Electrical Isolation of Thermocouples

[mityeltu]

I am working on a project to automate fuse testing. Per the test procedure, thermocouples are attached directly to the fuse furrules. A current is passed through the fuse until thermal stability is achieved. The TCs are going to be connected to an amplifier then to an A/D converter and finally to a microcontroller for analysis.

Here’s the issues I am having that I do not know how to solve.

The power supply we use to drive the current will drive it at whatever voltage is conveient to achieve the curent required. So, if the fuse should open, there is the possiblity of imposing severeal hundred volts onto the thermocouple for a brief time. I say brief, but the few milliseconds will be sufficient to damage/destroy the electronics downstream of the TCs. What I need is a way of electrically islotaing the TCs while permitting the 0-15mV signal from tem to reach the amplifier undisturbed.

I do not want to mechanically isolate the TCs from the fuse as the introduction of any medium between the fuse and TC will introduce errors in temperature measurement. While that is not necessarily a chritical attribute, it will make calibration traceable to NIST more difficult.

Do any of you brilliant guys out there know of a solution that is not off the shelf technology (in other words I need discrete component type stuff) or bulky to the point of needing it’s own desk (I need it all to fit in a box the size of a deck of cads)?

 

[GoldDigger]

My first thought is to put a voltage limiting element (MOV, zener doide, etc.) in parallel with the fuse holder.
The second is to use a power supply with a reasonable maximum output voltage set low enough to prevent damage.
If you were only measuring at one end of each fuse I would make that a grounded point in the circuit.
If you are making an electrical connection to a point that is even a fraction of a volt away from ground the entire TC sensing circuit would have to be floated free of ground anyway, so I am not sure how much isolation or immunity to bias voltage is available in the first place.

 

[gar]

151118-1650 EST

Do as GoldDigger suggested and make the fuse end that you are measuring be grounded. Not a good idea to directly bond the thermolcouple to the fuse. Use a very thin piece of mica and high thermal conductivity epoxy to mount the thermocouple to the fuse.

You are dealing with millivolts from the thermocouples. The multiple thermocouples also need isolation from one another. The mica will do that for any paths back thru the fuses.

Also consider voltage or thermocouple amplifiers with built-in dielectric isolation.

See:
http://www.ti.com/lit/an/sloa123/sloa123.pdf
http://cds.linear.com/docs/en/application-note/an28f.pdf
http://www.dataq.com/products/di-8b/di8b47.html

From here do your own searching. I used --- isolated thermocouple amplifiers

.

 

[n1ist]

I'd use one of the digital thermocouple chips like the MAX31855 and feed it to the micro through an isolator. The ADUM5401 is nice there since it also has an isolated power supply built in. You will need one of each per thermocouple, and make sure you lay out the board correctly for the isolation you need.
/mike

 

[JoeStillman]

...The second is to use a power supply with a reasonable maximum output voltage set low enough to prevent damage

This looks like the most practical solution to me. It doesn't take a lot of volts to blow a fuse - just amps.

 

[steve66]

I'd use one of the digital thermocouple chips like the MAX31855 and feed it to the micro through an isolator. The ADUM5401 is nice there since it also has an isolated power supply built in. You will need one of each per thermocouple, and make sure you lay out the board correctly for the isolation you need.
/mike

I agree. The power supply for the thermocouple should be isolated. :thumbsup: