Group buy: LTZ1000 precision resistor set

[MiDi]

Meeting point to organize group buy of precision resistor sets for LTZ1000 circuit.
Mainly for 10V pcb (branadic / Dr. Frank), but not limited to that.

I think group buy would only make sense when a couple of people want small quantity of same type (to save shipping cost and get volume cut in price - hopefully).

I am not familiar with precision resistors, so I do not really know where to go:
- Vishay Z/VH/VHP and equivalent as dividers (incredible long lead time?)
- Edwin G. Pettis Ultrohm Plus (seem to have a good price point) - specs, thermal shock resistance
- Rhopoint Econistors
- ?

[modified quote from pm]

I can appreciate the problems with shipping to various countries especially in the EU, for customers outside of the USA, the average 1st Class shipping charge is about $14,90, domestically it is about $4.15 for 1st Class.

If your group does a ‘bulk’ purchase, I will extend the discounts to everyone based on the total of each line item ordered, the minimum quantity for discount is 10 and that is 15%.  It would help if everyone contacts me directly through my e-mail (pettiseng@q.com) rather than the blog’s system.

I will leave it up to you and the group to determine which [e.g.] two weeks they want to order in; of course I’ll need to know the dates.

1.    Everyone will need to identify to me that they are part of the group purchase.
2.    I will determine the discounts based on the final tally of values ordered.
3.    I will notify everyone of the final pricing and individual shipping charges shortly after the [e.g.] 2-week period ends.
4.    Most customers use PayPal, my account is: pettiseng@q.com
5.   Production time will depend on the final quantities; I will apprise customers of the expected time.

Current base pricing for type 802 resistors, 1-9 is:

120R0   $7.19
1K      $6.65
12K      $7.72 [~45°C heater set point, only for NON A version at roomtemp]
12.5K   $7.85 [~52°C heater set point]
13K      $8.03 [~60°C heater set point]
70K      $9.07

±0.1%, ±3 PPM/°C maximum, 0.250” D x 0.375” L

I do not recommend using anything less than 12.5K for the LTZ circuit, you will not achieve any significant decrease in aging characteristics and it is better to have more than a few degrees overhead for temperature control.  It is the internal temperature of the LTZ that you are regulating, not the external environmental temperature; the LTZ is a power in – power out device.

Technically, it would have been best if the temperature ratio pair had been put next to each other on the PCB so they could be better thermally coupled but since that isn’t possible, thermal air drafts will need to be blocked to minimize differences.  NOTE: do not encase the LTZ to the point of no thermal exchange with the air as this will cause the LTZ to exceed the internal regulation loop and it will lose thermal regulation.  This is a known characteristic; you need to block air drafts around the leads, both top and bottom but not the body.  Do not use any hydroscopic material inside the LTZ box, you don’t need anything that absorbs humidity and holds it.  Of course you can also include a package of silicate to help with that.  I am not saying that any particular components are humidity sensitive; it just isn’t a good idea to have humidity building up.

A voltage booster circuit is much more complicated than the LTZ board; many things beyond the resistors contribute to the output voltage.  Thermals of course, the op-amp, the op-amp amplifies thermals and TCRs, thermal coupling between the resistors, and noise to name a few things that will affect the output.  While I have made resistors for this purpose that has exceptionally low tracking TCR, it took exceptional effort to achieve in the short term.  Tracking TCRs down to 0.1 PPM/°C were achieved but this was for an industrial customer where the effort and cost were justified, the customer actually did the final processing because he had the equipment to do it.  He also went to some extreme to minimize thermals.

For the average user and some reasonable processing, you could expect something around ≤0.5 PPM/°C or so, if the design of the booster circuit is done correctly and carefully you could achieve very good thermal equilibrium thereby minimizing further the TCR of the output.  One possible way is to use a small oven to stabilize the booster circuits so that temperature becomes a minor problem.  The voltage booster circuit could easily end up costing more than the LTZ circuits.  Look at the Fluke 732A/B to see the lengths they went to to stabilize the outputs, quite complex.  The LTZ board does not need this sort of effort because it is basically internally heat regulated and is not aided by external ovenizing.

