[Chiuso] Preheat Error Driving Me NUTS!  

Re: Preheat Error Driving Me NUTS!

Okay so after another couple hours of testing I think I have it narrowed down to the bed.
I can preheat to 240/90 no problem, and it will stay there for as long as I like. So I manually moved the temps one at a time. 240 up to 256 and all is well. But moving the bed up 1c at a time and letting it sit for 2-3 minutes at each step everything is good until I go PAST 97, then I get a preheat error within 30 seconds.

It seems that maybe the heater on the bed just can't keep up with the demand. Not to mention it takes a LONG time to even get to 97c.

Room temp is 26.5c, and no drafts near printer.

Re: Preheat Error Driving Me NUTS!

check the following three things at your assembly:

1. connector between printbed and rambo board. can you still unplug it, or is it already molten ?
2. connectors between rambo board and psu... even check the connection at the psu side !
3. check the voltage at the power connectors (at the rambo board) with a multimeter.

the heated printbed draws approximately 15 amps @ 12V, which is quite a decent amount of current and at the edge (wouldn't say beyond) of the specification of the connectors.

furthermore: at the psu, near the power connectors, there is a little trimpot, where you can adjust the output voltage.
if you can't reach the required temperature of the heatbed, you can increase the voltage a little bit (i would suggest between 12.2 and 12.5V). but i recommend to do this only while the rambo board is disconnected ! otherwise a short glitch at the trimpot might let your supply voltage exceed the limits of the rambo board.

Re: Preheat Error Driving Me NUTS!

Thanks for your reply!
All connections are fine, and nothing is melted.
When the board was cold but starting preheat there was 11.37v, right before reaching set temp it was 11.45v at the solder pads on the bed.
Getting 11.52v at the bed connector on the Rambo board. And 11.91v at the PS connector at the Rambo board. Looks like I could definitely up the PS voltage a little. This would also lower the amp draw slightly and possibly allow me to hit the 100c mark for ABS.

BTW, this is my very first printer, this is even the first time I have ever been around a 3d printer. Yes I have read that ABS is not for beginners, but I just finished my first real part using ABS at 255c/96c and it turned out absolutely perfect. SOOOO my earlier hypothesis appears to be correct about the heater simply not being able to output anything over 97c, because for the first time there were no preheat errors! Looks like Prusa needs to figure out how to get the heated bed to heat up a bit more. (maybe going to 16v?)

SOLVED!!!

Took your advice...
Took the PS off, measured where the hole is behind the printed housing, drilled a hole so I could easily get to the adjustment pot. Put it all back together. Adjusted the PS to 12.4v, plugged it back in and preheated to 255/100 with ZERO issues! Now I measure 11.92v on the solder pads of the bed. Huge improvement thanks to you!
Prusa SHOULD make checking the output voltage of the PS an important step in the assembly manual! This would have saved me DAYS of headaches and probably many other people too!

Thanks for your help! 😀 😀 😀 😀

Re: Preheat Error Driving Me NUTS!

.... Looks like Prusa needs to figure out how to get the heated bed to heat up a bit more. (maybe going to 16v?)
💡 i would strongly disadvise 16V.
➡ the heating power, and even the power loss at the mosfet at the rambo board, is a function to the power of 2 by the input voltage. usually adjusting the output voltage of the psu is limited to 13.8V (sometimes 14.4V). so if you drive your system with 13.8V instead of 12.0V (as designed), you'll get 32.25% more heating power... which should be more than enough for tuning.
but with it comes 32.25% more power loss at the mosfet at the rambo board.... so sooner or later (during summer ?) the board requires some cooling when rising the supply voltage.
furthermore you have to consider that increasing the supply voltage affects all the electronics (so the hotend is affected as well) and will speed up the deterioration of it.

😳 so if you crank up the voltage to 16v and you do the maths for the powers, it'll be 77.78% more as designed ! i'll bet this wont go well for a decent time of operation.

Re: Preheat Error Driving Me NUTS!

LOL, I never said to turn the existing design up to 16v... I mean maybe they should redesign the heater circuit to use 16v which will lower the amp draw and create less heat in places other than the heated bed.

I'm at 12.4v now and all is well. Printed for 3 hours last night with no more preheat errors!

Re: Preheat Error Driving Me NUTS!

... I mean maybe they should redesign the heater circuit to use 16v which will lower the amp draw and create less heat in places other than the heated bed...
this would require a redesign of the heatbed as well and a power supply with two voltages. the actual MK42 bed would draw 20 amps then...
but you are right, there are printer designs out there, that use 24V for the heatbed.

