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'[EE] Thermocouple PCB mount connectors'
2010\12\06@235947 by Jon Chandler

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I'm looking for PCB-mount type K thermocouple connectors but I'm not having much luck finding anything in the US.  I just need a small quantity for a prototype but if the project turns into something workable, I'll want to be able to order more.  As far as I can tell, my traditional suppliers like Digikey and Mouser don't have anything.  Omega has some that will work of a couple bucks a piece....in a package of 100.  Way more than I need.

Hamitherm in the Netherlands has some perfect options, but I don't think they have US distribution.

http://www.hamitherm.com/pdf/01.04.01_CMJ-xxPF%20Thermocouple%20Connector%20Miniature%20PCB.pdf

http://www.hamitherm.com/pdf/01.04.03_CMJ-xxPS%20Thermocouple%20Connector%20Miniature%20PCB.pdf


Any ideas for a US source?

Thanks,

Jo

2010\12\07@004303 by Spehro Pefhany

picon face
At 11:59 PM 12/6/2010, you wrote:
>I'm looking for PCB-mount type K thermocouple connectors but I'm not
>having much luck finding anything in the US.  I just need a small
>quantity for a prototype but if the project turns into something
>workable, I'll want to be able to order more.  As far as I can tell, my
>traditional suppliers like Digikey and Mouser don't have anything.
>Omega has some that will work of a couple bucks a piece....in a package
>of 100.  Way more than I need.
>
>Hamitherm in the Netherlands has some perfect options, but I don't think
>they have US distribution.
>
>http://www.hamitherm.com/pdf/01.04.01_CMJ-xxPF%20Thermocouple%20Connector%20Miniature%20PCB.pdf
>
>http://www.hamitherm.com/pdf/01.04.03_CMJ-xxPS%20Thermocouple%20Connector%20Miniature%20PCB.pdf

Of course the color code should be yellow for type "K" in US/Canada (ISA standard).

>Any ideas for a US source?

The Omega price of $19 for 5 pieces doesn't sound too bad to me.. but you could always
try asking Hamitherm's Baltimore office.

Hamitherm Inc.
484 Severnside Drive
Severna Park, MD 21146 USA
Tel: 410-7292919
Fax: 410-7292926
Email: spam_OUTinfoTakeThisOuTspamhamithermusainc.com

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2010\12\07@015057 by Jon Chandler

picon face
Spehro Pefhany wrote:
{Quote hidden}

The only PCB mount connectors I found at Omega were in a 100 pack.  Do you have an Omega part number?

Thanks,

Jon

2010\12\07@030003 by peter green

flavicon
face
Jon Chandler wrote:
> I'm looking for PCB-mount type K thermocouple connectors but I'm not
> having much luck finding anything in the US.  I just need a small
> quantity for a prototype but if the project turns into something
> workable, I'll want to be able to order more.  As far as I can tell, my
> traditional suppliers like Digikey and Mouser don't have anything.  
> Omega has some that will work of a couple bucks a piece....in a package
> of 100.  Way more than I need.
>   Do you actually need a special thermocouple connector? Last time I needed to hook up a thermocouple I just used an ordinary micro spox.

Afaict all you achive by using a special connector is moving the reference junction from the cable-connector interface to the connector-pcb interface. That gets the reference junction slightly closer to whatever temp sensor you are using as a reference but I doubt it will make much difference in practice.

{Quote hidden}

>

2010\12\07@031250 by Jon Chandler

picon face
peter green wrote:
> Do you actually need a special thermocouple connector? Last time I
> needed to hook up a thermocouple I just used an ordinary micro spox.
>
> Afaict all you achive by using a special connector is moving the
> reference junction from the cable-connector interface to the
> connector-pcb interface. That gets the reference junction slightly
> closer to whatever temp sensor you are using as a reference but I doubt
> it will make much difference in practice.
>
>    True enough.  I'd like to use a standard connector so that I can use standard probes.  Not so
important for my application but if others use my board it will be a nice feature.

The Omega connectors feature a clip for a TO-92 temperature sensor for the most accurate
temperature measurement - I'm not that concerned but it's a nifty feature.

