Hi Mark,

I would recommend two options:

1) Use a thermocouple with very little mass (true of most of them).
Then, wrap the very tip of it in a thin layer of Kapton tape or other
very thin insulating material. Finally, use aluminum or copper tape to
stick this insulated TC to the surface whose temperature you are
measuring. The metal tape does a very good job of adhering and it
provides a very good thermal connection, conducting heat around the
back of the TC and "bathing" it in the temperature it is supposed to
read. Because the mass of the thermocouple is so low, and the wire
connected to it so thin, very little heat can be conducted away from
the TC and the small amount of thermal insulation created by the
electrically insulating layer can be neglected.

2) I use a Pico Technologies TC-08 ( see
http://www.picotech.com/thermocouple.html ) and I don't recall ever
having a problem with common mode voltage. That said, I just looked it
up and it only has a +/-7.5V common range so I guess by the numbers it
does not meet your need.

Sean


On Thu, Apr 29, 2010 at 10:37 AM, Mark E. Skeels
<mskeels@competitionelectronics.com> wrote:
> Hi, List,
>
> I'm trying to make some temperature measurements on =A0a project I'm
> working on.
>
> My NI 4350 thermocouple interface has a max permissible common mode
> range of +-2.5VDC in order to make proper measurements, so that means
> that I can't just solder the thermocouple to a lead or tab of some of my
> TO-220 devices because the voltage on these parts will be too high to
> meet this requirement.
>
> Space is pretty limited around the TO-220 packages as they are kind of
> "buried" inside of a heat sink assembly.
>
> As you can imagine it would not be good if one or more thermocouples
> came loose during testing and started flopping around in close proximity
> to the PWM output stages.
>
> I am thinking I am going to have to try to use some kind of thermally
> conductive adhesive to attach my thermocouples to the epoxy part of the
> package.
>
> I have never done this.
>
> Can anybody recommend a suitable adhesive?
>
> Or some other idea?
>
> Or maybe somebody knows of a sub $200, 4 channel thermocouple interface
> that can make measurements in the presence of 26VDC common mode.
>
> I found a few, but they are expensive, especially considering that I
> already have a really nice National Instruments interface.
>
> --
> Mark Skeels
> Engineer
> Competition Electronics, Inc.
> TEL: 815-874-8001
> FAX: 815-874-8181
> www.competitionelectronics.com
> --
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