Can thermocouples be successfully used around noisy circuits? - Page 1
Forum section: electronics / general

author: Labrat127
author_rank: Contributor
timestamp: September 08, 2025, 06:55:28 pm
content: I deal with switching power supplies a lot. In the past, I used thermal imagers such as the Fluke 400i to measure component surface temperatures. Lately I've developed a strong interest in using thermocouples, as they are handy in some cases. The thermometer I use is a Fluke 52 II handheld. The thermocouples I use are unshielded untwisted 36-gauge K-type from Omega. I mount the thermocouples on top of power devices (molding compound) with epoxy. I thought high frequency noise picked up from (very) noisy nearby circuit would be easily filtered out by the thermometer. To my disappointment, whenever my circuit goes live, the thermometer readings become erroneous, or at least questionable. One time, I increased the DC load to my power supply circuit from 0A to 3A, and the thermometer reading jumped from 27C to 82C within a split of a second and stayed that way. Should've been 30C or something. Yes, my thermocouple lead was close and running in parallel to the load wire for some distance. I later corrected that. But still, the readings were fluctuating more than a few degrees within a few seconds. Sometimes the readings were more than 15C different from that reported by the thermal imager. Also, when I use the thermocouples around a DC circuit, the readings are very stable. In short, I found the thermal imager much more trustworthy than the thermocouples in my use case. I contacted Fluke tech support and they said thermocouples are not intended to be used around live circuits.My question is, has anyone here successfully used thermocouples around very noisy circuits? If so, was any measure taken to deal with noise coupling? In my case, I have to use a very small gauge TC because I don't want to cool down the top of my device too much. Any sharing of tricks and experience on this topic is much appreciated.

author: tooki
author_rank: Super Contributor
timestamp: September 08, 2025, 07:02:42 pm
content: Any way you can switch to an RTD like a PT100 or PT1000? I would expect that to be more resilient, not to mention much more accurate anyway.Otherwise, what about shielding? Both on the supply cables and thermocouple cables.

author: Slh
author_rank: Regular Contributor
timestamp: September 08, 2025, 07:52:26 pm
content: I've found that it depends a lot on the thermometer. I've embedded thermocouples successfully in 30kW motor windings and 800V, 4kW+ 50kHz transformers.We had a cheap handheld thermometer which gave negative readings when the switching started. We bought a fluke 52 II which we assumed would be better. The reading jumped +/-20C or more when switching. While we waited for better tech I used an HP bench multimeter (can't remember the model) to read the thermocouple voltage directly and did the zero point correction in excel. That had stable reliable readings but was awkward. That test was good enough for me to persuade the bosses to get a keysight DAQ970A that could do 20+ channels at good speed and no noise issues.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2025, 09:08:49 pm
content: I have never noticed EMI affecting my thermocouple meters, so I assume some are better filtered than others.

author: Labrat127
author_rank: Contributor
timestamp: September 08, 2025, 09:13:17 pm
content: Quote from: tooki on September 08, 2025, 07:02:42 pmI've never used an RTD based method. Will explore that. If the probe tip can be as small as the bead of a 36-gauge thermocouple, I'd definitely give it a try.Shielding is a pain for me. I can't imagine tying the shield to any net on my board out of safety concerns. At the thermometer end, there's no place for me to connect the shield to.I appreciate your advice.

author: Labrat127
author_rank: Contributor
timestamp: September 08, 2025, 09:24:15 pm
content: Quote from: Slh on September 08, 2025, 07:52:26 pmInvaluable sharing! I did suspect this 'cause I was under the impression that thermocouples WERE used around switching circuits. Will get the Keysight unit one way or another. Thanks much!

author: Labrat127
author_rank: Contributor
timestamp: September 08, 2025, 09:32:07 pm
content: Quote from: David Hess on September 08, 2025, 09:08:49 pmEither my Fluke 52 II has been modified by someone so that it's now prone to EMI or by design it's lacking in this regard. But per Fluke tech support and Slh's comment, and your comment as well, I'm convinced that the Fluke 52 II is the problem.Thank you for your input!