Thanks,

Jon

2010\12\07@074639 by Spehro Pefhany

picon face
At 01:50 AM 12/7/2010, you wrote:

>The only PCB mount connectors I found at Omega were in a 100 pack.  Do
>you have an Omega part number?

http://www.omega.com/Temperature/pdf/PCC-OST-SMP.pdf

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2010\12\07@082904 by PICdude

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face
Same-ish here.  I use regular screw-terminal blocks for this, and  ensure that the reference junction is inside the enclosure, with the  temp sensor next to the terminal block, on the side the wires connect  to.

Cheers,
-Neil.



Quoting peter green <plugwashspamspam_OUTp10link.net>:

> Do you actually need a special thermocouple connector? Last time I
> needed to hook up a thermocouple I just used an ordinary micro spox.
>
> Afaict all you achive by using a special connector is moving the
> reference junction from the cable-connector interface to the
> connector-pcb interface. That gets the reference junction slightly
> closer to whatever temp sensor you are using as a reference but I doubt
> it will make much difference in practice.
>

2010\12\07@092753 by Sean Breheny

face picon face
Is there really one reference junction, though? I am under the
impression that any joint of dis-similar metals in the thermocouple
circuit will cause a voltage difference which is temperature
dependent. This includes even things like bond wire connections inside
the IC used to read the thermocouple. So, I'm not sure that it makes
sense to couple the cold junction sensor so closely with the TC
connector unless you are also coupling it closely with the rest of the
electronics in the circuit which reads the TC.

I am speaking as a heavy user of TCs who has never designed a circuit
to interface with them so perhaps someone with more experience here
can chime in.

Sean


On Tue, Dec 7, 2010 at 8:29 AM, PICdude <@spam@picdude3KILLspamspamnarwani.org> wrote:
{Quote hidden}

>

2010\12\07@100916 by Mike Harrison

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face
On Tue, 7 Dec 2010 09:27:52 -0500, you wrote:

>Is there really one reference junction, though? I am under the
>impression that any joint of dis-similar metals in the thermocouple
>circuit will cause a voltage difference which is temperature
>dependent. This includes even things like bond wire connections inside
>the IC used to read the thermocouple. So, I'm not sure that it makes
>sense to couple the cold junction sensor so closely with the TC
>connector unless you are also coupling it closely with the rest of the
>electronics in the circuit which reads the TC.
>
>I am speaking as a heavy user of TCs who has never designed a circuit
>to interface with them so perhaps someone with more experience here
>can chime in.
>
>Sean


No - the voltage you get is the difference between the hot and  cold junctions. After the
cold-junction terminal block, all connections are symmetrical  - terminal to PCB, pcb to chip
package, bond wires etc., so any thermoelectric voltages get cancelled out.  Special connecitons are used where you don't wanr to createa cold junction, so the contatcts match
the TC metal composition. As long as you know the temp at the pont where the TC wires transition to
normal metals, you know everything needed to derive the hot junction temp.
{Quote hidden}

>> -

2010\12\07@110018 by Kerry Wentworth

flavicon
face
Imagine you had a thermocouple made of metal X and metal Y, and that metal X had a Seebeck coefficient of 1uV/degree and metal Y had a coefficient of 1.5uV/degree.  The coefficient of the XY thermocouple is therefor .5uV per degree.

The hot end is at 125 degrees and the cold end is at 25 degrees.

You define the cold Y end as 0 volts.  The hot Y end is at 150uV.  The hot X end must also be at 150uV, since they are connected.  The cold X end is at 100uV negative relative to the hot X end, and therefor at +50uV relative to the Y cold end.  You have an IC temperature sensor here.  You also transition to copper in the PC board traces.  The traces make their way across the board, which has a temperature differential that causes the copper to exhibit +10uV difference.  The signal from the X hot end is now 60uV, and the signal from the Y cold end is 10uV.  The difference is still 50uV, so when you divide the 50uV by .5uV per degree of our XY thermocouple, and add the 25 degrees of the cold junction, you get 125 degrees for the hot end.
As long as the X and Y inputs of the A/D are at the same temperature, and the X and Y junctions at the cold junction are the same, they will cancel out. regardless of the temperature difference between the cold junction and the A/D.

