Heated bed won't heat

Hi everyone,

Contributor here and I usually solve other people's problems. Well, today I have a problem of my own. I am totally stumped.

I have a Prusa i3 Rework that has been running like a champ for around 9 months. Recently I noticed that when I go to turn on the heated bed (Marlin firmware with Full Graphic Smart Controller), the screen dims. A lot. I thought nothing much of it over the past couple weeks until today.

I went to start up a print and the bed was around 80 degrees Celsius, with a target of 70. I accidentally set it too high and was looking to go back down to 70. The screen was quite dim at this point because the heated bed was in use. As I was waiting around, the printer inexplicably shut off and turned back on. The heated bed stopped being in use, so the screen was at the full brightness.

I was a bit confused but I reset the settings for the heated bed again back up to 70. However, even though the target was 70, the temperature kept on dropping from 58 down and down and down.

I turned it off and tried again to warm up the heated bed. Still no luck.

---

Now I began some sleuthing of my own. I took out a multimeter and measured the voltage at the top of the screw terminal (by pressing the probes into the screws). No voltage there. No voltage on the bed terminals either.

So I disconnected the bed from the RAMPS board and tried to set the heat up again. No voltage again. However, the screen dimmed even without the bed connected.

I tested the continuity of the wires and the resistance of the heatbed and it seems like it is in good shape. So I don't really know what the issue is.

---

My best guess is that it has something to do with the RAMPS board, perhaps one of the MOSFET chips? Anyone have any ideas?

Thanks!

Edited 2 time(s). Last edit at 02/08/2015 12:41AM by Blue Ice.

---

Blue Ice

blog at [contemporarymaker.com]

It sounds like there's a short circuit around the MOSFET switch and when the switch turns on the the short is pulling the power supply voltage down. Monitor the power supply voltage to the RAMPS board and see what it does when the switch turns on and off. Look for solder blobs on the RAMPS board, check the wires to make sure they aren't frayed and shorting or touching anything they shouldn't. When you ay the display dims do you mean the backlight dims or does the display contrast wash out or both? Does it do this with and without a USB port connection to your computer?

I had a problem where the 5V regulator on the Arduino board was bad and the display looked fine while I was plugged into the USB port on a computer (because the USB port supplied the 5V to the display), but when I disconnected from the computer I couldn't read the display.

Quote
the_digital_dentist
It sounds like there's a short circuit around the MOSFET switch and when the switch turns on the the short is pulling the power supply voltage down. Monitor the power supply voltage to the RAMPS board and see what it does when the switch turns on and off.

All right. It stays steady at about 14.92V with and without the bed "on" and the screen dimmed.

Quote
the_digital_dentist
Look for solder blobs on the RAMPS board, check the wires to make sure they aren't frayed and shorting or touching anything they shouldn't.

I didn't get a chance to take a peek at the bottom of the board yet- too many wires to deal with. I'll do that next. I just checked all of the wires and connections and they are not frayed in any way.

Quote
the_digital_dentist
When you ay the display dims do you mean the backlight dims or does the display contrast wash out or both? Does it do this with and without a USB port connection to your computer?

I mean that the contrast washes out. The backlight is fine. It does this with and without a USB connection.

Quote
the_digital_dentist
I had a problem where the 5V regulator on the Arduino board was bad and the display looked fine while I was plugged into the USB port on a computer (because the USB port supplied the 5V to the display), but when I disconnected from the computer I couldn't read the display.

Hmm. This doesn't seem to be exactly what I have.

However.

I tested heating up the ceramic cartridge for the hotend and one of the MOSFETs became very hot (finger test). But when the bed is "heating up", both of the other two MOSFETs are stone cold. When I flip the RAMPS board over for inspection, I'll test all three MOSFETs with the board running to find voltage and resistance. None of the MOSFETs are heatsinked, so there is a good chance that one burned out and died. You better bet that I'll extend the leads out in the future and heatsink all three suckers in the future if this is the case.

Thanks so much for the help, the_digital_dentist. Please let me know about any other suggestions that you have.

Edited 2 time(s). Last edit at 02/09/2015 01:16AM by Blue Ice.

---

Blue Ice

blog at [contemporarymaker.com]

I just took a video of the effect of the screen dimming:

https://www.youtube.com/watch?v=Qqm13lPNRaU

---

Blue Ice

blog at [contemporarymaker.com]

Turn the heater on. put a probe on + for the bed and - for your power supply. If you have 12V, then your bed mosfet is likely toast. If you don't have 12V, I'd check your polyfuse.

Also it sounds like you have a ~15V power supply? Most Arduinos don't like to be ran at that high of voltage. 12V is usually the recommended max as the power regulator has to dissipate the extra voltage as heat. If you're going to run at > 12V, I'd figure out a way to step that voltage down (external regulator, buck-boost power supply, etc) and remove D1 on the ramps board.

I'd also bet your dimming issue is due to the Arduino not being able to supply sufficient power.

MOSFETS were blown. After replacing them and heatsinking them, the printer runs like a champ. Thanks for the help.

---

Blue Ice

blog at [contemporarymaker.com]