DE/EU Simple LM399 Reference Board, improved second Version

[RoadRunner]

Hi,

i have two boards and realised, that the 10 V output voltage varies in the 100s uV if the 15V input change in 100s mV below the 15V checkpoint.

Karsten


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Thanks for pointing, yes power supply need to be as close as possible to 15V.

Regards

Well the 10V board also makes a good thermometer

Attached is the board when the temperature is 24.5C and the voltage is 15.0+- 0.05V

10uV out which is much more like what I expected.  I'm going to build a new 15v board and wrap the ref board and stick it in it's own box but with my reference resistors as well

EDIT: Just read that....  So is it better to be 0.1V over than a little under?  OR as close as possible..... and just HOW close?

ALSO....  Is it safe to wrap it up in foam?  I don't want to damage it by overheating the board or anything like that.

Thanks


This is now in the range of ..... which is out, the reference or the DMM? :-)

One volt difference in Supply voltage makes around 180uV difference in output because of reference zener diode current.

[cowasaki]

Hi,

i have two boards and realised, that the 10 V output voltage varies in the 100s uV if the 15V input change in 100s mV below the 15V checkpoint.

Karsten


Gesendet von iPhone mit Tapatalk

Thanks for pointing, yes power supply need to be as close as possible to 15V.

Regards

Well the 10V board also makes a good thermometer

Attached is the board when the temperature is 24.5C and the voltage is 15.0+- 0.05V

10uV out which is much more like what I expected.  I'm going to build a new 15v board and wrap the ref board and stick it in it's own box but with my reference resistors as well

EDIT: Just read that....  So is it better to be 0.1V over than a little under?  OR as close as possible..... and just HOW close?

ALSO....  Is it safe to wrap it up in foam?  I don't want to damage it by overheating the board or anything like that.

Thanks


This is now in the range of ..... which is out, the reference or the DMM? :-)

One volt difference in Supply voltage makes around 180uV difference in output because of reference zener diode current.

In that case I need to build another power supply.  I've got one on KiCAD at the moment which would do it based on the LT3080ET.....  Maybe that's the way to go.

[jpb]

My 15V was about 14.998V.

I used a LT3402 board from E-bay and a 150kohm Vishay resistor also from ebay (Note a LT3405 board provides more current but are about twice the price):

relatively expensive UK sourced which I used because I was in a hurry
https://www.ebay.co.uk/itm/LT3042-Ultra-low-Noise-Ultra-high-20V-200mA-PSRR-RFLinear-Regulator-Power-Module/142890461926?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649
much cheaper straight from China
https://www.ebay.co.uk/itm/20V-200mA-Ultra-Low-Noise-LT3042-PSRR-RF-Linear-Regulator-Power-Module-K/283132638364?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

NB I had to desolder the setting resistor from the board and also connect the enable pin with the wire as shown in the picture on my earlier post.
https://www.ebay.co.uk/itm/Vishay-S102C-150K-0-01-Metal-Foil-Resistors/352563629910?ssPageName=STRK%3AMEBIDX%3AIT&var=621889264013&_trksid=p2060353.m1438.l2649

[kado]

jbp

thanks for the detailed information and links!

Bye the way: where could one buy those fine dual banana cables? Never find them until now.

Karsten


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[jpb]

jbp

thanks for the detailed information and links!

Bye the way: where could one buy those fine dual banana cables? Never find them until now.

Karsten


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They are pomona ones - they are quite expensive new but I buy them if I see them going cheap on ebay - most of the time ebay is more expensive than digikey so beware!:
They come in different lengths e.g.
https://www.digikey.co.uk/product-detail/en/pomona-electronics/2BA-24/501-1696-ND/736429

[cowasaki]

It arrived today - thank you RoadRunner.

I've not insulated it but I've fed it through an LT3042 board I got from ebay to give it a low noise 15V supply.
My 2015 is dead on and my 2000 is reading between 2 and 3 in the last digit.

I'm very pleased though I'm not sure I like the power sockets which seem to require a fat probe to be stuck in rather than the thin wires I was trying to use - perhaps I'm using them wrong.

