Intermittent heat bed issue (D8 oscillating)

Hello reprapers!

I'm new to the forum and also with electronic and programming. Due to a unexpected resignation i became addicted to this new hobby.

I spent the last 2 years researching and adding tools and finally i built my first 3D Printer (I3 style watching TOM's 3D channel). It was just an amazing experience and joy, but it's normal that comes with some issues.

But man, a get an unbelievable print quality that i never expect!

I have been able to solve almost 90% of an incalculable number of problems, but there still 1 thing that i can't solve, so let's get to the point.

I'm using a MK2 210x210 heatbed (cheap Chinese PC with an external MOSFET 30a board with heatsink and a dedicated Industrial 400w 33.3a power supply for the bed. When I turn the bed on at Repetier i saw the bed output signal increasing but 0V at D8, the curious thing is that D8 output led starts a very weak glow and after ~20min increase the glow with the voltage too. This behavior keeps cyclical until D8 gets around 6.5V (at this point whe talk about 1~2 hours) only then it sends 12V to the external MOSFET and things started getting hot (after that the bed hit 110°C very fast, like 4min), but never stable. Every 4min the process restart and it never keep the temperature.

I'd tryied every thing, insulate under bed, replace all cables according specs, replace arduino, replace ramps, power supply, a lot of fans on ramps as MOSFET, but nothing seems to work.

My last attempt was replace the big polyfuse to a 15a automotive, and the only thing that changed was the D8 led now keep always on but when starts the bed it hits arroun 3V and keep the same old cycle (increasing very slowly the voltage and external MOSFET os off during that).

ps.: can't use PID bed tuning because the very long time needed to start heating the bed, always end with thernal runaway, so the only way that i can use the bed is with bang bang and thermal protection disabled.

I hope some one could help me, because it's very frustrating arrive at this point and can't have a 100% functional printer.

Sorry if my english wasn't very clear, this is not my native language.


Thank you so much!

So I will assume that you are using a RAMPS 1.4 and an Arduino Mega 2560.

You mention the dedicated power supply for the heated bed, what are you powering the RAMPS and Mega2560 with? The 400W 12V power supply should be able to power the RAMPS directly without the need for the external MOSFET going directly to the 12V inputs on the RAMPS board. There are 2 separate power circuits by default on the RAMPS board, one 5A setup which is used for the motors and hot end. The other power input is isolated and is used entirely to power the output to the heated bed. If you did not connect 12V to this, you will not see power on the output to your external MOSFET.

So you should have 12V power supplied to the RAMPS at both inputs, even if you are using the external MOSFET. If you do not supply power to the second (11A) power supply on the RAMPS, then the output of the MOSFET will be questionable at best. I would actually expect that you get nothing, as that circuit is supposed to be isolated.

---

MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

Thanks for reply, the other power supply is feeding the ramps 5V and 12V connectors (Industrial 250w 20a) and the other is just for heatbed.

But are you feeding BOTH of the 12V feeds on the RAMPS? there should be no 5V feed on the RAMPS, Only the Arduino would require 5V.

---

MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

Are you talking about these connectors(first attached pic)? All ramps are in 250w 20a and only the MOSFET (second attached pic) in 400w 33.3a.

Edited 1 time(s). Last edit at 08/20/2018 07:16PM by Kroyzus.

Yes, I am talking bout the connector in the first picture.

What you have wired to it is a little unclear. In this image:

you can see that there is power (red, black, red, black) going to that connector. your image is a little unclear that you have the same power going to both. Also in your second picture, I don't see anything connected to the external MOSFET's control circuit. Did you disconnect it for the photo?

Honestly, if you have a 400W+ power supply available, and you're using a readily available PCB|, I think that you should not need the external MOSFET at all. The more or less standard PCB heater board is ~250W dissipation, more than possible with the 11A fuse and on-board MOSFET that the RAMPS board has without the need for an external add-on. It seems to me that would make for less to go wrong.

---

MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

So, in short, I would recommend that you connect your RAMPS board much like that diagram. (Maybe not with the second extruder, maybe with a PWM fan instead of a second heater.) Use your high wattage 12V power supply only.

If you really want to use the external MOSFET, then the output to the headed bed instead goes to the control input (Usually via a pair of light gauge wires with a dupont connector into the socket on the external MOSFET board) I didn't see this in your image. An example would be like this:

The control board in that image isn't a RAMPS board, but one of the all-in-one boards that is compatable.

I would go over everything and make sure that connections are as they should be, observing polarity. you said that you replaced the RAMPS board. I wouldn't think that you'd have 2 bad MOSFETs which are bad int he same way in a row, so it's highly likely that the problem is with how you've got it wired.

---

MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

I would like to appreciate you interest in help me with this issue.

Maybe the pic wasn't very clear but I'm sure thats is exactly like the diagram that you post.
With the MOSFET i'm not using the dupont connectors because it was very fragil so i'd soldered under board connectors (it only work this way). I need to use them because even with 90mm fan over it, it heats like hell.

The good notice is that i finally figure out the problem, it was ramps native mosfet. Yep, I've got 2 bad ramps board but now i replace and the problem is gone.

Now the bed hits 110°C in 5min.
Thanks.



Edited 2 time(s). Last edit at 08/21/2018 03:28PM by Kroyzus.

Have you connected the GND Pins of the both power supllies? - Else it will never work!

Yes, both connected. But the problem was the leg of the MOSFET.

I've replaced D8 and also D10 as precaution. Now everything works better than i intended.