Author Topic: DIY Low Thermal EMF Switch/Scanner for Comparisons of Voltage and Res. Standard  (Read 14543 times)

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Offline Mickle T.

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This idea has been widely used in the Prema DMMs. I used a similar scheme to control the sealed relay (RPS-25) in the selfcal module of the 8.5-dig. DVM.
 

Offline Andreas

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Hello Lymex,

thanks for sharing also from my side.

There are many good ideas in the cirquits.
Especially using the DMM trigger as power supply was a fact that
I could not see from the link on volt nuts to 38hot.com.

8. 16*4 scanner for 4 wire standard resistors comparisons

believe or not, 32 latching relays and it's related circuitry power by multimeter VM and consume as little as 0.2mA (When 3458A choose NPLC=50 and AZ on)
I guess it is only for resistors and not for reference voltages.
Or do you have a possibility to get into some "reset" state for the relays after "power on".

The bad news is that Mouser is saying that the OMROM G5AK-237P is "discontinued"  :(  ...

But the good news is that I found an alternative relay, the Panasonic TX-S series  :) ::

I am using Panasonic/Matsushita TQ2 series (the TN2 series would work also) running for about 1-2 weeks from a 9V block including microprocessor.

With best regards

Andreas

 

Offline splin

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The bad news is that Mouser is saying that the OMROM G5AK-237P is "discontinued"  :(  ...

But the good news is that I found an alternative relay, the Panasonic TX-S series  :) ::

I am using Panasonic/Matsushita TQ2 series (the TN2 series would work also) running for about 1-2 weeks from a 9V block including microprocessor.

With best regards

Andreas

Panasonic specify a minimum switching load of 10uA, 10mV which might be a problem for comparing references - but probably not for a few thousand or 10s of thousands cycles! They recommend the SX series for low signal levels but even those specify a minimum of 10uA, 1mV.

I know that they ran into this problem at a company I used to work for - minimum loads aren't always quoted for relays but some contact materials rely on the current disruption to clean the contacts.

I wonder if the contacts get noisy over time or just suffer increasing contact resistance?

Are reed relays better in these regards being hermetically sealed?

The thermal EMF spec is for the TXQ is 5uV and .3uV for the TX-S but those are for the non-latching parts. Are you using latching versions and if so have you measured the thermal EMFs (or tried - I guess it's quite tricky to do)?

Thanks,  Splin
 

Online Vgkid

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What about switching a resistor to ground previous to making the measurement. Or as part of a self test sending a current through all of the relays.
If you own any North Hills Electronics gear, message me.
 

Offline DiligentMinds.com

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What about switching a resistor to ground previous to making the measurement. Or as part of a self test sending a current through all of the relays.

Interesting idea...  I need to think about that...
 

Offline Andreas

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The thermal EMF spec is for the TXQ is 5uV and .3uV for the TX-S but those are for the non-latching parts. Are you using latching versions and if so have you measured the thermal EMFs (or tried - I guess it's quite tricky to do)?

Hello,

I use the TQ2-L2-5V version (the latching type). Otherwise a 1-2 week operation would not be possible from a 9V block.
picture is here: http://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg567203/#msg567203

I am not able to measure thermal EMFs reliable since my 24-Bit ADC is quite noisy. (1uVpp with a integration time of 1 minute).
A partly compensation of the thermal EMFs is done in my setup since I am doing a offset measurement through the relays.
I can actually switch the +/- signal of the output to every + pin of the references or to the common ground. So its possible to do difference measurements between 2 references also.

Some cleaning of the contacts is also done by daily switching of the relays.

With best regards

Andreas
 

Offline quarks

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finally received the Caddock MG745 40MOhm 0.1% resistors.
In my first checks they are well within spec and spread positive and negative in value.
That should help in combining them to a my specific values.
 

Offline timb

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Wow, thanks so much for this thread! I'm working on a scanner myself and it never occurred to me I could do it self powered!


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Any sufficiently advanced technology is indistinguishable from magic; e.g., Cheez Whiz, Hot Dogs and RF.
 

Offline timb

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By the way, these might be a nice stand-in for those Omron relays: http://www.mouser.com/ProductDetail/TE-Connectivity-Axicom/V23079B1201B301/

$3/ea, 5V, DPDT 2-coil bistable in through hole. According to the datasheet thermal emf is <10uV, which is as good as the baseline version of the Coto Low Thermal relays (much better I imagine though as they are latching so no internal heat generation).


