DIY Low Thermal EMF Switch/Scanner for Comparisons of Voltage and Res. Standard

[doktor pyta]

Some photos showing details of RFS135B.

[doktor pyta]

++

[doktor pyta]

Just before putting into enclosure.

[branadic]

Time to measure the performance like thermal emf Nice build.

-branadic-

[2N3055]

I'm making doktor pyta scanner too.. Two boards for a 16ch/4W scanner config..
All parts are here and ready. Time to get busy.

[2N3055]

After few evenings of soldering i assembled scanner..
It has two boards, so I can do 4 wire x 16 channels..

All parts were easily ordered from TME and Farnell.
Boards are good quality, switching part with relays is isolation routed form rest of board..
Power consumption is very low, so no problem for DMM logic output to drive it...

Now I need to make cables. 
Will be testing it with my 7562 Yokogawa (I'm waiting for a connector for Yokogawa, it doesn't use BNC) and Rigol DM3068.
I will also do some timing testing to see what are parameter boundaries..

Regards,

Sinisa

[cellularmitosis]

Very cool!  Where did you order the kit?

[plesa]

RJ45 is OK for low EMF scanner? What type you have?

[2N3055]

It's not really a kit. It's Doktor pyta scanner from above, I just got PCBS , and front and back panels for a standard Hammond box from him. His schematic is above in this topic..
Regards,
Sinisa

[2N3055]

Hi Plesa, long time no hear...

Yeah, it looks too easy, but...

That connector is very compact, and isothermal by virtue of that, it is shielded (also acting metal thermal spreader on the outside). Contacts are very close, made of gold plated beryllium copper, very low thermal mass, attached directly to board, being isothermal there... Also SFTP cable is twisted pairs, pure copper, individually shielded with aluminium, that acts as shield and thermal spreader...

Generally, doktor pyta went with making everything as isothermal and low mass as it can be, instead of relying on exotic components. We shall see how well it does, but in his measurements so far, he got double digits nV TEMF..
I don't have equipment at the moment to measure whatever TEMF it makes now.. My crystal ball sees more equipment in my future... Nanovoltmeter would be nice....:-)

What these connectors are not is that they are not low leakage and are SELV only (max 60V)...
I would think it cannot be used for precision measurement of 100 Gohm with 500V..

My use for this scanner is measuring voltage refs (up to 20V), 4W resistor comparisons (up to 10Mohms), and 4W PT100/1000 and precision thermistor measurements. It should do this with less influence than 6.5 digit DMM can see.
That is more than OK for me now..

Of course, I will keep you guys posted.

Regards,

Sinisa

P.S. This is the connector type..

[2N3055]

I did a little test, scanner works with voltages of more than 4,5 V and low going pulses from longer than cca 950 nsecs.

So 4.5V and up and more than 1 usec pulse going low works reliably.

When I make cables I will also measure brake-before-make interval too...

Regards,

Sinisa

[doktor pyta]

Hi Sinisa,

few observations after some time of operation:

1. if You change D1  (BAS86) to eg. LL41 You may reduce minimum operating voltage by approx. 200mV.
2. as I warned before custom cable must be prepared. This is due to wire allocation in standard Ethernet cable (see RJ45 ethernet cable pair drawings in google)
3. Power-On-Reset is weak point of the design. It allows to charge capacitors and turns on channel 1. The problem is that You never know which channel remained active at the end of previous operation cycle. The best ad hoc solution after powering is to press NEXT button for 16 or 32 times to make sure all relays are in known position.
4. current revision of  the schematics below.

[2N3055]

Hi Sinisa,

few observations after some time of operation:

1. if You change D1  (BAS86) to eg. LL41 You may reduce minimum operating voltage by approx. 200mV.
2. as I warned before custom cable must be prepared. This is due to wire allocation in standard Ethernet cable (see RJ45 ethernet cable pair drawings in google)
3. Power-On-Reset is weak point of the design. It allows to charge capacitors and turns on channel 1. The problem is that You never know which channel remained active at previous operation cycle. The best ad hoc solution after powering is to press NEXT button for 16 or 32 times to make sure all relays are in known position.

1. Yes, of course..
2. Yes, I didn't mention this. Cables are not made as standard ethernet cables, were pairs are intermixed. Here pairs are in PAIRS, so cables have to be crimped 1-2  3-4  5-6  7-8 .. That's why you have to make them otherwise I would just cut few ethernet cables in half.
3. I did notice that from schematic. I also went for a scan through channels once on startup and then reset.... Not a problem for now. I have some ideas how to work around that . I'll let you know...

Thanks for your help..

Regards,

Sinisa

[MiDi]

When thinking about low thermal emf scanner and building my own, I came across with thermocouple scanner cards from Keithley (7402, 7057A) rated at <1µV thermal EMF.

They use Coto 3400-0066 dual reed relays which could be predecessor of low thermal EMF 3600-Series.
Connection to a µC should be quite easy as the coils are routed to edge card connector directly and are working with 5V supply.
The signal path is fully guarded - not shure if this is good or rather bad for this purpose (capacitance between guard and signal).

At first look they seem to be suitable for the job, what do you think?

