DIY Compact DC Voltage Standard - Page 1
Forum section: electronics / projects

author: ltz2000
author_rank: Regular Contributor
timestamp: November 22, 2015, 05:31:31 pm
content: Inspiration for the project came from this long but entertaining thread.https://www.eevblog.com/forum/testgear/calibratory-d-105-dc-precision-voltage-reference-standard/My PCB layout follows the component numbering of the schematic reverse engineered and posted by Jay_Diddy_B on page 8. Except the input protection diode which is placed in the positive supply lead instead. (Don't get stucked in the schematic component values.)https://www.eevblog.com/forum/testgear/calibratory-d-105-dc-precision-voltage-reference-standard/?action=dlattach;attach=134408;imageThe circuit is practically identical to the good old Geller SVR.http://www.gellerlabs.com/Voltage_References/SVRTH/SVR%20THR%20Sch.jpgThe PCB layout is quite flexible allowing to build the unit also without the temperature compensation or using other compensation schemes.The board is single sided with large pads, making it suitable for home etching. If anyone needs the Gerber files, please let me know.Binding posts mounted directly on the PCB, no wiring required. Good also from the mechanical rigidity and thermal EMF point of view.The standard is highly immune to changes in humidity, if built in a gasket sealed type enclosure and o-rings fitted under the binding posts. There is also space for a small silica gel bag (on the solder side of the PCB between the binding posts).The thermistor R3 should be glued on the top of the reference chip as shown in the drawing.

author: Andreas
author_rank: Super Contributor
timestamp: November 22, 2015, 05:56:47 pm
content: Quote from: ltz2000 on November 22, 2015, 05:31:31 pmThe geller SVR lacks of EMI-Filtering at the output.I would use a 10-100nF capacitor at the output if the reference is stable with capacitive loads. (most are).Humidity can only avoided by using a reference with hermetically package which is mechanically decoupled from the PCB.All other measures do only increase the time constant. (from days to weeks).But since humidity changes seasonal. You will have different output values in spring and autumn.For most references a stable power supply is also essential.If I would build this I would probably use 2 9V Blocks (no EMI from power supply)+ a 15V regulator built from a LT1763.With best regardsAndreas

author: TiN
author_rank: Super Contributor
timestamp: November 22, 2015, 07:12:02 pm
content: Alternative idea - make it in way, so binding posts coming from the side, to allow sticking reference directly into regular bench DMM inputs.Just like shorting plugs. Also second module could be battery supply (2 x 9V cells + low noise LDO, like LT3042 /spec 0.8uVrms/ or similar).

author: ltz2000
author_rank: Regular Contributor
timestamp: November 23, 2015, 09:16:32 pm
content: Quote from: Andreas on November 22, 2015, 05:56:47 pmGood point. Now there is an output capacitor "C4". I also added "R8" in series with the tempco trimmer.Quote from: Andreas on November 22, 2015, 05:56:47 pmDiecast enclosure and connectors fitted with good quality o-rings actually perform surprisingly well. Even some primary level instruments like JRL Kelvin Varley dividers use o-rings instead of glass-metal feedthroughs.

author: ltz2000
author_rank: Regular Contributor
timestamp: November 24, 2015, 08:25:09 pm
content: Quote from: TiN on November 22, 2015, 07:12:02 pmThere is no need to change anything. Just replace the output binding posts with these (and spacers between the enclosure and the printed circuit board).Quote from: TiN on November 22, 2015, 07:12:02 pmQuote from: Andreas on November 22, 2015, 05:56:47 pmThe original idea was to make a drop-in replacement for the rat nest version Awesome14 is selling in Ebay. He is free to use my design, even though I am not holding my breath that it is going to happen. And I also wanted to keep it so simple that anyone can build it.But I thought the power supply issue too. My solution was a small regulator and battery PCB mounted to the binding post holes with spacers and screws (also powered via the spacers). I can publish the layout, but first I need to find a PCB style 9V battery holder, which is low cost and widely available. Please feel free to suggest.

author: Andreas
author_rank: Super Contributor
timestamp: November 24, 2015, 10:14:55 pm
content: Hello,cheap are only the "standard" 9V clips (30 ct in single quantities).RS-Components has some Keystone parts for PP3 blocks.(search for "PP3 Keystone")But I do not find that they are "cheap" and "widely available"With best regardsAndreas