Author Topic: DIY Compact DC Voltage Standard  (Read 3316 times)

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Offline ltz2000

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DIY Compact DC Voltage Standard
« on: November 22, 2015, 05:31:31 pm »

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;image

The circuit is practically identical to the good old Geller SVR.

http://www.gellerlabs.com/Voltage_References/SVRTH/SVR%20THR%20Sch.jpg

The 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.

 

Offline Andreas

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Re: DIY Compact DC Voltage Standard
« Reply #1 on: November 22, 2015, 05:56:47 pm »

The circuit is practically identical to the good old Geller SVR.

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 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 regards

Andreas




 

Offline TiN

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Re: DIY Compact DC Voltage Standard
« Reply #2 on: November 22, 2015, 07:12:02 pm »
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).
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Offline ltz2000

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Re: DIY Compact DC Voltage Standard
« Reply #3 on: November 23, 2015, 09:16:32 pm »
I would use a 10-100nF capacitor at the output if the reference is stable with capacitive loads. (most are).

Good point. Now there is an output capacitor "C4". I also added "R8" in series with the tempco trimmer.

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.

Diecast 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.
 

Offline ltz2000

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Re: DIY Compact DC Voltage Standard
« Reply #4 on: November 24, 2015, 08:25:09 pm »
Alternative idea - make it in way, so binding posts coming from the side, to allow sticking reference directly into regular bench DMM inputs.

There is no need to change anything. Just replace the output binding posts with these (and spacers between the enclosure and the printed circuit board).




Also second module could be battery supply (2 x 9V cells + low noise LDO, like LT3042 /spec 0.8uVrms/ or similar).

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.

The 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.



 

Offline Andreas

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Re: DIY Compact DC Voltage Standard
« Reply #5 on: November 24, 2015, 10:14:55 pm »
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 regards

Andreas
 


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