<$30 eBay LM399+LT1001 2.5/5.0/7.5/10V Vref Module Hack- Step-1: PoC - Page 1
Forum section: electronics / general

author: GEOelectronics
author_rank: Regular Contributor
timestamp: May 12, 2018, 10:01:38 pm
content: I'm striving to build a practical, inexpensive 10.000000V DC reference for my Kelvin Varley Divider Calibrator project (ala poor man's Fluke 732A/752B) for my Mini Metrology LabMy thanks to Conrad Hoffman for the inspiration :http://conradhoffman.com/mini_metro_lab.htmlSeveral Vref ICs are being evaluated, this weekend's project focuses on a less than $30 USD eBay LM399H based / LT1001 buffered multi Voltage stable reference module which arrived last week. Preliminary testing showed it to be very stable, but also very inaccurate (no surprise with 5% variability of actual output Voltage between samples)This Asian version's design opted for a (not) precision fixed resistance divider scheme to supply any single output, jumper selected from 2.5, 5.0, 7.5, 10.0. The actual individual "calibration" chart showed all sorts of jumper selected values,but very few zeros present. Quite useless for my 10.00000 + KVD project. (see Fig-1)9.85773V (/=) 10.00000VThe other V selections held no interest to my project so the jumpers were set for 10V and the plan to add a trimmer to get the accuracy needed began. Basically I want to duplicate the "Portable Calibrator" from the National Semiconductors data sheet for the LM399 series. (see Fig-6)Pictures of the hack's first crude Proof-of-Concept (PoC) test, data sheets and the module's jumper schedule located at:http://www.qsl.net/k/k0ff/VOLT_NUTS/LM399H-LT1001_10.00_Vref/Updates will arrive in that same folder and eventually a project PDF.Before anything else, all the unneeded resistors and jumpers will be removed and grounding paths cleaned up as required. Once 10.0 V is achieved, all the other desired Vrefs will be selected by the KVD.The needed R7 resistance turned out to be 19.182K Ohms (vs. the original fixed 20K), the next step will be to build the trimmer as a low resistance precision 10-turn panel potentiometer ( I like the Spectrol 10T 1K WW and Beckman 15 turn counter for this) with a stable ~18.5K main resistor, and box the whole thing for the next round of experiments and testing.Have fun.George Dowell

author: eurofox
author_rank: Unknown
timestamp: May 13, 2018, 07:32:55 am
content: I order the same and expect it the next coming days.I got exactly the same idea how to proceed with it ....

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: May 13, 2018, 09:47:07 am
content: BTW : The "precious" components inside the metal shield appears to be second hand. Date code on the LM399 = 9518, date code on the LT1001 = 9329.

author: eurofox
author_rank: Unknown
timestamp: May 13, 2018, 01:29:04 pm
content: Quote from: EmmanuelFaure on May 13, 2018, 09:47:07 amthis way they are already "aged"

author: GEOelectronics
author_rank: Regular Contributor
timestamp: May 13, 2018, 01:35:22 pm
content: Quote from: EmmanuelFaure on May 13, 2018, 09:47:07 amYes and in person it is obvious that the LM399 is dirty looking.After removing the unwanted parts and redrawing the circuit, I see no reason this couldn't be duplicated quite easily, but frankly this seems to a good enough platform and cost effective for sure, especially considering the simple but effective "can" (the LT1001 spec sheet mentions it should be protected from air currents)George Dowell

author: glarsson
author_rank: Frequent Contributor
timestamp: May 13, 2018, 03:37:00 pm
content: Quote from: eurofox on May 13, 2018, 01:29:04 pmMost likely the aging must be "restarted" after thermal and mechanical chocks (desoldering from old PCB, bumping, cleaning and soldering to new PCB).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 13, 2018, 05:17:58 pm
content: The LM399 is getting rather hot in normal operation. So unless really brute force is used the thermal conditions on un-soldering and soldering are not that severe compared to a normal power cycle.For the LT1001 it is a little different - here a new one is likely better.A possible problem could however be ESD during recycling.

author: cellularmitosis
author_rank: Unknown
timestamp: May 15, 2018, 06:29:34 am
content: This appears to be the origin of these boards:https://item.taobao.com/item.htm?spm=a230r.1.14.59.6a91279aqTyH3R&id=564137743559&ns=1&abbucket=6#detailThere are a few variants available (TL431, AD584, LM399).On taoboa, it looks like the LM399 version is about $21.There's another LM399 board from another taobao seller:https://item.taobao.com/item.htm?spm=a230r.1.14.71.6a91279aqTyH3R&id=538328618275&ns=1&abbucket=6#detailI've ordered a few of that design, but he seems to either have difficulty with stock, or is a very lazy shipper (I had to harass him to get a second order of a few boards from him).https://www.eevblog.com/forum/metrology/cheap-ebay-ad584-voltage-references-my-experiences/msg1274955/#msg1274955

author: cellularmitosis
author_rank: Unknown
timestamp: May 15, 2018, 06:32:00 am
content: (If there is interest in importing these used / salvaged LM399's from taobao, I'd be happy to do a group buy and distribute the parts).

author: Kleinstein
author_rank: Super Contributor
timestamp: May 15, 2018, 07:43:35 am
content: The circuit around the LM399 looks a little odd and not the highest precision one. So what is the point of buying a board with used parts and a few standard resistors if the circuit has to be bodged somehow and not much more than the LM399 is really useful ?

author: EmmanuelFaure
author_rank: Regular Contributor
timestamp: May 15, 2018, 01:07:40 pm
content: +1A brand new LM399 costs ~$13 :https://www.digikey.com/product-detail/en/linear-technology-analog-devices/LM399AH-PBF/LM399AH-PBF-ND/4071473A precision 5ppm resistor costs ~$1 (Vishay PTF56)A tinned RFI shield < $2

author: GEOelectronics
author_rank: Regular Contributor
timestamp: May 15, 2018, 01:53:29 pm
content: Yes that looks correct.I've added a PDF to the project folder which also shows other eBay boards and parts going into the project:http://www.qsl.net/k/k0ff/VOLT_NUTS/LM399H-LT1001_10.00_Vref/LM399_Project_05_15_18.pdfUpdates will be added as they occur. I'll try to measure the chip capacitors and generate a schematic today. I see no reason it can't be duplicated in the Home Lab on a much smaller PCB.George Dowell

author: cellularmitosis
author_rank: Unknown
timestamp: May 15, 2018, 01:56:06 pm
content: Sorry, I meant importing the used LM399 chips themselves, if people were interested in making their own boards.

author: GEOelectronics
author_rank: Regular Contributor
timestamp: May 15, 2018, 03:33:30 pm
content: Quote from: cellularmitosis on May 15, 2018, 01:56:06 pmI like that thru-hole board you linked and ordered a few to play with.My first time ordering from that site. Hoping for good service like eBay.George Dowell

author: GEOelectronics
author_rank: Regular Contributor
timestamp: May 16, 2018, 12:45:02 pm
content: Quote from: cellularmitosis on May 15, 2018, 06:29:34 am

author: 1audio
author_rank: Frequent Contributor
timestamp: September 08, 2018, 01:52:47 am
content: Any updates? I need to make a new reference for a vintage JRL Volt-a-vider. It has a 6 digit KV divider an output amp with remote sensing etc. The original seleced Zener reference was pulled and a marginal reference using MC1469 was kludged in which is not up to the task. Modifying one of these may be the easiest way to get a suitable reference to patch in.