Heated bed temp drops when print starts

Sewyboowy

Hi there,

I have been using my printer to print PLA fine and it works well, with my heated bed at 50 degrees and extruder at 210 degrees.

I am now trying to print in TPE, and so have upped the bed temp to 70 degrees and the extruder temp to 250. Before the print starts, i can use repetier host to set both of these temperatures at the same time and both heat up together to the right temperature. However, when i start printing, the heated bed gradually starts dropping temperature and doesn't go back up.

I am using bang-bang for the heated bed and PID for the extruder.

Any ideas?

Thanks,
Tom.

RAyWB

What Power Supply do you use and whats the maximum reachable bed temperature?
Is Bed heater on the whole time when Temperature drops?

Sewyboowy

its an old 350W atx psu, it has 2 18A 12v rails. Maybe thats not enough?
Im seeing how hot i can get the bed now. Yes its always targeting 70 degrees during print

RAyWB

Can you measure Voltage at Heatbed ? Think Power supply is overloaded

Sewyboowy

it got to 88 degrees

Sewyboowy

sure

Sewyboowy

10.75V
it got to 88 degrees without the hotend on

Sewyboowy

i have a much newer corsair 600W power supply with a +12V 46A rail and -12V 0.8A rail.
Shall i change them over?
What is the -12V 0.8A rail for?
Thanks

RAyWB

it´s usually for RS232 Supply, there you need negative Voltage.
so if you see, you just can get 88 Degrees without Hotend on and without motors on... think that´s the reason.
Maybe you can do another measurement with all switched on and motors running, then i think voltage will drop beyond 10V.
From my experience you should change power supply

Sewyboowy

Ive just swapped out the power supply, ill see if it works!

Sewyboowy

Im getting the same problem unfortunately. I set the bed to 70 degrees and then turned the hotend on, then the bed started to lose its heat and the temp drops. Maybe the ramps board cant supply enough for the bed and hotend at the same time?

Sewyboowy

here's a graph of what happens.
if its not the ramps 1.4, maybe could it be that ive wired up my psu incorrectly? Ive used the positive and negatives from the CPU power cable coming from the psu

RAyWB

voltage measured directly at heatbed would be interesting, also check if connector supplying the ramps gets hot

Sewyboowy

voltage when heating (measured at 35 degrees) is 11.2V and drops to 11.07V when hotend is turned on.

The connector does feel a little hot actually

Sewyboowy

The two big square yellow components (capacitor?) are also very hot

Sewyboowy

After letting it sit for a while, i can make both the hotend and extruder get up to temperature, but when i start a print and the motors start moving, the temp drops again. Is it possible the ramps could be overheating and then thermal throttling somehow?

RAyWB

what kind of heatbed do you have? and can you post a pic of your wiring?

MartinH

The 2 big yellow things next to the power input socket are called resettable fuses, the larger one is rated 11amps, the smaller one is 5 amps. these are known to go high resistance and over heat.
A common mod for these is to replace with car type fuses or replace the poly fuse with a wire link and fit inline fuses in the power wires.

Sewyboowy

Its a glass bed 25cm in diameter. Im not sure what u mean about the type of heater but it was a stick on sheet. The wiring is the standard ramps 1.4V wiring, anything to look out for with regards to wiring?

I see, ive added a fan temporarily over the fuses and am seeing if that helps