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Online 3roomlab

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wow this is a good thread for DIY !
sometimes i understand, sometimes i don't. so i will post nonsense, likely i will spew more nonsense and not even know about it.
 

Offline Gyro

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It would be prudent to stock up on Mercury wetted bistable reed relays for this kind of use while they are still available on ebay / surplus market.

They're the only contact technology capable of truly dry switching, and with stable contact resistance and a cycle life long enough to never have to worry about.

Edit: ...and of course zero bounce.
« Last Edit: December 01, 2015, 11:21:11 PM by Gyro »
Chris

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Offline branadic

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The scanner of the Prema 5017 SC uses Omron G6AK-234P, just for the info. The scanner is specified with thermo forces typ. ±1µV, max. ±2 µV after 1,5 h warm-up. The Prema 2080 scanner, that I had, is specified the same way using the same relays.

http://ohh.de/5017.htm
http://ohh.de/2080.htm
Prema 5000 | Prema 5017 SC | Keithley 181 | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, HDO6054, Keysight 53230A and other goodies at work
 

Offline Mickle T.

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Prema has a nice and bulky series 2000 of precision scanners. The top MUX 2010, that I had, have a max 50 nV of thermo forces.
 

Offline branadic

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Prema has a nice and bulky series 2000 of precision scanners. The top MUX 2010, that I had, have a max 50 nV of thermo forces.

Never heard of that model, the only info I found is the manuel:

http://www.prema.com/index.php/de/service/messgeraetewartung/item/download/116_124e71e4309f5edffbaa2bfc370ef7b5

Smells like a teardown and a lot more photos of your unit, cause less then 50nV (measured on the plugs) is very interesting for the volt nuts here :)
Prema 5000 | Prema 5017 SC | Keithley 181 | Tek 2465A | VNWA2.x with TCXO upgrade and access to: Keysight 3458A, Keithley 2002, Prema 5017 SC, 34401A, 34410A, HDO6054, Keysight 53230A and other goodies at work
 

Online dr.diesel

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Smells like a teardown and a lot more photos of your unit, cause less then 50nV (measured on the plugs) is very interesting for the volt nuts here :)

And another saved eBay search!

Offline doktor pyta

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Inspired by Zlymex, I started to design my own scanner based on the same idea.
The goal is to make some improvements:

1. indicate active channel's number (1 of 16 LEDs). The measurements are made very slowly.
       If there is some bad connection etc. it will be easy to identify.
       I tested green LEDs that work satisfactory biased with 50uA of current.
2. provide Manual control of the channel
3. provide power-on reset (quite tricky part...)
4. use RJ45 connectors, so the user just cuts off-the-shelf CAT6 cable in two pieces and strips wires only on one side.
      This also makes testing of relays easy - just disconnect RJ45 cables and plug in RJ45 plugs with pairs shorted by loop of wire.
      The RJ45 connectors will be hidden inside metal enclosure to ensure thermal equilibrium.
5. use SMD components (prepared to reflow soldering, better EMF performance expected)
6. the board should fit standard TUFxx Fischer Elektronik enclosure
7. It's nice to have a spare relay on board (tribute to Data Proof scaners)
8. the design should be expandable to 2 wire 32 channels; 4 wire 16 channels; 4 wire 32 channels by using the same boards (with some PCB jumpers, desoldering of some components but basically same design)
9. last but not least: the scanner shall provide break-before-make action not to disturb expensive gear and save the relays.


The quiz is: Open the schematic pdf file and find as many bugs as possible.
If anyone is interested in factory made PCB for this project let me know ->PM.
Zlymex has a free PCB if he wants ;)
« Last Edit: December 05, 2017, 10:58:58 AM by doktor pyta »
 
The following users thanked this post: Andreas, Muxr, beanflying

Online beanflying

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Interested! Weighing up another 6 1/2 - 7 1/2 Meter or a 34970A and boards.  :)
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Offline doktor pyta

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First bug found: net NE0 added to J5 connector.
Without it the slave board would not break-before-make.
Schematic upgraded to rev 0.2

Edit:
SW1 pin 6 connected to wrong node.
Schematic upgraded to rev 0.3

Edit: Switched from 74HC4538 to 74HC123 for two reasons:
4538 has built-in power-on reset circuit which would disable generation of a pulse after turning scanner on. This is not wanted in this circuit.
4538 draws much more idle current.
Schematic upgraded to rev 0.4
« Last Edit: December 07, 2017, 09:06:00 AM by doktor pyta »
 

Online beanflying

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Now Interested for interests sake. Santa is bringing me an Agilent Box  8)
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