Some may point to the LTC5400 chip for very low tracking TCR, to some degree that is quite accurate but the type of resistors inside have wide tolerances from chip to chip, significant noise and 1/f noise as well, PWW resistors have neither.  So there is a tradeoff for getting low TCR with higher noise and you also have more trimming to deal with because of tolerance and limited values.

You are correct in that there will be a power imbalance between the ratio resistors in the booster circuit (just like the heater circuit in the LTZ), just having resistors with the same TCR will not get rid of that problem, that is where some of the thermal problems come from, they require very good thermal coupling between them to start with, such as copper tape wrapped around them.  If you decide to trim the output, there will be additional resistors to contend with including a trimmer, this should be a Bourn 3290 for best stability (yes you have to shop around, many places are asking a high price for it but you can find them at lower pricing if you look hard enough, the Vishay trimmers are inferior no matter what the specs claim).  If you can tolerate a more fixed output then you can eliminate the hassle of trimming, the drawback is you will need to calculate the booster ratio resistors values to a closer tolerance (no you really don’t have to go to ±10PPM).  Your resistor values will depend on your circuit design and what you want at the output so I can’t really advise on specific values, just how to implement them.

You do not need to have exactly 10.0000000Vs at the output to calibrate with, anything fairly close will do as long as the value is known, whether it is a manual or automatic adjustment calibration.  The stability is really more important which brings me to another topic, long term drift.  If you haven’t read the post I put in metrology section for definitions, it is a good place to start.  All freshly made resistors have drift which tends to decrease over time naturally, depending on the user’s patience, one can either wait some months for the drift to settle down or purchase the so-called PMO or enhanced processed resistor which takes some of the initial drift out of the resistors.  On average, again depending on circumstances, resistors will settle down to fairly low drift after about a year (it might be more or less) to low single digit drift without PMO, depending on the enhanced process, it could remove the lion’s share of the initial drift to possibly single digit drift per year.  Note that your LTZ is going to take at least that long or longer to really settle down to its lower drift rate per year.  Precision takes time and in many cases it cannot be rushed as rushing can have its own long term consequences.

For the two resistors [for 10V boost] you mentioned, I would suggest ±0.01% tolerance:

4.8K      $10.04
12K      $10.13

Ultrohm Plus
125 Vista Grande Dr.
Grand Junction, CO 81507-1427
970-242-4929
pettiseng@q.com

[BradC]

I would be interested in any of those. I have already contacted Edwin about some sets in addition to some other special resistors I need but I've struggled to figure out how to get a set of Vishay metal foils. Can't seem to find an economical or easy way to get them from Australia.

[Sprock]

Hi

sorry no experience too.
For hobby use i tend to Edwin´s resistors but will also
accept others. Majority will rule.   

mfg
Sprock

[GregDunn]

This company makes precision resistors and has a fairly short lead time for kits:

http://www.precisionresistor.com/MC-7-Calibrator.html

[RandallMcRee]

I vote for Edwin's resistors.

Have measured many s102k vishay resistors and most are not "typical" they have more tempco than expected. The VHP series are WAY pricey and have those very long lead times.

For other off-the-shelf I have good experience with Susumu RG series 2ppm, they all were close to spec. These are only available in SMD style so would either need another pcb or an adapter. Plus it would be a custom order for the non-traditional values.

So, in my limited experience, Edwin is the best bet.

[GregDunn]

I'm afraid I'm going to have to bow out of this group buy; at the moment, I won't be able to get the board or the reference chip, so it's moot.  Sorry.   

[Macbeth]

I vote for Edwin's resistors.

Have measured many s102k vishay resistors and most are not "typical" they have more tempco than expected. The VHP series are WAY pricey and have those very long lead times.

