[WANTED] Precision resistors for LTZ1000 [FOUND] - Page 1
Forum section: electronics / general

author: alex-sh
author_rank: Regular Contributor
timestamp: June 12, 2019, 12:27:52 pm
content: Hello,Unfortunately, digikey or any other distributor is not offering Vishay precision resistors unless buying in bulk 50 or 100 pieces.What's the best place to get them please? I need the following:120R01K13K70K (2 pieces)4.8K12KThanksAlex

author: jpb
author_rank: Super Contributor
timestamp: June 12, 2019, 08:38:03 pm
content: have a look at this recent thread:https://www.eevblog.com/forum/buysellwanted/group-buy-ltz1000-precision-resistor-set-(for-branadic-10v-pcb)/

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 12, 2019, 10:00:57 pm
content: If you need to avoid Edwin's long lead times, I have one ebay set (hifi-szjxic) that is sitting around doing nothing...2 75K RNC90Z 0.01%2 244R0 S102K 0.01%1 1K000 S102K 0.01%1 2K500 S102K 0.01%1 10K00 S102K 0.01%These were part of a stable reference that I zapped and never re-built (replaced by a set of Edwin's PWW). They have short leads. Probably a bit of hysteresis from soldering, but should still be serviceable.Will sell at cost, plus shipping, paypal fees and so on

author: babysitter
author_rank: Frequent Contributor
timestamp: June 13, 2019, 08:03:19 pm
content: Econistors. Available e.g. from rhopoint or farnell.

author: alex-sh
author_rank: Regular Contributor
timestamp: June 14, 2019, 09:10:45 am
content: Quote from: jpb on June 12, 2019, 08:38:03 pmI have been waiting these from October 2018. Enough is enough - Really long wait

author: alex-sh
author_rank: Regular Contributor
timestamp: June 14, 2019, 09:17:26 am
content: Quote from: babysitter on June 13, 2019, 08:03:19 pmSadly, the values I need are not available from Farnell.

author: alex-sh
author_rank: Regular Contributor
timestamp: June 14, 2019, 09:43:04 am
content: Quote from: RandallMcRee on June 12, 2019, 10:00:57 pmYou have quite a few interesting items.However, I only managed to find 120R, 1K and 12K (array) - these are most common ones.4.8k, 13K, and 70K are not available.

author: branadic
author_rank: Super Contributor
timestamp: June 14, 2019, 01:09:25 pm
content: Use NOMCA1603 resistor network for the temperature setpoint (12k:1k, 12.5k:1k or 13k:1k) and the 120R resistor you already found. The 70k are not critical, RC55Y with 68k or 69.8k will do the job. For voltage boost to 10V you can again use NOMCA1603 resistor network. That saves you a lot of money as well as lead time and you have the best you can get-branadic-

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2019, 02:27:36 pm
content: For the resistor networks Mouser also as the MORN series with better specs than NOMCA, though only 4 resistors in a case an smaller.The 70 K resistors don't really have to be 70 K. This is just a compromise between unheated TC (which would prefer a higher resistance) and noise (which would prefer a lower resistance). So 50 K can also be OK, with only marginal high unheated TC. The unheated now noise version even goes with 30 K. One of the 70 K resistors still has quite some influence.For a 7 V to 10 V step, one could in principle combine this with the temperature setpoint divider - so only one more precision resistor would be needed. So the divider would be some thing like 1 K - 13 K - some 6.8 K. The added error from an OPs offset at the 7.1 V point should not be that dramatic.

author: alex-sh
author_rank: Regular Contributor
timestamp: June 14, 2019, 08:52:55 pm
content: Quote from: Kleinstein on June 14, 2019, 02:27:36 pmThank you.These resistors have 25 ppm tempco. Are you sure this would be ok for LTZ1000?Unfortunately, they are not available on either mouser or digi-key in values I need.

