heating bed decreases while printing

Does anyone know what would cause the heating bed to decreases in temp while printing?

Polyfuse problems on RAMPS?

Not sure since it works before and after the print.

What slicer are you using ? Have you set the default bed temperature in the machine settings in the slicer ? Have you set the bed temperature under the filament settings in the slicer ? The slicer settings will over-ride anything that you do before hand in terms of preheating.

Edited 1 time(s). Last edit at 11/21/2016 12:02PM by Supermec.

Some history first.

I had my printer for over a year and decided to tune the motors for the first time. I then shorted the ramps board and I ordered a new one but, not from the company that I got it from originally. When I replaced the ramps boarded I moved the wire one at a time from one board to the other board. I only made a few adjustments using slice3. I adjusted the the extruder heater temp and the %on the first level only. Other then that I haven't change it that includes keeping the heating bed at 80C. I printed a object that took 40 min and eventually the heating bed came back up to temp. So now I thinking maybe a resistor or even possible loose wire some were might be the problem. Recap, so it seams like it powering up to 80c and when I print it looses some of its power (drops about 9c) and then over time ramps back up to 80c.

My luck I probable got a bad board but, have no way of figuring that out.

Measure the current from heat bed terminals or from the bed contacts when it starts to drop.
it should be 12V. if it is, says, 10V, 8V, 0V etc.. i'd suspect the polyfuse being triggered because it heats a lot.

Fuse will recover itself when it cools down enough and will be able to pass current -> raising the bed temperature back to normal.

Or PSU problem. If you measure voltage drop from the bed. You then measure the PSU voltage output to verify that the PSU is giving all the power.

How do I use the multimeter to measure the poly fuse?

I used voltage DC for the steppers so should I use the same settings for the poly fuse?

Same setting should work. You are measuring current. if you have 1, 10, 100 settings on the multimeter, set to 10. You probably used "1" to measure vref.

Polyfuse will break the circuit when it triggers.
So, you can measure the current before and after the fuse WHEN heat bed should be turned ON but it is not heating.

So, basically you can measure the fuse by first putting the black lead to ground (Black is the minus ( - ) )
You find this from the power connector that comes to the RAMPS. It is a screw terminal and you can just point the probe to the screw that has black wire under it.

Then use the red lead (This is plus ( +)) to measure current from both of the fuse legs.
Both should read 12V if the fuse is Ok. (When heating is ON)
If only one of the legs is 12V WHEN heating is ON, then the fuse has triggered.

When heating is turned OFF, both of the fuse legs should measure 0V

Edited 2 time(s). Last edit at 11/24/2016 07:44AM by Veesta.

Quote
Veesta
Same setting should work. You are measuring current. if you have 1, 10, 100 settings on the multimeter, set to 10. You probably used "1" to measure vref.

Polyfuse will break the circuit when it triggers.
So, you can measure the current before and after the fuse WHEN heat bed should be turned ON but it is not heating.

So, basically you can measure the fuse by first putting the black lead to ground (Black is the minus ( - ) )
You find this from the power connector that comes to the RAMPS. It is a screw terminal and you can just point the probe to the screw that has black wire under it.

Then use the red lead (This is plus ( +)) to measure current from both of the fuse legs.
Both should read 12V if the fuse is Ok. (When heating is ON)
If only one of the legs is 12V WHEN heating is ON, then the fuse has triggered.

When heating is turned OFF, both of the fuse legs should measure 0V

Please DO NOT measure current like this.

this is how to measure VOLTAGE on the meter