Re: Preheat Error Driving Me NUTS!

I mean maybe they should redesign the heater circuit to use 16v which will lower the amp draw

Err... No it won't, it will increase the current draw.

Say, the heated bed has a resistance of 1 Ohm. At 12 Volts, the current draw will be 12 Amps. At 16 Volts, the current draw would be 16 Amps.

The bed's resistance is actually somewhere between 0.8 and 1 Ohm. The stepper motors used within the system are "off-the-shelf" and run at 12 Volts. As are the hans and extruder heater A re-design would require the fans, motors, bed and extruder to all work at 16V and require all of those components to be proprietary - or in other words, excessively expensive. An alternative would be to have 2 PSUs (1 x 16V + 1 x 12V) which would also add significant cost (not many 16V PSUs around....

Peter

Re: Preheat Error Driving Me NUTS!

That is only if the resistance is a constant.

This is all just hypothetical anyways... It's not going to happen.

So why not just use a 24v bed and PS, then a coil to lower the voltage to the Rambo?

Re: Preheat Error Driving Me NUTS!

So why not just use a 24v bed and PS, then a coil to lower the voltage to the Rambo?

Why? RAMBo is fine up to 28V. I believe fans/motors/extruder heaters are also available in 24V version with similar power drain. It's only the bed that is not.

The RAMBo has a built-in buck regulator to generate the 5V rail (not linear regulator). The problem is that the system was designed around 12V.

Peter

Re: Preheat Error Driving Me NUTS!

THANKS!
I too was having Preheat Error at high 90's Deg C and duplicated randy.h fix - bumped source voltage from 11.8 to 12.0 and all is good.
Thanks folks.
Now if I could only get 1st layer ABS to stick.

Re: Preheat Error Driving Me NUTS!

Jeff.T3
I don't know why, you have an issue with your first layer not sticking, because mine is RIDICULOUS to get it off the bed, and I have no issue at all with layer separation.

This is my very first printer, and my very first time ever printing anything. I chose black ABS for my free filament with my printer. I use the preset ABS temps (255/100), and I set the nozzle/probe height per instructions in the kits build manual (using a paper and included zip tie) I then had to move the live Z down .36mm so that the filament was flat and looks smeared into the bed. The first layer doesn't look perfect, but once the 2nd layer is put on the first layer looks as it should to me. I am blown away at how easy and nice ABS prints on this printer! PM me if you need anymore info.

Re: Preheat Error Driving Me NUTS!

thanks randy.h

My problem was the z-live adjust. I went back to the calibration documentation/video, followed it and that is now good and I've had several great prints now...

However, my PREHEAT ERROR is BACK.
I can usually get something to print by quickly resetting, putting "paper towel blankets" on the bed to help it stay hot, and trying again one or two times.

Re: Preheat Error Driving Me NUTS!

This time the PREHEAT ERROR was being caused by a loose temperature sensor on the underside of the bed (it somehow got pulled outside the tape).

All is good.

Re: Preheat Error Driving Me NUTS!

I had the same problem with "PREHEAT ERROR" when trying my first "higher temp filaments"(PETG). The heatbed is not able to do the step from 85deg to 90deg...the LED on the heatbed shows "full power" when trying to make the stepup, but the temperature is climbing with lower than 1deg/min and after a minute or two(in my case around 86deg) it says PREHEAT ERROR.

My guess would also be that the heatbed is "underpowered" or maybe the powersupply is adjusted a little to the low side...I've speculated on this previous during my PLA print, since i think i can hear the steppers change "tone" when the heatbed is activated. That could be an indication of underpowered powersupply or too small cable diamensions...the problem with the ability to step-up the temperature just makes my suspicion stronger i'll investigate this later on... might be something that the Prusa-team should look into also for future hardware revisions...

FIX: I solved the issue at the moment by removing the stepup in temperature inside slicer for the PETG filament and did the entire print at 85deg...no problems...

Re: Preheat Error Driving Me NUTS!

I had the preheat error and chatted with customer support about the issue. One of the things he asked was "what height was the nozzle while being preheated?" It had been fairly close to the bed. He suggested raising it higher, 8-10 cm, and trying again. Problem solved! I haven't had a preheat error message since starting the practice of raising the nozzle each time before starting the preheat step.

RE: Preheat Error Driving Me NUTS!

@randy-h

I recently recieved the PREHEAT ERROR and am seeking a solution. I will go home today and take my machine apart but I struggle with some of the terms found in your post (ie... "took the ps off..."). I need to know what I'm looking for when I go in to locate issue.