Author Topic: DIY Low Thermal EMF Switch/Scanner for Comparisons of Voltage and Res. Standard  (Read 20937 times)

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Offline 2N3055

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Update. Try to find a typo (engraving company fault  :palm:)

C'mon, what's a two CH16 between friends...  ?     :-DD



 

Offline doktor pyta

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Some photos showing details of RFS135B.
 
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Offline doktor pyta

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++
 
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Offline doktor pyta

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Just before putting into enclosure.

Offline branadic

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Time to measure the performance like thermal emf :) Nice build.

-branadic-
Prema 5000 | 2x Prema 5017 SC | GenRad 1434-G | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, Keithley 2182A, HDO6054, Keysight 53230A and other goodies at work
 

Offline 2N3055

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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.

 

Offline 2N3055

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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
 

Offline cellularmitosis

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Very cool!  Where did you order the kit?
LTZs: KX FX MX CX PX Frank A9 QX
 

Offline plesa

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

Offline 2N3055

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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
 

Offline 2N3055

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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..

« Last Edit: June 17, 2018, 08:52:53 pm by 2N3055 »
 

Offline 2N3055

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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
 

Offline doktor pyta

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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.
« Last Edit: June 18, 2018, 04:31:05 pm by doktor pyta »
 

Offline 2N3055

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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
 

Offline MiDi

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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


 
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Offline niner_007

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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
« Last Edit: November 25, 2018, 09:01:21 pm by niner_007 »
 

Offline dkozel

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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?
 

Offline MiDi

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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...
« Last Edit: November 30, 2018, 10:24:00 pm by MiDi »
 

Offline niner_007

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I'm working on a 8 independent channels one
 


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