Thermistor temperature oscillating

[Tincho85]

This is a bit weird...

While the printer is idle, the thermistor readings from my heated bed are oscillating badly. It goes from 22ºc to 35º, then to -5º, etc.
First I thought it was a bad thermistor or a lose/bad wiring, but when I unplugged the 24v psu from the wall it starts reading perfectly.

From day one (when I did the 24v heated bed upgrade) I had some oscillations, but very low, like 1 or 2 degrees.
Didn't pay much attention at the time, but now it's something else.


Any ideas?

[geneb]

You have a loose wire somewhere I suspect.

g.

[JFettig]

Could also be shorting out somewhere

[Tincho85]

Thanks for your suggestions guys, I'm about to remove the onyx sandwich and see if something odd is going on there.
Don't have much hope though, because what's happening is very strange:

While not printing, 12v PSU ON (Rambo board ON), 24v PSU ON ------> Bad readings
While not printing, 12v PSU ON (Rambo board ON), 24v PSU OFF and plugged into the wall ------> Bad readings
While not printing, 12v PSU ON (Rambo board ON), 24v PSU OFF and un-plugged from the wall ------> Good readings

[Tincho85]

Fixed! I'm glad I can't leave home for a few days, viral conjunctivitis! YAY

I think it was noise interfering with the measurements because there was nothing visibly wrong with the wiring.
This is what was done:


[img]https://c2.staticflickr.com/2/1532/2437 ... d033_b.jpg[/img]
Just to avoid variables, I changed the wires, the connector and then twisted them.
Did some reading... Never thought that twisting wires had a purpose, it acts as a low pass filter.


[img]https://c2.staticflickr.com/2/1545/2470 ... 55dc_b.jpg[/img]
Then I used some aluminum IPSolar tape to wrap it up. Great stuff, it comes with a very strong adhesive. I think air conditioner and gas installer guys use them.
The little ring in the middle is a ferrite bead that was salvaged from an old inkjet printer. It suppresses high frequency noise in electronic circuits (thank you again google!).


[img]https://c2.staticflickr.com/2/1530/2488 ... 5ca5_b.jpg[/img]
To finish up, shrink tube to protect everything.



I will never know what did the trick, the new wiring, the ferrite bead, the aluminum or the whole package.
But well, hope it helps to someone in the same situation.

[geneb]

Well so much for only changing one thing at a time. *laughs*

g.

[Jrjones]

FYI you should ground the aluminum shielding as well since you went through the effort of putting it on there.