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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.

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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):

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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).

1 Answer 1

From the decay curve of the temperature graph, it appears that the firmware is filtering the temperature signal. Since it jumps up so quickly, I suspect that the thermister is indicating a much higher than 10-degree temperature change, but after filtering the result is a ten-degree change.

If the problem were a bad connection to the typical thermistor, the temperature would appear to be much lower, not much higher.

An electrical fault that would cause a typical negative temperature coefficient thermistor to indicate an intermittent high temperature could be a short circuit or some other condition that could cause 0 voltage across the thermister.

Your circuitry may be different, but I would suggest that several circuits I've seen on 3D printers to sense temperature have one side of the thermistor connected to ground, with the other side connected both to a pull-up resistor to Vcc (+5 or +3.3 depending on the electronics) and to an analog input which samples (measures) the voltage. If the powered thermistor line shorts to ground anywhere, it will show 0 volts. If the pull-up resistor is disconnected from the power source, it will show 0 volts. If either thermistor line has extra connection resistance or has a bad connection, the voltage will be closer to Vcc than it should be.

You could check this possibility by connecting a voltmeter or oscilloscope to the non-grounded thermister line, and then monitoring the voltage. The voltage should always appear to smoothly change (limited by the thermal physics of the bed. Try moving the bed and/or head through the range. Try tapping the bed, cables, and electronics while looking for a change. Any sudden change indicated a connection problem. The voltage going up indicates an open circuit to the thermistor. The voltage going down indicates a short.