For other off-the-shelf I have good experience with Susumu RG series 2ppm, they all were close to spec. These are only available in SMD style so would either need another pcb or an adapter. Plus it would be a custom order for the non-traditional values.

So, in my limited experience, Edwin is the best bet.

If you are in the USA then there is no need for a group buy. I think MiDi is based in Germany, the EU - so every single shipment of a handful of Edwins resistors are subjected to extreme "Let me see your paperz" at the Merkel border, potentially held up for months, then finally released having had massive surcharges, duties and taxes stamped on them.

A group buy with a destination outside of Herr Merkels border zone, like Switzerland, could be advantageous when EU cross border shipping no longer attracts the UK HMRC or German Customs Gestapo.

But I think MiDi is based in Germany and so is really the worst EU member to arrange such a transaction. Even Dave refused to ship his DMM's to Germany.

For some reason we don't have this with stuff imported from China. As a Brit I look forward to Brexit so we can drop this insane EU shit and import stuff as freely from the USA as we get with China.

[MiDi]

If you are in the USA then there is no need for a group buy. I think MiDi is based in Germany, the EU - so every single shipment of a handful of Edwins resistors are subjected to extreme "Let me see your paperz" at the Merkel border, potentially held up for months, then finally released having had massive surcharges, duties and taxes stamped on them.

A group buy with a destination outside of Herr Merkels border zone, like Switzerland, could be advantageous when EU cross border shipping no longer attracts the UK HMRC or German Customs Gestapo.

This is advised to be a meeting point only.

You guessed right, I did not fake my country with Frau Merkel as the Chancellorette 

I have no experience with customs so far (luckily), but afaik there are no customs for resistors into EU if declared correctly (not sure if same applies for electrical measurement equipment).
But afaik there is a tax applied (for imports outside EU with >= 22Eur (~25USD) value incl. shipping 19%VAT for germany), so yes, it would not make much sense to import to EU and then send them around the world outside EU.

[BradC]

Right, in for a penny, in for a pound. I've just ordered 3 sets of Ultrohm Plus PWW, 2 sets of SFERNICE foils from E-bay and a set of 3 KX boards from OSHPark to go with the boards branadic is producing. If I'm building one, I might as well build 5 or 6 right?

The SFERNICE foils are not particularly an optimum set with the values being : 123R / 965R / 12K4 / 68K1 so the set temp will be somewhere around that provided by a 12K85/1K divider. They are also only 5ppm, so we'll see. A start to experiment with anyway. They were about $40 / set. The PWW were about $65 a set including shipping (our exchange rate is horrid right now) but at least they're the right values and reputedly far more stable.

I wouldn't mind spending some real money on a (single) set of hermetic Vishays including a 12.5K/1K divider if someone was going to organise a group buy though.

The LTZ1000ACH at Digikey is backordered until November, so I'll be waiting a while until those land. So far I've put a few bucks over the counter, so there's some incentive to make this more than a pile of parts sitting in a box in the corner

[BradC]

Has anyone done a laymans writeup of the 5 critical resistors in an LTZ1000 circuit and potential effects of deviating from the datasheet/recommended values?

[Macbeth]

Has anyone done a laymans writeup of the 5 critical resistors in an LTZ1000 circuit and potential effects of deviating from the datasheet/recommended values?

Have you read this thread?

https://www.eevblog.com/forum/metrology/resistor-set-for-ltz1000-positive-standard-7v-circuit/

Essentially it's R4/R5 (the 12-13:1 divider) and then R2 (one of the "70K" pair) that are most important.

Apparently the Polish ebay surplus SFERNICE resistors are for Space Applications, ESA 4001/011. http://www.vishaypg.com/docs/63132/rckhr.pdf - I wonder if Prof. Colin Pillinger relied on them for Beagle 2? 

[BradC]

Has anyone done a laymans writeup of the 5 critical resistors in an LTZ1000 circuit and potential effects of deviating from the datasheet/recommended values?

