Issue with weird heated bed temperature fluctuations I have a strange issue with my heated bed. It has been working well for a long time, but recently it has developed an issue where the temperature reported by the thermistor will occasionally jump by around 10 degrees.





My setup is a 600W, 240V silicone heater mat, with integrated 100k thermistor, that is switched with a solid state relay. The mat is fixed to a 4mm thick aluminium plate. It seems unlikely that these fluctuations are due to bad PID tuning since the reported temperature changes much more quickly than is physically possible.

I've checked the wiring, tried using a different thermistor port and making sure the connector was properly plugged in, but to no avail. The fluctuations are brief enough that they don't cause any problems when printing, but I'm worried about the issue getting worse.

I have a RAMBo board 1.1b, and I've tried using both the integrated 5V SMPS supply and an external USB supply.

The issue is not limited to the start of printing, the temperature reported can be stable for a long time before the issue pops up. The issue also occurs during cooldown (further confirming that PID has nothing to do with it):



In fact, the issue seems slightly more common during heat up and cool down, but is not limited to these times.

I would like to know what might be causing this issue, and if there's a way to solve it without replacing the thermistor (which would be a pain, since it's integrated into the heater mat).
It really looks like either a bad thermistor or bad electrical joints. Are the connections to the thermistor itself crimped, or soldered? Are there connectors near the thermistor that can get heated up by the bed?

Electrical connections that are mechanical in nature (such as crimped, or using a connector) can degrade quickly in heated situations, particularly when the heating is intermittent.  Consider replacing crimped connections with soldered joints where possible, and where not possible use gold plated connectors and crimps to avoid oxidation issues. Oxidation and poor mechanical connections will change resistance based on movement, giving you apparent increases or decreases in sensor readings even when the sensor output is unchanged.

Chances are good that the thermistor itself is fine, but that the connection nearest the thermistor, between the wires and the thermistor, is faulty. If it's crimped, you may be able to simply add flux, then solder it to increase the bonding between the wires and the thermistor and decrease future issues with oxidation.

Note that soldering is really only useful for the thermistor on the bed.  The head thermistor must be crimped, as the head temperatures can exceed solder temperatures depending on the target temperature and the solder alloy.