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Build plate heater problems solved PT100


mhatay

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Posted · Build plate heater problems solved PT100

For about a year I have been fighting with a build plate heater problem that comes and goes.

The intermittent nature and various errors have made it hard to pinpoint. 

However I think I found the error and it makes sense.

Symptoms,

The mainboard thinks the bed heater is at the correct temperature.  when It is actually not. 
Over temperature errors. ER14
Build plate taking to long to heat up error.
If I go to the menu - system / build plate / set temperature / enter, and set the temperature with the dial, the left display (actual temperature) immediately goes up to whatever I set the right display to, without any heat-up time.  
Some of this looked like a software error. Although I suspect the software is not handling the situation well, it is a PT100 hardware problem. 
 
What I have tried.
I read the FAQs 
Removed the heater plate and cable. With an ohmmeter rung out the cable. they appears to be ok, even with flexing and moving it around.
Checked the heating element, good, 
Checked the PT 100 reads ~ 100 ohms @ 20 degC. appeared to be OK.    
Checked the connectors, look good and are tight
I did a firmware upgrade.
A factory reset.
(used some bad words!)
 
None of this helped or gave me any indication of what is wrong.
 
I decided to take a better look at the PT100. I set up an ohmmeter, a heater and monitored with an infrared thermometer. I intended to plot the resistance vs temperature but never got that far, as soon as I started to heat the plate I got erratic reading. I experimented for a while to see if the errors were reproducible but they were not. 
I let it cool down and it again gave me ~ 100 ohms @ 20degC  
Then I used a small wooden dowel and pushed on the PT100 SMD chip. and the ohmmeter went crazy. I found that one side of the chip was sensitive to pressure.
Take home, Single point, (resistance at ambient) measurement is not enough to verify PT100s functionality. 
 
I re-soldered the chip and repeated the measurement. Everything looked good, with no erratic behavior  
It would appear that in my case the PT100 chip had developed a bad connection at the board that was OK at ambient but when heated would introduce erroneous resistance. 
Most likely thermal stress crack.
 
Note.
The board acts as a heat sink, preheat the board to ~80degC to get a good quick clean solder and not damage the chip. 
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