Have you read this thread?

https://www.eevblog.com/forum/metrology/resistor-set-for-ltz1000-positive-standard-7v-circuit/

Essentially it's R4/R5 (the 12-13:1 divider) and then R2 (one of the "70K" pair) that are most important.

Apparently the Polish ebay surplus SFERNICE resistors are for Space Applications, ESA 4001/011. http://www.vishaypg.com/docs/63132/rckhr.pdf - I wonder if Prof. Colin Pillinger relied on them for Beagle 2? 

Thanks, I had read that thread but forgotten completely about it. when looking for information on the LTZ in threads, there is an awful lot of chaff to sort through to get to the wheat. Much appreciated.

[Edwin G. Pettis]

Since I've shipped into a number of EU countries.....on the whole, yes the German customs tends to be the most difficult to get packages through, it also depends to some degree on which customs in which city it goes through, some are much more difficult than others.  I'm told there are fees on top of fees and the ever present VAT.  The dollar value for no significant fees is about $20, that is meant to make sure very little comes into the EU without being nailed and makes it more difficult to fudge on the value a lot.  They are also very suspicious of too many samples or of what looks like very low pricing, that may or may not trigger a customs battle.  I do not know of any customs free duty on PWW resistors in any EU country, if there is none of my customers has mentioned it.  You might be able to get away with shaving some of the actual costs off of the customs paperwork for reduced fees/taxes but it can't be excessive.

Each country tends to be different in how they handle customs, there doesn't appear to be any universal rules being applied outside of perhaps the VAT.  I would presume once the resistors are 'inside' the EU, there would be no further problems/charges/fees on distributing them to other countries outside of postage.  It would only be a matter of checking around with EEVblog members to find the lowest cost entry country.

I also give pretty good quantity discounts......end of ad.

[Andreas]

I'm told there are fees on top of fees and the ever present VAT. 

Hello,

but shurely not by the customs.
Maybe if you send it by a parcel service they call for a handling fee for the customs.

The normal customs process (sending by USPS/Deutsche Post) up to 2kg in a jiffy bag is relative easy.
The package after import goes directly to the local customs store.
After around one week you get a letter that you have to pick up your package at the local customs.

You should have the "Zolltarifnummer" (TARIC) or HTSUS number at hand so that they can calculate correctly the duty rate
(which is 0% for resistors below 20W at the moment).

http://ec.europa.eu/taxation_customs/dds2/taric/measures.jsp?Lang=de&SimDate=20181001&Area=US&MeasType=&StartPub=&EndPub=&MeasText=&GoodsText=&op=&Taric=8533210000&search_text=goods&textSearch=&LangDescr=de&OrderNum=&Regulation=&measStartDat=&measEndDat=

HTSUS: 8533.21.0080
Description: Electrical fixed resistors, other than composition or film type carbon resistors, for a power handling capacity not exceeding 20 W
MFN Duty Rate: Free

Of course you have to pay the VAT (Einfuhrumsatzsteuer) which is 19% at the moment in Germany. (So mostly lower than in other european countries).
So you need a invoice and the paypal/paying receipt.

And there is a speciality if you import: Customs has a own (monthly fixed) exchange rate when importing things so the 19% are not exact 19%
(ok this may appear as fee on top of a fee).
http://www.zoll.de/SiteGlobals/Forms/KursSuche/KurseSuche_Formular_Initial.html;jsessionid=1D3765BD6DA32B62D74B2C5DE084508B.live4411?nn=298534

Disclaimer:
Above are experiences on 2 small orders of around 5 LTZ (7V) resistor sets each.
If ordering large quantities it might be necessary to have a customs EORI-number.

With best regards

Andreas

[MiDi]

Has anyone done a laymans writeup of the 5 critical resistors in an LTZ1000 circuit and potential effects of deviating from the datasheet/recommended values?

Cellularmitosis has collected several data from users.
The 12k/1k goes for heater set point of ~45°C.

