heatbed dropping to zero for a few seconds?

So im new to the whole 3D printing and purchased a Folger Tech Prusa i3 reprap that came with a RAMPS 1.4 board.
I had taken the new marlin source code and modified it to work with my reprap and also enabled thermal protection for both the extruder and bed.
But i keep hitting the bet thermal protection and it kills my print due to the bed's temperature "disconnecting".
In repetier host, it looks as the bed dropped to zero then comes back up.
I've tried adjusting the time period and temp for the thermal protection but it seems to still hit that protection...
Is this normal?

Been heating up the bed to 80 degrees celsius and it does maintain that temperature.

this is pretty much what is happening...

Going to print without the bed thermal protection though...

Looks like your thermistor connection is bad . . . the temp should be steady . . .

And what happens if you load Folger's firmware?

- Tim

folger firmware, i believe, is the same issue.
heck, folger firmware was marlin 1.0 version.

I did have a much wilder temperature but it was nipped in the bud by switching the pins on the thermistor.

I was able to print fine without the thermal protection though and my extruder temp is consistent.

Edit:
I dont know why, but i think my thermistor might be shorting out...

Edited 1 time(s). Last edit at 07/06/2015 01:19AM by alatnet.

That pretty much rules out a config oops changing firmware . . . and leaves electrical. As you have noted, nothing is shutting down, but since Folger seems to ship a 40w heater, the temp is likely going *up* when this happens, and if it hard fails, you get a runaway . . .

- Tim

i think it might be the thermistor itself. might have bent a wire to much or the heat is doing something to the thermistor.
i might have fixed it as it now isnt going down to zero and returning to normal but im going to see about taping the thermistor in kapton tape at the wires to see if that will ensure it not disconnect or something.

Edit:
here's how it's looking now:


Edited 1 time(s). Last edit at 07/06/2015 02:00AM by alatnet.

Do you use bang-bang or PID control for the heated bed ?
If you use PID, did you run a autotune ( M303 E-1 S90 C8 ) and wrote the new values in FW and stored them in EEprom?
-Olaf

i have no idea...
i've just got this a few days ago and used the settings that folger tech had for their configuration.h.

Edit:
here's their gdrive of their prusa i3 stuff including firmware: [drive.google.com]

Edited 1 time(s). Last edit at 07/06/2015 02:16AM by alatnet.

#define TEMP_SENSOR_0 6
#define TEMP_SENSOR_1 0
#define TEMP_SENSOR_2 0
#define TEMP_SENSOR_BED 6

That might be the problem, because sensor 6:

Quote

// 6 is 100k EPCOS - Not as accurate as table 1

Try sensor 1 instead of 6 and see if it better.
-Olaf

for both extruder and bed or just bed?

Edit:
ok so here's the heated bed: [folgertech.com]
and here's the extruder: [folgertech.com]
i believe they are the same.
so the extruder should be using a 100k EPOS NTC thermistor and the bed is using a 100k thermistor (whatever that means)...

Edit 2:
i think this is the thermistor for the bed: [folgertech.com]

Edited 2 time(s). Last edit at 07/06/2015 02:57AM by alatnet.

I would do both, you can reflash it anytime, if it gets worse.
-Olaf

so i changed the temp sensors from 6 to 1, looking fine so far.
Also enabled pid temp for the bed and tuned both the bed and the extruder.
Everything seems to be going fine.
also had a bit of a problem with my x-axis belt but fixed that quite nicely.