Kerry


Sean Breheny wrote:
{Quote hidden}

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2010\12\07@111434 by Spehro Pefhany

picon face
At 09:27 AM 07/12/2010, you wrote:
>Is there really one reference junction, though?

The main effect is from the pair of junctions where the thermocouple
materials connect to (typically) copper.

>I am under the
>impression that any joint of dis-similar metals in the thermocouple
>circuit will cause a voltage difference which is temperature
>dependent. This includes even things like bond wire connections inside
>the IC used to read the thermocouple.

Usually that is a relatively small effect, and usually it (mostly)
cancels out in a well-designed circuit. If you want to invite problems,
place high power devices (eg. an SSR or a hot linear regulator) on the
same PCB such that there are large gradients.

You need a good 'cold junction' (which might actually be the warmer
junction in some applications) measurement to get an accurate reading.
You'll get around one degree error for every degree your sensor and
measurement of the sensor does not match the actual 'cold junction'
temperature (with most thermocouples- pathological examples like 'B'
notwithstanding).

>  So, I'm not sure that it makes
>sense to couple the cold junction sensor so closely with the TC
>connector unless you are also coupling it closely with the rest of the
>electronics in the circuit which reads the TC.

You need to measure the points where the T/C alloys connect to
copper-- ideally all three are exactly the same temperature. It
depends on the accuracy you need. For better than a few degrees
C you probably shouldn't be using the PCB type of connector at all.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
TakeThisOuTspeffEraseMEspamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2010\12\07@125436 by Olin Lathrop

face picon face
Sean Breheny wrote:
> Is there really one reference junction, though? I am under the
> impression that any joint of dis-similar metals in the thermocouple
> circuit will cause a voltage difference which is temperature
> dependent. This includes even things like bond wire connections inside
> the IC used to read the thermocouple. So, I'm not sure that it makes
> sense to couple the cold junction sensor so closely with the TC
> connector unless you are also coupling it closely with the rest of the
> electronics in the circuit which reads the TC.

Temperature differences don't matter anymore when both conductors are made
of the same material, like both become copper traces on a PC board.  For
best accuracy you want both junctions, each between one of the thermocouple
wires and the PC board, to be at the same known temperature.  The
thermocouple voltage then tells you the difference between this temperature
and the junction at the other end of the cable where the two dissimilar
conductors meet.  Add that to the known absolute temperature and you get the
absolute temperature at the end of the thermocouple.



********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000

2010\12\07@133447 by Dave Tweed

face
flavicon
face
Olin Lathrop wrote:
> Sean Breheny wrote:
> > Is there really one reference junction, though? I am under the
> > impression that any joint of dis-similar metals in the thermocouple
> > circuit will cause a voltage difference which is temperature
> > dependent. This includes even things like bond wire connections inside
> > the IC used to read the thermocouple.
>
> Temperature differences don't matter anymore when both conductors are
> made of the same material, like both become copper traces on a PC board.

Sean is right. You have many different junctions throughout the circuit, and
you need to make sure that corresponding junctions are at the same temperature
throughout. It never becomes a "don't care". However, in most cases,
corresponding junctions are pretty much implicitly at the same temperature
because of the physical constraints of building the circuit.

For example, there isn't going to be much temperature difference between the
two junctions of gold bond wires to aluminum pads on the surface of the die
unless there's a significant source of localized heat on the die itself. A
similar argument holds for the junctions between the chip lead frame and the
pads on the PCB, and so on throughout the circuit.

One feature of speciallized thermocouple connectors is that they have a low
thermal resistance path (but high electrical resistance) between the two
contacts, in order to keep them at the same temperature. As long as that's
true, it doesn't matter what that temperature actually is.