[Edwin G. Pettis]

!2k is cutting it a bit close unless you are in a cooler controlled environment such as a lab at 23°C-25°C or located in a cooler country.  I do not recommend using less than 12.5K as a minimum and I regularly recommend 13K as the universal value to assure correct operation of the LTZ.  Remember the LTZ is a power in, power out device and the real temperature you need to pay attention to is the internal temperature of the LTZ, that is where the regulation of temperature occurs, it is not the same temperature as the environment.  You are not really gaining anything in long term stability by trying to shave off a few degrees.  HP/Keysight uses 15K in their modules and that includes the 'golden' 3458As and they are perfectly stable for years at that temperature.  You definitely do not need to run it with 15K, unless you're running it in a rack for instance, but you aren't going to get better stability by running it too cool either.

[Edwin G. Pettis]

Thank you for clearing up what that HTSUS number does, I do indeed include that on the customs when I ship to Germany and Andreas was the one who originally supplied it to me, thank you.  Since I am not on the 'receiving' end, I can only go by what my customers have told me about their customs experience, in Germany it has varied some depending on where the package went, some local customs were exceedingly picky about paperwork and contents while others not so much.

[MiDi]

As most people - me included - want to go for the Ultrohm Plus Resistors from Edwin, I asked him for a special deal.

Edwin is so kind and gives us a special offer, I put the details in the first post, but here is the short version of the rules for 'bulk' order (hopefully I got it right):
- valid only within a given time span (tbd)
- valid only for orders marked for this special offer
- individual order with individual shipping (like normal order)
- after time expired the orders will be treated as one big order and the discounts are added for each individual resistor value

My suggestion for the time span: sunday 2018-10-07 till sunday 2018-10-21.
So we have a bit of time to talk about questions and to prepare.
If you agree with this, please leave a "thanks" as feedback.

If everything is prepared, I will do the advertisement here and give the details (time span & passcode).

[BradC]

The LTZ1000ACH at Digikey is backordered until November

... and now due to arrive on Monday.  Yay Digikey!

I just priced the Bourns 3290 trimmers. I could buy 3 Ultrohm Plus  0.01% resistors for the price of one trimmer.
I've ordered some 3292W for the boards in question, but I'm starting to think I'll just use the op-amp to track the LTZ and spit out a buffered 7.x volts.

[MiDi]

For those that are not on a rush for buying Ultrohm Plus can now benefit from special offer from Edwin:

- Add discount code "Halloween" to your order in E-Mail to pettiseng@q.com
- The discount code expires on sunday 2018-10-21

The orders are collected till 2018-10-21 and then treated as one order for discount & manufacturing.
Everyone then gets the final pricing and individual shipping charges.
See details in first post.

Your order should include:

Code: [Select]

- Discount code: "Halloween"
- Quantity and value of resistors (±0.1%, ±3 PPM/°C maximum, 0.250” D x 0.375” L is standard, add for other specs)
- Complete shipping address
Edit:
Expired, but feel free to organize new one.

[VK5RC]

Thanks MiDI, Edwin, DrFrank and Brandiac 

[meandeev]

Thanks from me, too!

but I tried to send an e-mail and got an error. Does anybody has the same problem?
--------------------------------
A message that you sent could not be delivered to one or more of
its recipients. This is a permanent error. The following address(es)
failed:

pettisieng@q.com:
SMTP error from remote server for RCPT TO command, host: mx.centurylink.net (206.152.134.65) reason: 550 5.1.1 [R2] Recipient pettisieng@q.com does not exist here.
---------------------------------

[SvanGool]

You should drop the last "i" out of the address you used, Edwin's email is working fine today 

[meandeev]

Looks better, no error.

I copied the address from the first post.

@MiDi: was this intended (to block e-mail grabbers)?

You should drop the last "i" out of the address you used, Edwin's email is working fine today 

[MiDi]

I copied the address from the first post.
@MiDi: was this intended (to block e-mail grabbers)?

Where did you got it? Do not see it.
In the announcement is the linked mail, no c&p needed.