What is this blog about?
Back in June rscasny was looking for people to experiment with thermocouple connectors. I got selected and connectors were dispatched. However they did not arrive until mid August.
Meanwhile I was designing a PCB for the connectors (they are designed to be PCB mounted). I designed the connector footprint according to the datasheet, so they fit, but now that I see how they fit, I would design the foot print slightly differently to make them easier to hand solder.
When I went to order this card, the price had jumped from the last time I ordered PCBs by almost a factor of 4. The shipping cost had increased by a factor of 5, and the shipping time paradoxically also more than doubled. So I didn't receive the PCBs until late September.
The circuit to handle thermocouples needs a temperature reference at the connector, so I decided to use a Maxim 31855 thermocouple interface chip which included the reference temperature. I ordered these from Newark along with some other components using a shopping cart I had just won. Unfortunately these also took months to arrive, but finally made it here in mid September.
I have to apologize for the tardiness of this blog - it has been a long slow journey - I actually was losing hope that any of the parts would arrive at all.
The experiment itself is fairly straight forward - solder 3 thermocouple connectors on a PCB with some signal conditioning, a microcontroller and a display, write some firmware and see how well they work.
Here is a schematic and a picture of the circuit card with a kit of its main components:
The Thermocouple Connectors
The Omega PCC-SMD thermocouple connectors are individual spring socket contacts without a housing. They have a unique design that allows the exact same part to be used for both the smaller and larger pins of a standard mini thermocouple connector, and at the same time provide keying so the larger negative pin cannot be plugged into the socket intended for the smaller positive pin. This is accomplished by rotating one socket 180 degrees where the back has a smaller opening.
This arrangement requires a PCB footprint that is a mirror of its mate. It also requires 2 slots in the PCB for each socket, one of which is plated. Implementing slots on my CAD system is non-trivial and this is the first time I have attempted it, so that is part of the experiment.
Normally the connectors and cold junction reference would be enclosed in an isothermal block to ensure the reference and the connectors are at the same temperature, but I am leaving them exposed in this blog so they can be viewed. The error introduced by doing this is going to be small anyway.
Here are pictures showing how reversing the same socket presents a different socket width:
Here is a picture of the assembled PCB which includes an arduino Pro Micro and a 5110 LCD:
Obviously this is a close together as the connectors can be spaced - the external plastic shells are touching each other.
Here is a short video of the system in operation although the software is still a work in progress:
The connectors work well and are quite secure. I would probably design an isothermal block that would also minimize stress on the sockets if It was going to be used in a product.
Conclusions & Discussion
These Omega connectors are an extremely small thermocouple connector solution that provides a lot of flexibility when designing a product.
The clever keying design allows the same part to distinguish between different pins based on which side is used for pin entry.
This was a lot of work to go to to test a couple of connectors, but the system is designed to be a building block for other projects, so look for this circuit board to show up in another project. The next one (linked below) is measuring the temperature of a flame, which is a good application for a thermocouple.
The Saga of Kemet Tokin - a follow-on project that uses this sensor module