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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).

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    $\begingroup$ Do you have a cooling fan installed that potentially could change speed mid-print and cool the bed in the process? $\endgroup$ – Tormod Haugene Jan 14 '16 at 6:57
  • $\begingroup$ I don't, and it seems unlikely that a cooling fan could make the temperature jump up by 10 degrees in a matter of seconds. $\endgroup$ – Tom van der Zanden Jan 14 '16 at 10:08
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    $\begingroup$ You are probably right about that, Tom. Could it be that the actual temperature sensor attached to the bed is exposed to cooling? And btw, what is the time scale on the x-axis of the graphs you are have posted? $\endgroup$ – Tormod Haugene Jan 14 '16 at 11:14
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    $\begingroup$ I would be concerned about the routing of wires within and outside the heating pad. Perhaps there is inductive or other coupling between the thermistor wire and the 240V supply for the bed? $\endgroup$ – nanofarad Jan 14 '16 at 11:26
  • $\begingroup$ @hexafraction Yes, I'm worried about interference, the thermistor cable runs right next to the power wires and also right next to stepper motor wires. However, the issue also occurs when the bed is cooling down after a print, when those cables are not in use. $\endgroup$ – Tom van der Zanden Jan 14 '16 at 12:33
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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.

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It sounds like EM interference to me, though Adam's points about connections are also a good bet. I would try re-routing the thermistor wires far away from other wires. If that helps, then either leave them re-routed, or shield them. In any case, please let us know what you find.

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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.

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So it seems it's neither the PID, nor the outside wiring. It also seems unreasonable that these temperature jumps reflect reality (too quick both up and down).

You say that you have a heater mat with:

with integrated 100k thermistor

This seems to be a thing you haven't checked out, if it's broken / got its wires pulled out a bit, it could explain these jumps in temperature.

Try using another thermistor, at least to see if those temperature jumps occurs with it or not.

HTH

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