-- Dave Twee

2010\12\07@144839 by Spehro Pefhany

picon face
At 12:55 PM 07/12/2010, you wrote:
>Sean Breheny wrote:
> > Is there really one reference junction, though? I am under the
> > impression that any joint of dis-similar metals in the thermocouple
> > circuit will cause a voltage difference which is temperature
> > dependent. This includes even things like bond wire connections inside
> > the IC used to read the thermocouple. So, I'm not sure that it makes
> > sense to couple the cold junction sensor so closely with the TC
> > connector unless you are also coupling it closely with the rest of the
> > electronics in the circuit which reads the TC.
>
>Temperature differences don't matter anymore when both conductors are made
>of the same material, like both become copper traces on a PC board.  For
>best accuracy you want both junctions, each between one of the thermocouple
>wires and the PC board, to be at the same known temperature.  The
>thermocouple voltage then tells you the difference between this temperature
>and the junction at the other end of the cable where the two dissimilar
>conductors meet.  Add that to the known absolute temperature and you get the
>absolute temperature at the end of the thermocouple.

Because of nonlinearity, that's a bit of an oversimplification if you
really want accurate readings from a thermocouple.

To get an accurate reading you need to read the TEMPERATURE at the
cold junction, then transform that (nonlinear) into a VOLTAGE, which
is then added to the VOLTAGE read from the thermocouple, and then use
the inverse (nonlinear) transform to find the corresponding TEMPERATURE
at the hot end. The only fly in the ointment is a T/C which is not
monotonic (eg. B).

Analog implementations cut corners and assume linearity around room
temperature (a really poor assumption if things get cold, for many
thermocouple types) and limit variations about room temperature so
you get a sort-of accurate reading, but it's not exact and there is
little excuse for doing this sort of hack in the digital realm.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2010\12\07@172058 by Mike Harrison

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face
On Tue, 07 Dec 2010 13:34:46 -0500, you wrote:

>Olin Lathrop wrote:
>> Sean Breheny wrote:
>> > Is there really one reference junction, though? I am under the
>> > impression that any joint of dis-similar metals in the thermocouple
>> > circuit will cause a voltage difference which is temperature
>> > dependent. This includes even things like bond wire connections inside
>> > the IC used to read the thermocouple.
>>
>> Temperature differences don't matter anymore when both conductors are
>> made of the same material, like both become copper traces on a PC board.
>
>Sean is right. You have many different junctions throughout the circuit, and
>you need to make sure that corresponding junctions are at the same temperature
>throughout. It never becomes a "don't care". However, in most cases,
>corresponding junctions are pretty much implicitly at the same temperature
>because of the physical constraints of building the circuit.

...and if your required accuracy is such  that these things matter, you probably should be  using
PT100s instead of thermocouples  anyway!

2010\12\08@043102 by alan.b.pearce

face picon face
> ..and if your required accuracy is such  that these things matter, you
probably
> should be  using PT100s instead of thermocouples  anyway!

With 4 wire connections, and maybe even constantan or other wiring that
doesn't have good thermal conductivity for all 4 wires.
-- Scanned by iCritical.

2010\12\08@060657 by Xiaofan Chen

face picon face
On Wed, Dec 8, 2010 at 3:49 AM, Spehro Pefhany <speffEraseMEspam.....interlog.com> wrote:
> Because of nonlinearity, that's a bit of an oversimplification if you
> really want accurate readings from a thermocouple.
>
> To get an accurate reading you need to read the TEMPERATURE at the
> cold junction, then transform that (nonlinear) into a VOLTAGE, which
> is then added to the VOLTAGE read from the thermocouple, and then use
> the inverse (nonlinear) transform to find the corresponding TEMPERATURE
> at the hot end. The only fly in the ointment is a T/C which is not
> monotonic (eg. B).

There is some argument here. I am with you that the Cold Junction
temperature needs to be read, but there are two ways to deal with
that, one is like what you mentioned, transform that into a voltage.
The other is to use it as a temperature and added that to the final reading..

Which one is more correct?

Believe it or not, we have spent a lot of efforts on high accuracy
TC/RTD modules design, but still they receive a lot of complaints
from demanding customers. It is very difficult to get it right for
all use cases.


