 am
content: Quote from: DiligentMinds.com on August 19, 2016, 02:12:10 pmVPG has "Precision Centers", where customers can get immediate samples. Maybe they simply scrape off the trimming points with a scalpel, as demonstrated here:http://www.vishaypg.com/foil-resistors/videos/?video=33. It's also explained, that trimming can be done by several different practises, including LASER.As far as I remember, the representative of VPG explained me (years ago), that the regular orders, were manufactured in Israel, and that they would use LASER trimming, what makes sense, especially in mass production.I also think, that these small round pattern in some photos must have been done by multiple LASER shots.Frank