How to connect a thermocouple with a plug connector to a Sensor IC? - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 07, 2016, 07:57:10 pm
content: I am making my own Toaster-Oven SMD Reflow/Electric Hotplate Combo solution to stick teeny-weeny SMD parts to PCB's.My idea, after reading some postings, was to try the tag-team approach of using a Hotplate to pre-warm and a Toaster-Oven to seal the deal.I see many very low-cost small pre-built thermocouples assem. that can be purchased on Ebay that have a dual-banana type connector. If I want to connect one of these to a MAX6675 IC, what is the best way..cut off the connector and solder the wires directly to the pcb pins of the 6675? Or are the screw-tighten connectors on the pre-built modules from ebay already correct for this job?BTW: Why do some posters here prefer an Analog Dev. AD595A (much more expensive) for the job?My super-duper Toaster Oven has a snaked heater element at the top and another heating element on the bottom. It has one shelf that can be inserted at various heights..wonder which position is best to tan my models on this hot ride?How to mount the thermocouple tip to a Hotplate and Toaster-Oven and to the PCB being cooked to get the best temperature readings?

author: mstoer
author_rank: Contributor
timestamp: February 07, 2016, 08:42:54 pm
content: You can just cut the connector off, but you will need to know which wire is which. Those two ICs are for K-type thermocouples, so verify the ones you are looking at are of the right type. The banana type connector is likely of the flat spade type with each blade marked as to the type of wire used. For K-type they might be marked AL (Alumel) and CH (Chromel). The ICs have pins matching these wire types as well.When you solder the thermocouple wires to the PCB, keep the distance to the IC short and the two paths as near identical as you can. Differences will show up as voltage offsets (errors in the temperature readings). For example, take care not have one one wire trace too close to a heat source. The solder joint itself will introduce some error, but likely small in your case. I believe the datasheets for both chips give some design hints when it comes to assembly as well. Be sure to read those.I think many choose the AD59x series over the Maxim chips since they have been around longer. I know I used the AD595/594 many years ago, long before I knew of the MAX6675 (and others they now make).

author: SuzyC
author_rank: Frequent Contributor
timestamp: February 07, 2016, 10:47:08 pm
content: Thanks for the hot tip, mstoer. All of the thermocouple modules shown on ebay that i've seen are for use with type K.If these banana-like connectors are connecting wires made of Alumel and Chromel, aren't these wires basically not able to be soldered..require some sort of crimp connection? It seems that these connectors are standard in size because all of the thermocouples shown seem to have the same type of connector.Since posting, I've browsed some YouTube and I see one guy uses just one thermocouple that just opens the door and shuts off power when the temperature profile reaches a target temperature after raising the temperature gradually according to profile.Others have cautioned that those ovens using radiant IR heating elements will always overheat the black-body surface of a IC sooner than the leads protruding to be soldered causing sunburn where not even a tan is desired.Can I assume that IR heating elements can be identified as those being cylindrical and made of quartz and glow quite a beautiful shade of orange in use, but it is that my oven's flat metal serpentine-shaped heating element suspended at the top is not so much radiant as conductive?Would it be wise to suspend by wires a thin sheet of shade, a large rectangular, but thin aluminum plate above the target board to be cooked to prevent radiant heat effects and feed the thermocouple's wires through it using slightly oversize holes to dangle at the same level, perhaps to the left and right of the PCB being heated to measure the target board temperature? The aluminum plate would not be so large as to prevent easy airflow on all four sides. In this way the PCB could wallow in the warmth where the sun never shines.

author: mstoer
author_rank: Contributor
timestamp: February 08, 2016, 05:25:13 am
content: In the connectors I have (also mostly K-type), the connector can be disassembled with two screws and the wires detached from the spade contacts by loosening the contact screws. If you take a look at the wiki page:https://en.wikipedia.org/wiki/Thermocoupleand scroll down to the table of type comparisons you can see there is a colour coding standard (which is not always followed unfortunately) for the wires. There is also a pic of a K-type thermocouple in a green (IEC standard) connector. If you have that type it will be relatively easy to disassemble and identify the wire types. If it is a molded connector it might prove to be more difficult, since disassembly might be too destructive.Here is the type I have (mine are off amazon though):http://www.dataq.com/resources/techinfo/thermocouple-assembly.htmThe other thing you will have to be cautious of is the temperature rating of the insulation. That Wiki page has the info there as well. I would suggest studying the temperature profile of the oven first before putting any PCBs in it. You may need more than one thermocouple to do this.Yes, those orange glowing rods are the heating elements. You will get a combination of radiative and conductive heating and that is why I suggest you study the temperature profile first. You can then figure out of a heat shield (or deflector) is needed or not.