Power Supplies and Thermocouples

[WBegg]

Recently, I've been making my own thermocouples, and today stumbled upon something odd. I make my thermocouples un-grounded, meaning the wire braid and metal sleeve are not connected to the thermocouple wire. I noticed, however, that noise would be induced on the TC when I touched the braid while barefoot, and also induce noise when hooked up to my WSM which sits on the ground (grounded). This occured with several different TCs I had.

Normally the thermocouple looks like this checking it with the hm-noise firmware.

When touching the wire braid (essentially attaching to house ground), it generates noise in the form of a cyclical (sinusoidal) wave.

So I thought, what would happen if I grounded the Braid to the thermocouple? I did, and the noise disappeared with no ill effect or deviation from the temp readings.

Now here's the strange thing. This noise only occurs with several different SWITCHING power supplies I had laying around. When I tested with a LINEAR supply, there were no ill effects grounded or un-grounded.

Should we all be using grounded thermocouples?

 

[SteveCK]

Now here's the strange thing. This noise only occurs with several different SWITCHING power supplies I had laying around. When I tested with a LINEAR supply, there were no ill effects grounded or un-grounded.

Should we all be using grounded thermocouples?

Maybe I misread everything, but wouldn't the appropriate deductive question be: Should we all be using a linear power supplies?

 

[Steve_M]

Maybe I misread everything, but wouldn't the appropriate deductive question be: Should we all be using a linear power supplies?

I don't think so. Any half decent (pretty much all of them) switch mode power supply should work just fine and not cause any noticeable noise. The top noise graph that wbegg posted has very little noise on it to begin with. All you really do by introducing a linear power supply is waste more electricity due to its inefficiency.

 

[WBegg]

Maybe I misread everything, but wouldn't the appropriate deductive question be: Should we all be using a linear power supplies?

I agree, but my thinking is most wall wart and "brick" style PSs that people have, and the most readily available, are switching.

Also, the DOE has just passed regulations regarding the efficiency levels required by compact power supplies. They must now be a class VI or better. Quite a few manufacturers are dis-continuing linear products that don't meet this requirement.

http://www.metlabs.com/blog/energy-star-2/external-power-supplies-must-meet-level-vi-energy-efficiency-requirements-for-u-s-doe-by-february-2016/