author: Smokey
author_rank: Super Contributor
timestamp: September 08, 2025, 09:36:10 pm
content: I've been doing a bit of thermal testing using a Keithley scanner and a bunch of thermocouples superglued to various points on the thermal stackup of a power supply module I'm working on (FET case, pcb top, pcb bottom, heatsink component side, heatsink ambient side, ambient). The module is a custom board with a 10A switcher converting 12V to 5V using LM25148 switching at about 600kHz. I'm doing a scan (all thermocouple channels in sequence) once a second. My main goal was to watch the thermal gradients from off-at-ambient to constant current steps at various current levels (watch the heat flow through the stackup when a load is applied) and see where the system thermally stabilizes for various currents and how long it takes.Once the temperatures stabilize for a given current I have some switching steady state measurements and I would expect to see a much higher noise floor on the thermocouple glued directly to the switching FET compared to the ambient thermocouple, but they look like they show up about the same in the plots. I wasn't looking for this specifically so I didn't run any statistics for noise but it certainly wasn't jumping around or giving obviously wacked out measurements that are wildly different when he FET was switching or not.

author: Stray Electron
author_rank: Super Contributor
timestamp: September 08, 2025, 09:46:14 pm
content: Quote from: Labrat127 on September 08, 2025, 09:13:17 pmI'm not certain here but aren't you supposed to leave one end of the shield unconnected? If you connect both ends of the shield to "Ground" then you can get current flow through it and that will interfere with the signal. I had that problem when I was using differential transformers for weight and balance sensing and leaving one end of the shield disconnected solved the problem. The lesson here, for me, was that one "ground" is not always the same as another "ground" and funny things happen at low signal levels.

author: Smokey
author_rank: Super Contributor
timestamp: September 08, 2025, 11:38:49 pm
content: Code:https://www.youtube.com/watch?v=Hj-HVve1QC4Cable shielding configurations demo with actual measurements.....

author: David Hess
author_rank: Super Contributor
timestamp: September 09, 2025, 12:29:28 am
content: Quote from: Stray Electron on September 08, 2025, 09:46:14 pmThat is right; Analog Devices application note 347 has a good discussion of shield connections, excerpted below.Thermocouples have additional requirements because signal levels are small, and thermocouples often exist in a difficult industrial environment that adds further restrictions, for instance differential operation may be required to either suppress noise or for safety. Linear Technology application note 28 discusses many of the issues:https://www.analog.com/en/resources/app-notes/an-28f.html

author: Labrat127
author_rank: Contributor
timestamp: September 09, 2025, 12:50:36 am
content: Quote from: Stray Electron on September 08, 2025, 09:46:14 pmYou're right. I meant to say I can't connect the shield at either end. Thanks for your comment.

author: Labrat127
author_rank: Contributor
timestamp: September 09, 2025, 12:57:22 am
content: Quote from: Smokey on September 08, 2025, 09:36:10 pmThanks for sharing your experience. Yes, I was talking about either a huge offset or fluctuating readings or big differences from the readings of a thermal imager. I think your Keithley scanner was what gave you stable sane results.

author: Smokey
author_rank: Super Contributor
timestamp: September 09, 2025, 01:31:18 am
content: Quote from: Labrat127 on September 09, 2025, 12:57:22 amMaybe. I didn't have it set to crazy long integration times or anything though. And if there was a crazy offset I would expect to see it between the direct FET connection and another thermocouple on the heatsink away from the FET.

author: shabaz
author_rank: Frequent Contributor
timestamp: September 09, 2025, 01:54:57 am
content: I have seen strange readings with RF circuits, but then the thermocouple was tucked in the tape insulation around a transformer toward the transmitter output. I don't recall the resolution, but from memory for that circumstance, ferrite cores may have helped, I don't recall shielding the thermocouple. This was with a very old measurement device. Or we might have stopped the transmitter for a second and then quickly measured, i.e. just during the good periods! Non-ideal for sure.

author: Slh
author_rank: Regular Contributor
timestamp: September 09, 2025, 05:27:49 am
content: I should also mention that we just used the cheapest K-type thermocouples from RS. No shield but they were always twisted.We would bundle up a perhaps 8 or more for more than 50 cm. Some of them were in the transformers and inductors, others would be on SiC MOSFETs.For the motors, I used the predecessor to the DAQ970A which worked just as well apart from being prone to USB dropouts.

