RepRapPro Mendel Heated Bed Temp Jumping

All has been going fine until I changed to use ABS. The problem seems to start at a higher temperature for the heated bed.

If I monitor the bed temperature it is cycling between a true reading and something like 220

This cycle is pretty uniform and is giving a square wave. This starts off with a duration of around 2 seconds and as the bed gets hotter it changes to around 1 second up and 1 second down.

I don't think that its is a probe failure since it is cycling.

Now I have set my bed temp to 55 for PLA again the readings are not jumping around.

I measured the voltage across the sensor while it was running at around 80 degrees and the voltage was steady.
I measured the resistance of the sensor while the machine was turned off and it was 3.5K.

I don't think it is anything to do with a bad joint or something like that since the temp measured oscillates even without a print running.

Has anyone else seen a similar problem or can suggest something to test further.

Question: Does the power to your bed turn off when you hit the 220C reading?

Yes, the LED goes off.

Please see below output. Blips are in red.

Really appreciate your help on this one. has got me totally floored.

Connecting...
start
Printer is now online.
echo:Marlin 1.0.1 RRP
echo: Last Updated: 2013-01-07-JMG | Author: RepRapPro
echo: Free Memory: 11527 PlannerBufferBytes: 1232
echo:Using Default settings:
echoteps per unit:
echo: M92 X91.429 Y91.429 Z4000.000 E875.000
echo:Maximum feedrates (mm/s):
echo: M203 X300.000 Y300.000 Z3.000 E45.000
echo:Maximum Acceleration (mm/s2):
echo: M201 X800 Y800 Z30 E800
echo:Acceleration: S=acceleration, T=retract acceleration
echo: M204 S1000.000 T1000.000
echo:Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=
minimum segment time (ms), X=maximum xY jerk (mm/s), Z=maximum Z jerk (mm/s), K
=advance_k
echo: M205 S0.000 T0.000 B20000 X15.000 Z0.400 E15.000
echo: M206 X0.000 Y0.000 Z0.000
echo: M208 X210.000 Y210.000 Z140.000
echoID settings:
echo: M301 P12.000 I2.200 D80.000 W125
echo:Thermistor settings: M304 Hh Bb Rr Tt, H0=Bed, H1..n=nozzle, b=thermistor b
eta value, r=series resistor, t=thermistor resistance as 25C
echo: M304 H0
B3480 R4700 T10000 M304 H1
B3960 R4700 T100000
FPU Enabled no
transform correction not enabledechoD card ok
ok T:18.7 /-273.1 B:19.4 /-273.1 @:0
ok T:18.7 /-273.1 B:19.4 /-273.1 @:0
ok T:18.7 /-273.1 B:19.4 /-273.1 @:0
ok T:18.7 /-273.1 B:19.4 /-273.1 @:0
ok T:18.7 /-273.1 B:19.4 /-273.1 @:0
Setting bed temperature to 120.000000 degrees Celsius.
ok T:18.7 /-273.1 B:19.4 /120.0 @:0
ok T:18.7 /-273.1 B:19.5 /120.0 @:0
ok T:18.7 /-273.1 B:19.8 /120.0 @:0
ok T:18.7 /-273.1 B:20.1 /120.0 @:0
ok T:18.7 /-273.1 B:20.5 /120.0 @:0
ok T:18.7 /-273.1 B:20.9 /120.0 @:0
ok T:18.7 /-273.1 B:21.2 /120.0 @:0
ok T:18.7 /-273.1 B:21.7 /120.0 @:0
ok T:18.7 /-273.1 B:22.0 /120.0 @:0
ok T:18.7 /-273.1 B:22.4 /120.0 @:0
ok T:18.7 /-273.1 B:22.8 /120.0 @:0
ok T:18.7 /-273.1 B:23.2 /120.0 @:0
ok T:18.7 /-273.1 B:23.5 /120.0 @:0
ok T:18.7 /-273.1 B:23.9 /120.0 @:0
ok T:18.7 /-273.1 B:24.3 /120.0 @:0
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ok T:18.7 /-273.1 B:24.8 /120.0 @:0
ok T:18.7 /-273.1 B:25.2 /120.0 @:0
ok T:18.7 /-273.1 B:25.5 /120.0 @:0
ok T:18.7 /-273.1 B:25.9 /120.0 @:0
ok T:18.7 /-273.1 B:26.2 /120.0 @:0
ok T:18.7 /-273.1 B:26.5 /120.0 @:0
ok T:18.7 /-273.1 B:26.8 /120.0 @:0
ok T:18.7 /-273.1 B:27.1 /120.0 @:0
ok T:18.7 /-273.1 B:27.5 /120.0 @:0
ok T:18.7 /-273.1 B:27.8 /120.0 @:0
ok T:18.7 /-273.1 B:245.6 /120.0 @:0
ok T:18.7 /-273.1 B:245.6 /120.0 @:0
ok T:18.7 /-273.1 B:234.3 /120.0 @:0
ok T:18.7 /-273.1 B:236.0 /120.0 @:0
ok T:18.7 /-273.1 B:231.7 /120.0 @:0
ok T:18.7 /-273.1 B:28.4 /120.0 @:0
ok T:18.7 /-273.1 B:28.5 /120.0 @:0
ok T:18.7 /-273.1 B:28.7 /120.0 @:0
ok T:18.7 /-273.1 B:29.0 /120.0 @:0
ok T:18.7 /-273.1 B:29.5 /120.0 @:0
ok T:18.7 /-273.1 B:30.2 /120.0 @:0
ok T:18.7 /-273.1 B:30.1 /120.0 @:0
ok T:18.8 /-273.1 B:257.1 /120.0 @:0
ok T:18.8 /-273.1 B:261.1 /120.0 @:0
ok T:18.8 /-273.1 B:247.9 /120.0 @:0
ok T:18.7 /-273.1 B:119.5 /120.0 @:0
ok T:18.8 /-273.1 B:195.5 /120.0 @:0
ok T:18.7 /-273.1 B:30.8 /120.0 @:0
ok T:18.7 /-273.1 B:30.9 /120.0 @:0
Disconnected.

