Thermocouple mux

[Spehro Pefhany]

You must be _very_ lonely, if you have to date yorself. ;-)

But I've always kinda been partial to calling myself up on the phone
and asking myself out, you know? Oh yeah, you call yourself up too
huh? Yeah, well one thing about it, your always around. Yeah I know,
yeah you ask yourself out, you know, some class joint somewhere. The
Burrito King or something, you know. Well I ain't cheap you know. Take
yourself out for a couple of drinks maybe. Then there'd be some
provocative conversation on the way home. Park in front of the house
you know. Oh yeah, you smoothly put a little nice music on, maybe you
put on like uh, you know, like shopping music, something thats not too
interruptive you know and then uh slide over real nice and say 'Oh I
think you have something in your eye'.

-- Tom Waits

(Burrito King in LA is pretty good, IMHO).

 

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Well, not much actual use. I designed a killer FTMS preamp in May of
2009, the goal being to detect a single molecule in orbit in a Fourier
Transform Mass Spectrometer. I used the BFT25 diodes in the T/R
section, to get the nanovolt receiver going fast after the kilovolt RF
transmit chirp. But Varian bought IonSpec, and then Agilent bought
Varian, and then somebody decided to get out of the FTMS business. So
I never got to try it.

https://dl.dropbox.com/u/53724080/Parts/BFT25.JPG

i(c) = 5 fA, Vbe=0. Nice. I bet i(cbo) is even lower.

Back-to-back LEDs would have gain. We were just before our time.

And still are.

 

[Marco Trapanese]

Il 26/11/2012 21:58, Spehro Pefhany ha scritto:

Careful, that's not quite correct.

To be a bit more explicit, the thermocouple transfer function or
tables assume a certain reference (cold-junction) temperature, usually
(but not always) 0°C (the temperature of an distilled ice-water
slurry, generally close enough anyway).

The inverse transfer function gives you the voltage you would read if
the "hot" junction was at the terminal block temperature and the cold
junction was at the reference temperature (say 0°C). For terminal
block temperature > 0°C that will be a positive voltage.

You then ADD that voltage to the measured voltage from the
thermocouple, and turn the crank attached to the transfer function to
get the temperature at the hot junction.

For example, suppose the terminal block is at 20°C we measure 4.360mV
from a type R (Platinum-Rhodium) thermocouple.

For 20°C. the corresponding voltage is +111uV

Add 4.360mV + 0.111mV = 4.471mV

Turn the crank, and get +500°C for the hot junction temperature.

Thank you for your clarification!
Marco

 

[josephkk]

Hello!

I'm working on a simple board that uses the recommended Analog's design
to acquire a tc:

http://www.analog.com/en/circuits-from-the-lab/CN0271/vc.html

1. what do you think about this circuit? IMO, it's quiet expensive but
it should be reliable

2. I need to acquire two tc: how to mux both sensors? I mean: I want to
use the same front-end and select what tc to acquire.

Thanks!
Marco

First question, how often do you want to switch versus the full settling
time of the meter? BTW nothing is all that bad with magnetically
latching to-99 relays for this application.

?-)

 

[Marco Trapanese]

Il 28/11/2012 10:26, josephkk ha scritto:

First question, how often do you want to switch versus the full settling
time of the meter? BTW nothing is all that bad with magnetically
latching to-99 relays for this application.

I need to acquire both tcs every second. What do you mean about the
settling time? The electrical settling time should be negligible, and
the thermal one shouldn't matter.

I'm going to use the solution with an AD773 to easily read both tcs and
an ambient temperature sensor for cold-junction compensation.

Marco

 

[John Devereux]

Me too! I thought there must be some trick; so I used zener as C-B junction.

And me. I had a kids electronics breadboard kit thing that came with two
2N2222 or some such, TO92 packages anyway. Within two minutes I had
snapped a leg off both of them, so tried to make new ones out of the
diodes.

That worked really well.

That was my first encounter with electronics, looking back can't believe
I got anywhere with it!

 

[Marco Trapanese]

Il 28/11/2012 12:26, Spuckle ha scritto:

Just out of interest I googled the AD773.

AD says its discontinued, may not matter if your're doing a one off -
just thought I'd pass it on.

http://www.analog.com/en/obsolete/ad773/products/product.html

AD7793

Marco

 

[Spehro Pefhany]

Il 28/11/2012 10:26, josephkk ha scritto:



I need to acquire both tcs every second. What do you mean about the
settling time? The electrical settling time should be negligible, and
the thermal one shouldn't matter.

The digital filter can take hundreds of milliseconds to settle,
depending on the configuration. It's all in the data sheet.

I'm going to use the solution with an AD773 to easily read both tcs and
an ambient temperature sensor for cold-junction compensation.

Marco

Best regards,
Spehro Pefhany

 

[[email protected]]

MMBT3906 vs BAV70: same package, same multimeter test result but not same working... http://www.hqew.net/product-data/BAV70

 

[Fred Abse]

MMBT3906 vs BAV70: same package, same multimeter test result but not same
working... http://www.hqew.net/product-data/BAV70

The former is a transistor, the latter a diode pair.

Not surprising they don't work the same.