author: Overspeed
author_rank: Frequent Contributor
timestamp: September 09, 2025, 09:29:58 am
content: HelloFirst you have two types of thermocoupleone is : The weld is in the air so isolated from the part or parts you measure , end can be very close , closer is the weld faster is the temp reading but you have no electrical contact with your part .second is : The weld is in contact with the part or parts up to be weld on the partThermocouple reaction time is a parameter to take in consideration as that not instantI agree that thermocouple can be thin as an hair which allow easy setup in narrow room / access.You can also use a low pass filter to remove what you don t need / wantI have make a lot of measurements by using thermocouples on industrial furnaces for heat treatment , I never see any problemsRegardsOS

author: CalibrationGuy
author_rank: Unknown
timestamp: September 10, 2025, 01:36:21 am
content: I second what OS said. We still use the stable workhorse 34970a DAQ unit, 10PLC setting (AMS2750 TUS) with no shielding on heat treating and vacuum brazing furnaces. The SCR drives produce tons of RF noise and the readings are rock stable.TomG.PS - The 34970a can be had for cheap on eBay.

author: Overspeed
author_rank: Frequent Contributor
timestamp: September 10, 2025, 07:26:37 am
content: Quote from: CalibrationGuy on September 10, 2025, 01:36:21 amHelloseems to work in the same domainI have tested 34970A no problem , Keithley 2700 no problem , Fluke Hydra no problem , AOIP SA 32 ( french made 6.5 digit data acquisition ) no problem .I agree that 34970A is a good value for the price , just carefully check the relay wear on multiplexer boards , and if you want to improve the system , cover them with a metal foil as shield , that a delicate work but that a real improvement if you want accurate measrementsseehttps://www.eevblog.com/forum/metrology/scannermultiplexers-for-voltage-references/100/reply 114 AronakeThat also possible if not already installed to install 34970-00603 DMM Shield I link some picture that a hard to find part but can be made bespoke by laser cutting and break press , note to avoid to spead aluminium oxide inside the 34970 please anodize or use 304 SS rather alumium alloyRegardsOS

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 21, 2025, 03:56:58 pm
content: Filtering is an important precaution, but there is more. Metal to metal transitions (as used in any connector, either screwed or plugged) can have diode like behavior, and if this rectifies a part of the noise, you get a DC voltage that would go though any low pass filter.

author: CalibrationGuy
author_rank: Unknown
timestamp: September 22, 2025, 07:34:53 pm
content: It is very important to have connectors of same material composition for connections. For thermocouple interfaces it is a must. Thermocouple connectors are made of same type of thermocouple metals.TomG.

author: TUMEMBER
author_rank: Regular Contributor
timestamp: September 23, 2025, 09:51:10 am
content: Quote from: CalibrationGuy on September 22, 2025, 07:34:53 pmThat's not true. So-called extension cables are made of cheaper materials with characteristics similar to the thermocouple's base material. It must be compatible up to 150°C (base metals). In the case of a type B thermocouple (noble metals – platinum group), it can even be pure copper wire, as the thermoelectric power of this thermocouple is negligible (single microvolts) up to 300°C. Similar principles apply to connectors – can you imagine how much platinum plugs and sockets would cost for type S, R, and B thermocouples? An ounce of platinum for plugs with pin diameters of 6 mm. How long would they wait until someone stole them? Everything that comes out of the "thermocouple head" is made of substitute materials that are many times cheaper. In combined heat and power plants, the distance from the measurement point to the control cabinet connection point is sometimes several dozen meters, and you'd put platinum in the cable every few dozen meters? Multiply this by the dozens of measurement locations on the turbogenerator.

author: Overspeed
author_rank: Frequent Contributor
timestamp: September 23, 2025, 01:46:36 pm
content: Quote from: CalibrationGuy on September 22, 2025, 07:34:53 pmHelloand also to carefully check corrosion as some thermocouple like T and J are very sensible to corrosionRegardsOS

author: CalibrationGuy
author_rank: Unknown
timestamp: September 23, 2025, 11:05:45 pm
content: I never even said the words "extension cable". Everyone serious in this industry knows that quality tc connectors are important to keeps errors to a minimum. I have a cable in the lab that is one foot in length with cheap tc connectors that contributes 0.8 degf error. I use it to demonstrate sources of error to new techs. We use thermocouple grade SLE wire, never "extension grade" wire. Having said that, you have no choice with type S sensors for vacuum furnaces. Nobody in their right mind would use platinum extension wire. It would probably be stolen anyway.TomG.