Keithley 7057A:


Keithley 7402 top


Keithley 7402 bottom

[niner_007]

this is an interesting idea and an interesting project, I'm working on something similar but my goal is a little bit different, I just need a compact (think 5cm x 8cm) standalone scanner card, in my case I need it for monitoring cell voltages and general power electronics projects, likely an handheld DMM (289, U1270 etc) will be used as well; somehow I independently end up choosing the exact same relays, G6K, but I'm using an MAX4821 relay driver, all controlled by an STM32 with an USB interface; power is supplied over USB; the general idea is that the scanner, and the DMM (over GPIB, or USB/serial etc) are both connected to a raspberry pi, which does the logging and uses the scanner to cycle the channels, then read the DMM samples

[dkozel]

Thanks to everyone who's shared information here! I'm interested in switching four references between two DMMs so i suspect I'll have to make something custom based on all the parts and circuits described here.

doktor pyta, thank you for sharing your design. Have you or 2N3055 measured the thermal EMF in use? Any chance you have more PCBs available, I could do the end plates of the enclosure myself.

MiDi, that looks like a really interesting option. Did you end up getting one and testing it?

niner_007, How many inputs are you designing for? I'm also using a RPi as the brains of the operation and a USB based scanner would be ideal. I'm considering getting the Keithley Thermocouple card that MiDi mentioned and designing a little STM board to interface with it, but it would be much better to have a complete design.

cellularmitosis, did you continue on with your design?

[MiDi]

MiDi, that looks like a really interesting option. Did you end up getting one and testing it?

Got two 7402 a while back, but since then I had other things going on, so no test so far.
Want to build custom case with custom drive incl. an Arduino nano.
Have to make design and order parts, so this will hopefully go for christmas holiday...

[niner_007]

I'm working on a 8 independent channels one

[niner_007]

Here is some progress on my scanner card, this one is not designed for low EMF, I'm working on a similar design, with same features, larger size and with COTO 3500 relays with < 0.5uV EMF noise

Some features:

- tiny size
- USB powered
- communication/control over USB
- daisy channable, one scanner via 1 USB port can control up to 10 other scanner (power included), thus expandable to 80 or more channels
- outputs can be connected

[dietert1]

Another DIY low thermal scanner including EMF measurement is here: https://www.eevblog.com/forum/metrology/scannermultiplexers-for-voltage-references/msg3569019/#msg3569019.

Regards, Dieter

[branadic]

I this week received a low thermal scanner from doktor pyta. I wonder if someone with such a device has already implemented a remote control for a fully automated setup?

-branadic-

[branadic]

This is my take on the "DIY Low Thermal EMF Switch/Scanner for Comparisons of Voltage and Res. Standard".
In Dezember of last year I received a unit from doktor pyta, again, thanks for that. It took me a while to carefully prepare the test setup and to perform the measurement.
The scanner was configured as 4x16 (jumper D and F on Master PCB, jumper F on Slave PCB).

I equipped the output cable with a LEMO FFA.1S.304.CLAC62 connector that I had at hand, with the top board of the scanner connected to channel A and the bottom board connected to channel B of a 34420A Nanovoltmeter.
To control the scanner I've added a level shifter based on 2x 10k resistor and 1x 2N7000 to my Raspberry Pi and used one of the GPIO pins to control the scanner via Python:

   import RPi.GPIO as GPIO
   GPIO.setmode(GPIO.BCM)
   pin = 17
   GPIO.setup(pin,GPIO.OUT)
   GPIO.output(pin, GPIO.HIGH) #set Pin HIGH (ON)
   time.sleep(0.2)
   GPIO.output(pin, GPIO.LOW) #set Pin LOW (OFF)

while capturing the readings of the 34420A via GPIB (20 NPLC).
The 32 input channels of the scanner were shorted using clean bare wires. I twisted together HI/LO of each channel and crimped them with some 10 mm long copper tubes.
I then ran a program capturing 20 samples per channel before switching to the next and repeated the round 20 times.

Attached are very first results, blue channel A of the 34420A and red channel B of the 34420A, with each additional round plotted in lighter color.
We can see some offset and an offset difference between channel A and B of the 34420A, while it is yet unclear where this offset and the difference between both is coming from, could also be an offset from the Nanovoltmeter itself, I have to check on that.
Apart from that yet unkown offset, the differences between channel 1-16 of top and channel 1-16 of the bottom scanner board are fairly small. Note, these are just very first results and not a comprehensive study or analysis, but the results look pretty amazing to me.

-branadic-

[iMo]

Double check your cables (those twisted pairs) are made of copper (a lot of cheap copper clad aluminum used in those patch cables).. 

[branadic]

No worries here, the twisted cables are bare copper I need to create a short to rule out the initial offset, connectors are on their way.

-branadic-

[branadic]

I today created a short myself using a FFA.1S.304.CLAC62 connector. I then performed a short measurement, with the results below (100 samples @ 20 NPLC and 100 samples @ 100 NPLC, blue again Ch1 of 34420A and red Ch2 of 34420A).
I corrected the measurements taken before with the scanner at the 34420A and subtracted the measured offsets, result below too.
That is as far as I can go for this week.

-branadic-