-- Xiaofa

2010\12\08@070329 by Spehro Pefhany

picon face
At 06:06 AM 12/8/2010, you wrote:
>On Wed, Dec 8, 2010 at 3:49 AM, Spehro Pefhany <EraseMEspeffspaminterlog.com> wrote:
> > Because of nonlinearity, that's a bit of an oversimplification if you
> > really want accurate readings from a thermocouple.
> >
> > To get an accurate reading you need to read the TEMPERATURE at the
> > cold junction, then transform that (nonlinear) into a VOLTAGE, which
> > is then added to the VOLTAGE read from the thermocouple, and then use
> > the inverse (nonlinear) transform to find the corresponding TEMPERATURE
> > at the hot end. The only fly in the ointment is a T/C which is not
> > monotonic (eg. B).
>
>There is some argument here. I am with you that the Cold Junction
>temperature needs to be read, but there are two ways to deal with
>that, one is like what you mentioned, transform that into a voltage.
>The other is to use it as a temperature and added that to the final reading.
>
>Which one is more correct?

The one that yields the exactly correct temperature reading in theory.

The voltage read on the thermocouple is only roughly related to the
difference from ambient temperature because the thermocouple is nonlinear.

You can't just read the T/C voltage and convert it to temperature like we used to do
in the olden days of tables and ice baths unless you maintain the cold junction
at a constant temperature (the old ice bath method) without errors. Imagine
(thought experiment) the hot junction is at 1000 degrees and the cold is at 500..
what will the  error be?

Analog instruments (and I've done far more than my share of those) like the analog
chips take an average mV/K coefficient around room temperature and either add
the mV to the mV read or add the temperature. It's only a rough linear correction.

>Believe it or not, we have spent a lot of efforts on high accuracy
>TC/RTD modules design, but still they receive a lot of complaints
>from demanding customers. It is very difficult to get it right for
>all use cases.

I have spent a lot of effort on designing high accuracy thermocouple
and RTD instruments. I do think I got it right. ;-) In particular the input
T/C type could be switched without any noticeable change in the reading, even
at extreme ambients.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffEraseMEspamEraseMEinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

2010\12\08@072650 by Xiaofan Chen

face picon face
On Wed, Dec 8, 2010 at 8:03 PM, Spehro Pefhany <RemoveMEspeffspam_OUTspamKILLspaminterlog.com> wrote:
>>Believe it or not, we have spent a lot of efforts on high accuracy
>>TC/RTD modules design, but still they receive a lot of complaints
>>from demanding customers. It is very difficult to get it right for
>>all use cases.
>
> I have spent a lot of effort on designing high accuracy thermocouple
> and RTD instruments. I do think I got it right. ;-) In particular the input
> T/C type could be switched without any noticeable change in the reading, even
> at extreme ambients.
>

The main problem is that you may indeed have some temperature
difference between the cold junctions. For a multiple channel device,
this is one of the major issues.



-- Xiaofa

2010\12\08@080040 by Spehro Pefhany

picon face
At 07:26 AM 08/12/2010, you wrote:
>On Wed, Dec 8, 2010 at 8:03 PM, Spehro Pefhany <RemoveMEspeffTakeThisOuTspamspaminterlog.com> wrote:
> >>Believe it or not, we have spent a lot of efforts on high accuracy
> >>TC/RTD modules design, but still they receive a lot of complaints
> >>from demanding customers. It is very difficult to get it right for
> >>all use cases.
> >
> > I have spent a lot of effort on designing high accuracy thermocouple
> > and RTD instruments. I do think I got it right. ;-) In particular the input
> > T/C type could be switched without any noticeable change in the
> reading, even
> > at extreme ambients.
> >
>
>The main problem is that you may indeed have some temperature
>difference between the cold junctions. For a multiple channel device,
>this is one of the major issues.

Indeed. It can be done, but the cost is quite high to do it highly
accurately, and it limits the ability to switch sensor types if
you maintain matching alloy connections deeper into the instrument,
again increasing the cost and inconvenience to the end user.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspamspamspamBeGoneinterlog.com             Info for manufacturers: http://www.trexon.com
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