Keenovo Heater with Built in Thermistor Questions/Concerns

[ctilley79]

I've been using a 600w Keenovo bed heater with built in thermistor for the last year or so. I decided to discontinue use of the built in thermistor, so I added one between the aluminum build plate and print surface. I left the built in thermistor hooked up so I could monitor both sensors. My goal was to improve the accuracy of the bed thermistor by placing it closer to the print surface.

I knew the new thermistor would read lower due to the thickness of the build plate, but I was surprised just how much. I started a PID tune set to 110C, and as the heater was approaching that temperature, I checked the built in sensor. It was reading as high as 147C. This concerned me because I worried that the heater pad adhesive would break down at that temp. I also noticed a slight smell probably from the gasket silicone I used to reinforce the heater.

I need to know if this is normal or whether I need to be concerned. How is everyone else placing their thermistors with this type of heater pad with a thick aluminum plate? Should I go back to using the built in thermistor?

[deckingman]

@ctilley79 I have my sensor in a small hole drilled into edge of the plate, as deep as possible and as close to the top surface as possible. I think I first the first person to do this many years ago. I never had a built in thermistor by it sounds to me as if the sensor is in contact with the heating element but not with the aluminium plate (another good reason not to use it). I wouldn't worry about the adhesive because the plate will conduct the heat away so it'll never reach the temperatures you are seeing (unless you heat the bed to 147degC ) IIRC, I had my bed heater arranged that way before I started my blog and that was January 2016 so it's been like that since 2015 - about 7 years without any problems.

[3DPMicro]

@ctilley79 I've seen a similar thing with a glass bed. I attached a thermistor to the top of the bed surface and it wouldn't auto tune. Just about killed the adhesive trying. The actual surface will be significantly cooler than the heaters integrated sensor reads and if you look at a thermal image of the bed it's far from consistent. Mine is using a 500w Keenovo on a/c

[Norder]

@ctilley79

I have a 330x330x8mm aluminum plate.
Including a standard 24V silicone heating mat (max 120°C) very similar to the Keenovo.
I don't even remember the wattage.
The print bed is insulated underneath with a 10mm cork plate which allows the heating plate to cool down very slowly.
I glued the cork together with silicone glue that can withstand temperatures of up to 350°C.

In order to get a closed border that also protects the cork plate and also holds it, I screwed a 20x20mm aluminum L profile to the aluminum plate (see photo).
To be able to see it better, I put a mirror on the body of the printer.

I use the thermistor welded into the heating mat and screwed a PT100 thermistor to the bottom of the aluminum plate about 3cm from the edge, which I use to check.
So exactly the other way around as you do it.

The way you connected it, i.e. the thermistor in the heating mat for checking and the thermistor on the edge for switching off... there is a risk that the heating mat can be damaged or, in the worst case, a fire can occur.

It takes time for the entire aluminum plate to heat up.
And when a few minutes have passed, you can use the thermistor attached to the edge (or wherever it is mounted) to compare the temperature with that from the heating mat and, if necessary, increase the temperature by the difference.

I'll link you to a thread where the thermistor was clamped at the edge between the glass plate and the normal aluminum plate.
This thread was started because the printed parts collapsed and took on strange curvatures.
At first it was assumed that the Delta printer had some problems with the geometry.
But in the end it turned out that the printing plate, which had a glass pane as its surface (glass is a poor heat conductor) had a temperature of around 140°C - 150°C and the printed parts had simply melted away.

So be careful with thermistors that are mounted on the edge or on the surface.
A thermistor directly in the middle of the heating mat is the safest.
There are also special safety switches that can be attached there, which then cut off the power to the heating mat above a certain temperature.
Unfortunately, I can't think of the name of these switches, which are hardly larger than a thermistor itself.

P.S.:
They are called "Bimetal Temperature Switch Thermostat" and they come in a wide variety of designs and for different temperatures and volts, at which they interrupt the circuit and thus prevent overheating.

Google Translate
-- Original Text --

Ich habe eine 330x330x8mm Aluplatte.
Darunter eine handelsübliche 24V Silikon-Heizmatte (max 120°C) sehr ähnlich der Keenovo.
Die Watt-Zahl weiß ich gar nicht mehr.
Das Druckbett ist nach unten mit einer 10mm Platte aus Kork isoliert die ein sehr langsames abkühlen der Heizplatte mitbringt.
Den Kork habe ich mit Silikonkleber zusammen geklebt der Temperaturen bis zu 350°C stand hält.

Um eine geschlossene Umrandung zu bekommen die zudem die Korkplatte schützt und zusätzlich noch hält, habe ich ein 20x20mm Aluminium L Profil an die Aluplatte geschraubt (Siehe Foto)
Um es besser sehen zu können, habe ich einen Spiegel auf das Gehäuse des Druckers gelegt.