author: alex-sh
author_rank: Regular Contributor
timestamp: June 14, 2019, 08:57:13 pm
content: Quote from: branadic on June 14, 2019, 01:09:25 pmThank you25ppm tempo? Is this fine?They are not available either in 12:1 or 13:1. Additionally, soldering these to your board would be a challenge - the footprint is completely wrong.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2019, 09:28:17 pm
content: The NOMCA resistors are 25 ppm/K, but the ratio TC is specified at 5 ppm/K. The divider tends to use 2 of the resistors in parallel so one has a chance to get even better matching from statistics. It may not be the absolute best ratio stability, but at least not bad for the price (some $4,x per chip)The arrays may not be a good choice for the 70 K resistors - these are not used in a kind of ratio. So matching these does not help. So even mixing 50 K and 100 K should be OK. Using 70 K each is more like a BOM simplification.Anyway with the resistors I would not consider the TC really critical - it is more the long term drift and maybe humidity sensitivity that is important. There is about a 100 fold attenuation of the TC, so that 10 ppm/k of the resistors result in some 0.1 ppm/K total and this is about the range that is compensated by R9 ( some 400 K nominally) for the non A version.For the 7 to 10 V step the NOMCA Array(s) (it would need something like 2 Array with 8x2 K each) would not be really high end, more like a low const. version. A possible configuration would be 2 in parallel for 1 K, 2 parallel + 6 in series for 13 K and some 2P2S+2 in series für some 6 K). So this would be a divider chain of 1 K : 13 K : 6 K to give slightly more than 10 V and the 1 K + 13 K divider for the temperature set point. The resistors can be split to symmetric use of the 2 chips - so no need for matching across the chips.This solution may need a lock out to turn on the heater only when the supply is good and the 10 V are OK.Mouser has 1 K and 13 K BMF resistors (S102..), though at a price.

author: dietert1
author_rank: Super Contributor
timestamp: June 15, 2019, 10:55:27 am
content: When looking for those resistors i discovered that for some families of small precision resistors there seems to be an upper limit around 50 KOhm or so. In the meantime i found some cheap 34 KOhm S102 that will give 68K instead of 70K. When i think about this, i will take the chance and put a capacitor from the middle point to Gnd, hopefully reducing noise. One might also think about splitting 20K + 50K intentionally. The 20KOhm could then be a usual 0.1 % +/- 5 ppm MF resistor that comes around 1 €.Regards, Dieter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 15, 2019, 11:48:55 am
content: Additional filtering at the 70 K resistors should not have much of an impact. There should not be much RF junk around at the output. So I don't see much hope for lower noise there. At least it should not hurt.The 70 K number for the emitter resistors is no magic - chances are 50 K will work about as good (in theory a tiny bit higher TC as the unheated TC would be slightly higher and a little less noise).One of the 70 K resistors has a pretty low impact - so a 5 ppm/K MF resistor should be OK for this one.

author: exe
author_rank: Unknown
timestamp: June 15, 2019, 11:53:59 am
content: Quote from: dietert1 on June 15, 2019, 10:55:27 amI was thinking about the same. Rumors say that putting a capacitor across zener may make it less stable due to current spikes. But a resistor should reduce the magnitude. So, please share your results!

author: branadic
author_rank: Super Contributor
timestamp: June 15, 2019, 09:24:32 pm
content: Quotehttps://www.eevblog.com/forum/metrology/statistical-arrays/-branadic-

author: alex-sh
author_rank: Regular Contributor
timestamp: June 15, 2019, 09:41:31 pm
content: Quote from: Kleinstein on June 14, 2019, 02:27:36 pmHello Kleinstein,My LTZ1000CH voltage output is 7.187987VWhat do you think would be my best resistor values to go to 10V?Thank you

author: dietert1
author_rank: Super Contributor
timestamp: June 16, 2019, 05:51:11 am
content: Elsewhere NOMCA resistor arrays were recommended for the 10V to 7 V divider, for example like this. Any other resistor you need to add into the network for fine tuning will have its TC attenuated by about 500. Current through this divider will be a little more than 1 mA. Output impedance is about 1.8 KOhm, low enough to avoid extra noise.Regards, Dieter

author: dietert1
author_rank: Super Contributor
timestamp: June 16, 2019, 07:23:42 am
content: A pretty perfect 14:1 divider for the temperature sensor can be made from the same part plus a MF resistor.Regards, DieterPS: The extra resistor can be made 25KOhm 10 ppm to adjust the ratio as 13.5 :1 for a lower temperature as recommended by some experts.

author: alex-sh
author_rank: Regular Contributor
timestamp: June 24, 2019, 03:14:11 pm
content: Ended up purchasing a few resistors 1k and 12k from hifi-szjxic on the ebay: