Vin vs 3V3 input MAX31855 Moderators: adafruit_support_bill, adafruit

[adafruit_support_bill]

We can try replacing it. Please contact support@adafruit.com with a link to this thread.

[Steve1445]

thank you

[Steve1445]

Got the new board, once I grounded out the board to the actual ground (of the kiln to ground) not just Raspberry Pi, the errors stopped. So thank you for sending along the hardware.

New question:

Receiving this error:

File "/home/pi/kiln-controller/lib/oven.py", line 20, in <module>
import Adafruit_GPIO.SPI as SPI
ImportError: No module named Adafruit_GPIO.SPI

Then:

(venv) pi@raspberrypi:~/kiln-controller $ sudo pip install Adafruit_GPIO.SPI
Collecting Adafruit_GPIO.SPI
Could not find a version that satisfies the requirement Adafruit_GPIO.SPI (from versions: )
No matching distribution found for Adafruit_GPIO.SPI

I have SPI enabled in the Raspberry, but it seems the software needs more.

[adafruit_support_bill]

Python is not my native language. Probably best to post this question over in the CircuitPython forum. viewforum.php?f=60

[Steve1445]

will do, thanks again.

[virtualbuck]

Hello. Maybe I can piggyback on this thread? I am having the same problems that Steve1445 was reporting.

I have a kiln controller that I built from a github project. This one >> https://github.com/jbruce12000/kiln-controller

I have two completely different RPIs (a 3B+ and a zero), two different max31855s and many different thermocouples. No matter what, both setups are getting spurious out of bounds temp readings and both occasionally report short to ground or short to vcc.

It has nothing to do with electrical interference from the heating coils. Coils are off and the max31855 throws errors when I pinch any thermocouple with my fingers to warm the probe.

Now Steve1445 says he grounded the thermocouple breakout board to the kiln ground and was able to stop the errors but I am uncertain about his wiring and how to apply it to my situation.

There are users that have implemented the kiln controller project successfully and get no errors. I am wondering if there is a chance that there is a bad batch of max31855 breakout boards...especially after reading through the other postings about the max31855 and error codes 2 and 4.

Thoughts?

[adafruit_support_bill]

Coils are off and the max31855 throws errors when I pinch any thermocouple with my fingers to warm the probe.

There are many possible sources of interference other than your heating coils. And watch what happens when you touch an oscilloscope probe with your fingers: https://www.youtube.com/watch?v=c3SRuX9lW58

Rule number 1 of electromagnetic interference is "everything is an antenna". That includes you, your thermocouple probe and all your wiring. Thermocouples work in the microvolt range so thermocouple amplifiers are very sensitive to noise of any kind.

To minimize noise, you can try shielding the probe and ground the shield at the amplifier end.

Even with shielding and other noise mitigation strategies, occasional spurious readings are a fact of life in any real system Your software should be able to recognize and ignore questionable readings.

[virtualbuck]

I really wasn't so worried about the spurious readings. I was worried that some component was heading down the road to failure and it was signaled by random short to ground and short to vcc errors. I can't ignore an imminent failure in a component in a kiln when I can't be certain that the failure won't lead to overheating.

I just want to be clear before I ignore the short to vcc and short to ground errors...are you saying that noise can cause these errors to report? I can imagine that spurious temps might be a result of emi but I have a hard time buying into the notion that the chip would be falsely reporting shorts in the TC based upon line noise.

Why would OP's problem resolve by grounding the breakout board to the kiln?

Thank you for your help. I look forward to working this problem with you.

[adafruit_support_bill]

I can't ignore an imminent failure in a component in a kiln when I can't be certain that the failure won't lead to overheating.

For industrial controls, we address risks like that in a few ways:
1) Avoid single points of failure. In some cases, this means redundant sensors.
2) Scheduled preventive maintenance - components with finite lifetimes (i.e. thermocouple probes) are replaced regularly.
3) Design the system to 'fail safe'. In the case of temperature controls, in the absence of credible feedback, you bring the system to a safe state. Electric kilns are fairly straightforward systems in that regard. Things get much trickier when you are dealing with combustible gasses.

are you saying that noise can cause these errors to report?

Yes, noise can cause these types of errors. The sensor is simply looking at voltages on pins. Errors are generated when certain thresholds are exceeded. The 'human antenna' test of pinching the probe tip demonstrates that nicely,

Why would OP's problem resolve by grounding the breakout board to the kiln?

Noise couples into systems in a variety of ways - some of which are not at all obvious. For example: if noise radiated from the kiln is picked up inductively by the probe, there will be voltage fluctuations at the inputs to the amp. If you ground the amp to the source of the voltage fluctuations, the signal are measured relative to that - cancelling out the noise.

[virtualbuck]

Is your recommendation to ground the breakout board to the kiln? My kiln's chassis is grounded to earth via mains ground. How would I ground the breakout board to the kiln? Just solder another lead onto the GND pin on the breakout board and take it to the kiln chassis...at the same time leaving the lead between GND on max31855 and the RPI header ground?

Or is you recommendation to ignore the errors?

[adafruit_support_bill]

Is your recommendation to ground the breakout board to the kiln?

Unfortunately, there is no one-size-fits-all solution for EMI problems. Even full-time EMI specialists will admit that a large part of their job is trial-and-error.

Grounding to the kiln is certainly worth trying. Many modern switching power supplies are isolated. So their 'ground' will be floating relative to the mains ground. And in older 2-wire system where ground and neutral were combined, current flow will cause voltage drops in the ground, so the ground potential can vary on different branches of the circuit.

Or is you recommendation to ignore the errors?

The goal of course is to minimize the interference so you have fewer errors to ignore and more confidence in the overall quality of the data.

[Steve1445]

Virtualbuck, when I worked through my issues I never came away fully convinced that there may be something up with the board. Still today at cone 6 temp my reading will suddenly go to zero degrees. I have used two different boards purchased during the same time period with same results.

As for grounding, I did not initially ground my RPI to earth or main ground. Once I did that, those errors cleared up.

[virtualbuck]

I added an earth ground to my project. I did this by taking a three prong AC plug and isolating the ground wire. I ran a lead from that ground wire to the ground bus on the breadboard. As soon as I plugged the "ground only" plug into a mains receptacle, the errors stopped. My problems are resolved. Thanks for your help!

[adafruit_support_bill]

That is good to hear. Thanks for the follow-up.