(Siehe Foto oben)

Ich nutze den in der Heizmatte eingeschweißten Thermistor und habe etwa 3cm vom Rand entfernt einen PT100 Thermistor unten an die Aluplatte geschraubt den ich zur Kontrolle nehme.
Also genau anders herum wie Du es machst.

So wie Du es angeschlossen hast, also den Thermistor in der Heizmatte zur Kontrolle und den Thermistor am Rand für die Abschaltung... da besteht die Gefahr dass die Heizmatte einen Schaden nehmen kann oder im schlimmsten Fall es zu einem Brand kommt.

Es braucht nun einmal seine Zeit bis sich die gesamte Aluplatte aufgeheizt hat.
Und wenn einige Minuten vergangen sind, kann man mit dem am Rand (oder wo auch immer er montiert ist) befestigten Thermistor die Temperatur mit der aus der Heizmatte vergleichen und zur Not die Temperatur um die Differenz erhöhen.

Ich verlinke Dir mal einen Thread, da wurde der Thermistor am Rand zwischen Glasplatte und normaler Aluplatte geklemmt.
Dieser Thread wurde gestartet weil die Druckteile zusammen fielen und merkwürdige Wölbungen annahmen.
Erst wurde vermutet das der Delta-Drucker einige Probleme mit der Geometrie hätte.
Aber am Ende stellte sich heraus dass die Druckplatte die eine Glasscheibe als Oberfläche hatte (Glas ist ein schlechter Wärmeleiter) eine Temperatur von etwa 140°C - 150°C hatte und die Druckteile schlicht weg geschmolzen waren.

Also Vorsicht mit Thermistoren die am Rand oder an der Oberfläche angebracht sind und für die Schaltung genutzt werden.
Am sichersten ist ein Thermistor direkt in der Mitte der Heizmatte der die Temperatur steuert.
Es gibt auch spezielle Sicherheitsschalter die man dort anbringen kann, die dann ab einer gewissen Temperatur den Strom zur Heizmatte trennen.
Leider komme ich gerade nicht auf den Namen dieser Schalter die kaum größer sind wie ein Thermistor selber.

P.S.:
Die nennen sich "Bimetal Temperature Switch Thermostat" und es gibt sie in verschiedensten Bauformen und für unterschiedliche Temperaturen und Volts, bei denen sie den Stromkreis unterbrechen und somit eine Überhitzung vermeiden.

[jens55]

@norder said in Keenovo Heater with Built in Thermistor Questions/Concerns:

There are also special safety switches that can be attached there, which then cut off the power to the heating mat above a certain temperature.
Unfortunately, I can't think of the name of these switches, which are hardly larger than a thermistor itself.
P.S.:
They are called "Bimetal Temperature Switch Thermostat" and they come in a wide variety of designs and for different temperatures and volts, at which they interrupt the circuit and thus prevent overheating.

I am not sure I have ever seen a "bimetal temperature switch thermostat" small enough to attach to the build plate but I suppose they must be available.
What most of us use is a "thermal fuse" which is available in every temperature imaginable and does not reset like a thermostat. If you exceed the rated temperature it opens the circuit and stays that way. It needs to be replaced at that point.
I don't think you want a 'thermostat' kind of re settable action when all your other safeties have failed - you want that thing to be off, period.

[Norder]

@jens55 said in Keenovo Heater with Built in Thermistor Questions/Concerns:

...you want that thing to be off, period.

It depends what you want.
If it is an overheating protection, then you want a certain temperature not to be exceeded but the heating bed stays alive.

If you want a panic OFF, then the circuit should remain permanently interrupted to prevent a fire, for example

But you could also install both...
A thermostat to prevent overheating and a thermal fuse to intervene should the thermostat fail to prevent a fire.

I am not sure I have ever seen a "bimetal temperature switch thermostat" small enough to attach to the build plate...

Of course there are. They are hardly bigger than a heating element in the HotEnd.
YouTube Video

[dc42]

@ctilley79 I suggest you configure the built in bed heater thermistor as an additional temperature sensor, and use the M143 command to set a temperature limit for that thermistor. That will give you some protection if the centre of the bed heater detaches from the aluminium plate.

[jens55]

@norder said in Keenovo Heater with Built in Thermistor Questions/Concerns:

Of course there are. They are hardly bigger than a heating element in the HotEnd.
YouTube Video

Cool .... learn something new every day! I still wouldn't want a thermostat as a safety device in case all my other safeties (that are built into the Duet) failed.

[ctilley79]

@dc42 Good idea. Made that change. Should supplement the thermal fuse I already have installed.