connectorpcbthermocouple
Has anyone had good/bad experiences with particular PCB-mount thermocouples?
I need to design a circuit board with a connector I can plug a standard thermocouple connector into.
(and yes, I am aware that when you connect thermocouple wires to copper traces, you create junctions of dissimilar metals that can cause errors if the junctions are at different temperatures)
I've decided that breadboarding ahead of time is a waste of time unless there are aspects of a part's performance that you need to measure yourself.
I even think building the board in pieces is a waste of time. The initial hardware debug just takes a day or two. Any subtle issues won't show up until software integration anyway. I'd rather debug one circuit (the complete one) than several ad-hoc reductions of the circuit.
Just build the thing! Assemble the whole board and power it up. Look at the supply voltage. How many mA does it draw? Which part is burning hot? Which part is warm? Then try to burn firmware on to whatever microcontroller is on the board. Then start writing firmware. Get the clock going and toggle a pin. Get serial (or whatever) communication up. Now write test programs for each peripheral. Then build the production test fixture and start writing the "real" firmware.
Edit:
If a supply rail is stuck to ground, (measures 0 ohms with an ohmmeter), power it up on a bench supply. Set the voltage normally and the current limit to a few hundred mA. Print out the PCB design on paper, and find a DMM that measures microvolts. Measure microvolts starting at the supply terminals, and write down the voltage drops on the PCB printout. By looking at microvolt differences, you can trace exactly where the current is going without depopulating or chopping up the PCB. This technique is a better than using an ohmmeter because you can pump a lot of current through the circuit, more than any normal ohmmeter will supply.
A similar technique works for finding shorted traces on an otherwise populated board. Run the board a use a scope to find "digital" traces that are in an in-between voltage range.
Use suitable male and female indirect connectors, such as those used on these boards:
The boards may be daisy-chained.
They are available from Digi-Key. Mating connectors are on the same page.
Best Answer
The most important part of how/where/when for connecting a thermocouple is to measure the temperature as close as possible to the bimetalic joint.
From here feed that temperature into your uP and then you can use software to perform the CJC compensation and use the known polynomials (or simplified linearised equation for smaller ranges) for converting your ADC value into temperature.
In the past we have got a big chunk of aluminium and performed wire to wire terminations around it and installed a LM35 for temperature measurement. This was performed off the PCB.
Having said that you can freely do it with connections n stuff on the PCB as long as for are able to measure the temperature as close as possible to the bimetalic junction.
So to keep the distance as short as possible consider using screw terminals and mount the LM35 right near the screw terminals themselves for the most accurate reading. If you really want to use a connector try DB9 with the metal back shell - cheap as chips and being metal will conduct heat well for accurate measurement of the back shell temperature. Use male pins on the PCB.
I assumed you are using a uP and software, if not you can do the CJC in hardware but thats a bit trickier.
Hope this helps.