[redwire]

in reply to: osPID malfunction using internal switch regulator #4014

Here’s a few suggestions on things to check- I’m also an EE.
-Try run it grounded. When you connect the USB cable, you providing the osPID an earth ground path, which may not be happening with your (2-prong) AC adapters. Then EMI can get into the MCU via the thermocouple.
-For the osPID regulator, check if C3 is correct and not cracked. I would measure the 5V rail and see if it’s off, or add 100uF and see if that fixes it.
-I’ve seen the fusebits overlooked in the ATmega328 – that the correct crystal/resonator is chosen- if wrong it can make the osc. weak and sensitive to EMI.
Also that the brownout detector is enabled and set to high threshold. I’m not sure if the watchdog is used.

[redwire]

in reply to: Thermocouple calibration/Adjustment #4011

I use a simple offset correction, subtract/add a degree or two inside the source code MAX31855.cpp
There are several places for temperature error and you should fix it in the right spot:

1. The cold-junction temperature can be off
2. The thermocouple temperature reading can be off
3. The MAX31855 uses an (internal) linear formula, whereas a termocouple is parabolic curve.
Variations between thermocouples are much less a source of error, because it’s the alloy.

Ideally the UI would have a screen to set the cal factor and save it in EEPROM.

[redwire]

in reply to: Arduino Reflow Oven Controller Shield #3926

Dan, here is what I suggest for grounding. For the opPID controller, it would be the same.[attachment=0:3tqi1pzm]Reflow Controller grounding001.PNG[/attachment:3tqi1pzm]

[redwire]

in reply to: PCB Prototyping #3705

It really depends on a few things…. how big the pcb is, how long you can wait…
I use https://batchpcb.com/ they are $2.50 a sq. inch and very slow delivery, but low cost. Once you get up to ~16in^2, they are not competitive but great for small smt breakouts etc.
This is a great cost tool to compare proto pcb houses: http://www.ladyada.net/library/pcb/costcalc.html

[redwire]

in reply to: Arduino Reflow Oven Controller Shield #3924

If one thermocouple-leg is grounded, the MAX31855 sets the fault detection (bits). I’m not sure what the osPID software then does- keep the reading, go to a default, halt etc.

[redwire]

in reply to: Arduino Reflow Oven Controller Shield #3921

I’ll post about some thermocouple chip issues I know about, I hope this helps.

The older thermocouple chip (MAX6675) had a grounded thermocouple input (T-) and could not read temperatures below 0°C.
The newer thermocouple chip (MAX31855) does not ground the thermocouple (in order to read negative voltages/temperatures) so it is more sensitive to noise. Adding the ferrite beads/10nF cap gets rid of a little RF noise but not other interference.

Your power supply for the OpPid (especially if it’s a SMPS wall-wart) adds noise, so you need to have a ground wire from the OpPid to the metal chassis and earth-ground of your oven/pump etc. But thermocouple junction must not connect to ground (=isolated) and best to run the wires away from high current (i.e. heater/fan/pump) wiring.

You could add an extra filter and cap for the MAX31855 to help with things.

Type-K polarity is yellow (+) (nonmagnetic wire) & red (-) (magnetic wire).

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