And I have just switched off the bed and the same oscillations are occurring as it cools down.

And once it had cooled down to 46 or so then all readings were 205 degrees but then changed to 44 after some 50 readings.

I did use Arctic silver to give good thermal contact, I do feel however that it is more likely that there is a problem with the ADC.

Edited 1 time(s). Last edit at 02/01/2013 05:39PM by Splashy.

I think you might have an intermittant short. Check your wires and see if they ever come close to an actual heating element in your bed. It will be much hotter in that area and cause the wires to expand, allowing possible bowing and warpage all along the wire.

I once had a similar problem where a wire to a thermistor was heating up and warping until it shorted to the other wire, which would turn off the heating element and allow the wire to shrink back and break the short.

If that is the case, I have no idea why it wouldn't happen when you're heating the bed up for the lower PLA temp.

Just saw your second post. The short as I described it would not happen if the bed is continuously off, so never mind.

I don't know what would cause your thermistor to behave as you are describing it, sorry. Perhaps some of the wiser heads on this forum could help out?

One more thought: I looked up Arctic Silver's site and they had this to say about their conductivity:

Negligible Electrical Conductivity:
Arctic Silver Thermal Adhesive was formulated to conduct heat, not electricity. NOTE: Even though Arctic Silver Thermal Adhesive is specifically engineered for high electrical resistance, it should be kept away from electrical traces, pins, and leads. The cured adhesive is slightly capacitive and could potentially cause problems if it bridged two close-proximity electrical paths.

Could this be your problem?

Yes, I think you are correct. I did wonder about that so I pulled out the thermistor, cleaned it up and placed it very carefully in the hole. Now the readings are rock steady. It would seem that it behaved exactly as you described in that it did create some sort of bridge and that I was seeing the capacitive effect of charging and discharging causing the fluctuations.

Thanks so much for all your help, I will now try to clean out the hole and just use cotton wool.

Maybe worth revising the Wiki for the RepRapPro build.