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thermocouple amp max6675 uusing digital isolator as means to isolate thermocouple

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HI,

I'm having problems with getting good readings on my 3d printer.

I'm seeing HUGE excursions when reading the tempterature whilest printing.

IT appears to be stable when I preheat the hotend without starting a print.

It's so bad I observed the mosfet that switches on the heater is on constantly via the led connected to the mosfet.

qestions:
1)How does one check for switching noise? I have a 60mhz digital scope.
2)Can switching noise increase if you draw more power from a dc-dc converter ?
3)Is there a cheap i.e. not £100+ linear psu that's around 400w 24v
 

tomizett

Active Member
For less then the price of a linear supply you could probably build a really substantial LC filter to clean up your existing DC supply. Alternatively, it may be possible to filter out the switching noise from the signal coming from the thermocouple, rather than trying to stop it getting into the signal in the first place.

2)Can switching noise increase if you draw more power from a dc-dc converter ?
You're right - it wouldn't be unusual to see more switching noise the more current you draw.

But, there may be something funny going on with your grounding or elsewhere in the system. Are you currently using an optoisolator (as you mention in the title) or is that just an idea? Do you think you could draw up some kind of system level block diagram showing how the system is interconnected?
I don't know, but I doubt that the answer to your problem is anything as drastic as replacing the entire power supply. In fact, it's probably more likely that the noise you're picking up (if that's what is happening) is coming from switching in the motor drivers rather than the power supply.
 

spec

Well-Known Member
Most Helpful Member
As Tomizett indicated noise may be a problem, but the noise may not be coming from the power supply.

There has been a number of similar questions to this on ETO.

The sensor output is quite low level so noise incursion is not unusual- the answer is to fit a simple low pass filter or filters to your system.

If you could post a schematic of your system perhaps we can advise.

spec
 
For less then the price of a linear supply you could probably build a really substantial LC filter to clean up your existing DC supply. Alternatively, it may be possible to filter out the switching noise from the signal coming from the thermocouple, rather than trying to stop it getting into the signal in the first place.


You're right - it wouldn't be unusual to see more switching noise the more current you draw.

But, there may be something funny going on with your grounding or elsewhere in the system. Are you currently using an optoisolator (as you mention in the title) or is that just an idea? Do you think you could draw up some kind of system level block diagram showing how the system is interconnected?
I don't know, but I doubt that the answer to your problem is anything as drastic as replacing the entire power supply. In fact, it's probably more likely that the noise you're picking up (if that's what is happening) is coming from switching in the motor drivers rather than the power supply.
Hi

I'm a beginner if you've not worked that one out yet. So I dont have much experience with system level diagrams.

Plus I dont have a clue so this is about as much as I know.

I was using a adum5401 with a max6675 before I tried the adafruit max3180 breakout.

I was told by another enginner on a diffrent forum that the thermocouple I have isn't a floating thermocouple.
And that the max66675 only works with floating thermocouples. He suggested I use a digital isolator with isolated power. So I opted for a adum5401.

I also read somehwere about ground loops on the hotend causing bad thermocouple readings. IN my case do they mean I have a connection from my hotend to gnd?

If so wouldn't the isolator prevent that?

Also am I correct in saying having a long thermocouple, can cause noise as well because it acts as an atenna ?

schematic for the mAX31850 breakout
https://cdn-learn.adafruit.com/asse...dafruit_products_max31850schem.png?1393454843

Can the the filtering on the breakout be beefed up without making a new board all together. I dont fancy trying to hand solder the 31850.
My little finger is bigger than the die.
 

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The limit switches are 3 pin jst connector (i think xh) it has 5v, GND, micro pin) so I have a logic level shiftier that shifts the 5v logic coming from my micro to the breakout.

As for the source of the 3.3v, the breakout has a 3.3v regulator on it so I get it from there then feed it to the shifter
 
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