thermocouple calibration

[spacemoose]

#3565

same problem here. exhibited on 2 osPIDs with 3 different thermocouples.

[Brett]

#3898

we have heard of this issue from one other person, but we haven’t had enough information to nail down a solution. one possible cause that was identified was that it only seems to occur when both the usb and DC power are applied.

can you guys confirm or dispute this?

[spacemoose]

#3899

The temperature reading is nonsensical whenever the rear power plug is utilized, regardless of whether or not USB is plugged in. I have both a 9V and 12V supply handy and neither works properly.

I did just try USB only and it seems to read more normally. It’s reading about 16C instead of what my other sensors are reading is more like 24C here in my office. Still, that’s a sight better.

[Brett]

#3900

Ok. We have found the cause of the issue: we screwed up. Buried in the MAX31855 data sheet there is a reference to a filtering capacitor, even though it’s not on their schematic. We didn’t put it in our design. Luckilly it’s an easy fix, and all units shipped after today will have this issue corrected. However, there are 22 units in the wild that have this issue. We have sent emails to everyone affected, but I want to post this here as well to close out the thread.

If you are affected, you can follow these instructions to correct the issue.

[rocketscream]

#3901

Hi Dominic,

I’m unable to reproduce your case with either or both USB and DC adapter connected after installing the capacitor.
May I know whether you inserted a through hole capacitor on the terminal or a direct solder?
I experience fluctuating reading before when using the insertion method because my thermocouple lead is thin and although it looks like connected on the terminal block, it is not firmly connected.

[rocketscream]

#3903

Dominic,

I use a switching AC-DC adapter 12V 2A and several other switching adapter too. So, my supply itself can be quite noisy.

Is your AC-DC cable overlap with the thermocouple lead? Before I solder the 10nF capacitor, if I have them overlap, the noise is awful.
But, once I solder the capacitor, if I have them overlap, there’s a tiny noise (0.3 C).
I only notice this after reading the last portion of the MAX31855 datasheet.

Let us know if it doesn’t work out. We’ll do a RMA in this case.

[spacemoose]

#3902

I tried adding a cap to one of the osPIDs with the terminal method and while the noise is less, the accuracy is still off. It’s reading negative temperatures and bouncing around, just not as quickly as before.

I have some SMD caps on order and I’ll try some other power supplies, and I will report back when I test them out.

[rocketscream]

#3904

I tried adding a cap to one of the osPIDs with the terminal method and while the noise is less, the accuracy is still off. It’s reading negative temperatures and bouncing around, just not as quickly as before.

It didn’t work well with mine too as all my thermocouple leads are way too thin compared to the capacitor lead. So, it’s kinda loose on the terminal block.

[KC6QHP]

#3905

I have also noticed this problem…. Mine was one of the 22 boards without the filter cap, so I added this (X7R 0603 cap across the leads on the part) and I get stable but low temperature readings. Initially I had use a 12 nF (NPO maybe?) cap and it was very noisy, so I swapped it out for the above mentioned X7R, but still does not indicate proper temperature when running off non-USB power.

Tony

[rocketscream]

#3906

Tony,

NPO capacitor supposed to perform better than X7R and the rest, across the whole temperature range. But, check the tolerance.

I swapped with as many AC-DC adapter I have here (switching based & transformer based) but I can’t get what you guys are seeing.
Do you guys have the part number of the thermocouple you guys are using?

[rocketscream]

#3907

Hi Dominic,

In the datasheet (page 11 Rev 2 latest from Maxim website), they recommend 10nF, but I also read some people using 100nF. It seems that this happens to a lot of other design even on simple breakout boards. If you look at their evaluation kit datasheet they also put a 10nF capacitor. On top of that a ferrite bead for each line (not sure whether it will affect the reading accuracy as it has some resistance).

I don’t think the twisted power leads makes any different (not differential signal). I’m using an old batch of wires. The one you are having, is the newer ones custom made for us by a connector company. But, you try.

I’m currently using the thermocouple bought from Adafruit (newer thinner ones and older thicker ones) and Lab Facility’s.

Please let us the outcome, we are eager to know.

[KC6QHP]

#3908

I am using a Fluke brand thermocouple. I cut the connector off, so it is bare wires going to the terminal block. I will try out putting a larger capacitor on the input and see if that helps. The noise is gone with the 10 nF cap, but the value is low by 5-10 degrees C when running off of the rear power input (going through the switching regulator as opposed to the direct USB +5V input). Have you checked the output of the switcher to see if it has sufficiently small ripple? I have used linear regulators after switchers in sensitive circuits before to reduce ripple, but part selection is important as well.

-Tony

[rocketscream]

#3909

Hi Tony,

The MAX31855 chip is being power off by an LDO (just like your approach) after a switcher. The last time I check it was around ~25mV for the switcher output.
I will measure again this week (someone took the scope).

[rocketscream]

#3910

Dominic,

We are looking into that option and would probably do that in the next release of the firmware & front end.
At the same time, we want to hear more from everyone on their units.

[dstorey]

#3911

Woops – I got deleted along with the SPAM! Brett’s put me back.

I’ve calmed down a bit now and am going to go for an RMA not a refund.

I’m interested in one thing though – anyone else running reliably with 2 relays on their boards and an external DC supply?

Dominic

[Author]

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