Sorry for the out-of-focus shot of the board.

Edit : the 2000 now seems steady at 2 and occasionally down to 1 in the last digit.

Do you have the schematic or other information regarding the ebay board?

[jpb]

Do you have the schematic or other information regarding the ebay board?

I don't but it there is a lot of information on the LT3042 datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/3042fb.pdf
I think the board is pretty close to the schematic on the datasheet first page.

I just blundered around unsoldering resistors (and accidentally unsoldering and resoldering a capacitor!) until the output was determined by the external resistor I had soldered. The board is clearly labelled underneath as shown in the attached photo. I think it was R6 I had to unsolder. You can also follow the traces from the chip if your eyesight is up to it (mine is getting a little dodgy now-a-days!)

You also need to connect the enable to positive VIN (check on the datasheet).

It would be neater to just connect enable to VIN on the underside of the board.

I left the resistor legs very long for two reasons - one I thought I might want to use it for something else if it didn't work well on the board and secondly to avoid over-heating it when soldering (I also used a copper crocodile clip as a heat sink).

Of course, you can also fit a switch between VIN and Enable if you want to turn the thing on and off.

[jpb]

I finally got around to putting this in a case:
https://www.eevblog.com/forum/metrology/encasing-of-roadrunner-lm399-10v-reference/

[grizewald]

I've cased mine as well, but as I always power the reference with a bench supply, I didn't need to add any other components, just a plug for the supply and a pair of 4mm plugs for the output.

The dimensions of the board are a little unfortunate as it doesn't fit inside any of the standard Hammond cases which are also tall enough. So, all I did was make a little foam igloo to keep the LM399 and it's leads warm and cosy and 3D print a couple of plastic rails to adapt the board to the case.

[alex-sh]

The dimensions of the board are a little unfortunate as it doesn't fit inside any of the standard Hammond cases which are also tall enough. So, all I did was make a little foam igloo to keep the LM399 and it's leads warm and cosy and 3D print a couple of plastic rails to adapt the board to the case.

Well, I am not so sure about unfortunate dimension. Did you measure it correctly?
This is the same LM399 board cased into Hammond 1455C1201. It is a tight fit though - I do not think you can squeeze in anything else.

[grizewald]

Well, I am not so sure about unfortunate dimension. Did you measure it correctly?

 

With the insulation I added around the LM399 and allowing for the board to be held in the lowest slots on the inside of the case, the remaining 18.5mm in height was not enough. This is why I included the phrase "which are also tall enough".

Maybe your LM399 is mounted closer to the PCB than mine. Did you add any insulation around the reference to quickly stabilise the temperature and avoid convection effects?

[alex-sh]

Maybe your LM399 is mounted closer to the PCB than mine. Did you add any insulation around the reference to quickly stabilise the temperature and avoid convection effects?

I am not sure how close my LM399 is mounted to the board compare to yours.
Yes, mine is fully insulated. In fact I put in a lot of insulation material. However, the problem I had was not height. The problem I had was length. With the banana plugs I have used, it was difficult to put it all together.

[TA2AWX]

Hi, good project. I recently salvaged 2 LM399AH2s from a junkyard board and thinking of making this. I readily have OP07CP opamps in my parts bin, do you think they could replace LT1001 in this project?

Thanks!

[ZhuraYuk]

Won't variable resistor make tuned voltage incorrect due high TCR and overall instability of element, like vibrations? Maybe better decision to measure it resistance and then replace with some vishay constant value resistors?

[pizzigri]

Roadrunner, do you still have these boards?

[RoadRunner]

Roadrunner, do you still have these boards?

Yes only few boards are still available , As there is worldwide shortage of LM399AH.

[pizzigri]

Received my board a couple of days ago, and placing it in a box with good insulation.
A question: where to get GOOD female contacts for input/output?
Another thing, is there a problem in using an all-plastic ox or do I need also shielding?
Heres a pic of how the ref is coming along.
I made a double insulation chamber, and suspended the board using vibration absorbing rubber loops used in microphones.

[coromonadalix]

Good setup,  the board suspenders are killer 

I dont think the lm399 is prone to emi interferences ?  personally i would have used an metallic box,  but if the box  is not mishandled or thrown on the floor it should be ok ?

gurus here may have another view(s) ?

[cowasaki]

I have a metal box within a rack unit in my workshop which contains the reference.  I'm have built a decent 15v power supply and a circuit which has Botha lower and higher temperature setting.  The temperatures are correct with my platinum probe to the keithley and the heater kicks in as soon as the temperature dips below the minimum and turns off as the temperature goes above the maximum.  BUT I have a big over shoot in temperature which I believe is probably due to the heater element I am using which is probably too large.

Question 1 - What is the best range to use?

Question 2 - What is a reasonable heating element?

[vutt]

I'm one of your customers who bought it this month. Package however is still in transit. But bunch of Hamon Resistor Divider Network components are already waiting on my desk.

Question: Do we have long term specification for this open source solution? LM399 specification is one part of it, but this particular board have also other components. Original revision was built 3 year ago? Do you RoadRunner have any feedback regarding long term spec drifts?
Would it be as precise let's say 5 years from now after initial calibration?

[jpb]

I'm one of your customers who bought it this month. Package however is still in transit. But bunch of Hamon Resistor Divider Network components are already waiting on my desk.

Question: Do we have long term specification for this open source solution? LM399 specification is one part of it, but this particular board have also other components. Original revision was built 3 year ago? Do you RoadRunner have any feedback regarding long term spec drifts?
Would it be as precise let's say 5 years from now after initial calibration?

I use mine to compare my two 6.5 digit meters and also my other meters.
To do a proper drift analysis would probably require measurements with a 7.5 or 8.5 digit meter - my anecdotal experience is that mine has drifted much less than the variation I see during my meters warm up period or the variation with temperature when measured on different days.

[vutt]

Thanks for your feedback jpb!

I think it addresses my concerns regarding this reference as long term investment. I recently acquired vintage Advantest R6551 5 1/2 digit 319999 count unit with basic 0.01%-ish 1y VDC precision with manual calibration possibility. It looks like this vref would be perfect match to calibrate that.
I'm also not ruling out in longer future getting 6.5 DMM. While this ref is not meant to be calibration standard for 6.5 level DMM-s it still work out as basic DMM check.

[jpb]

If you look at this thread:

https://www.eevblog.com/forum/metrology/encasing-of-roadrunner-lm399-10v-reference/msg2996906/#msg2996906

I show some measurements when I first got the device and some taken much more recently which will give you some idea of the drift I've seen, though, as I say there is quite a lot of variation from temperature differences and warm up time so I cannot claim any great accuracy in what I report.

I'll stick my meters on to warm up today and give you a new reading (I've left the reference powered on for the last several months now - I'm trying to get it to a settled state).

I should add that I've not had my meters calibrated, the most recently calibrated one is the Keithley 2000 which I bought from Top Loser which came with a calibration. The 2015 is one I bought off ebay and re-capped and is probably a few decades old but I find they both seem to agree with each other to a difference of only about 40 microVolts once they have warmed up i.e. a difference of 4 in the last place unless you extend the digits as described in this thread:
https://www.eevblog.com/forum/metrology/getting-one-more-digit-from-a-6-5-digit-meter-without-using-gpib/msg1300529/#msg1300529

[jpb]

I've now added a reading from today to the other thread linked above.
I think that the reference can be considered to be within about 100 microVolts (I know this isn't very scientific and will raise the ire of proper metriologists) but that seems to be the range of my readings over a longish time and varying temperatures and humidity levels.

The actual accuracy/stability might well be much better than that but I think 100 microVolts is 10^-5 so good for 4.5 digit meters and it is good to check the agreement between my two 6.5 digit meters if not their absolute accuracy.

For 5.5 digit meters a more serious set of measurements alongside temperature would need to be made and I'm not dedicated enough to do so at the present time.

[vutt]

My board arrived. Working as advertised.
btw. is that normal that LM399H cover is slightly loose.

Anyways it's working, so I'm not complaining. Decided to build little volt ref magic box with loose components I had at hand in lockdown situation. Pic attached.