Intermittent heated bed issue

I had a working heated print bed until recently, when the bed heating became quite intermittent and unpredictable.

When I power up the printer and turn the heat on the print bed in Repetier-Host, the bed light turns on and things start to warm up. But by the time the heat reaches ~20C the larger polyfuse on the RAMPS 1.4 board gets blisteringly hot and the bed shuts down. Fairly standard stuff well covered in other threads.

But with some powering on/off of the various parts, a little time and sacrificing a chicken in a volcano, I can sometimes get the bed to heat properly and the fuse remains cool.

Should I be replacing the larger polyfuse? Is the MOSFET likely to be the culprit? Why intermittent?

Thanks for any help.

- RepRap Prusa i3 (Hephestos)
- Ramps 1.4
- Repetier-Host
- XBox power supply on D8 for the MK2a print bed
- Separate 12V power supply for the printer

Polyfuses rely on the temperature of the internal material in them to determine how much current is being pulled. This means that if the ambient temperature changes (eg: the temp of the room, temp around the board, the amount of airflow, etc), the current at which it trips will change.

Even with a fan cooling the polyfuse, you can't guarantee that the fuse will trip at a reliable fixed value, as it depends on how much heat the polyfuse has built up over time. This is due to the fact that in over-current scenarios, the fuse takes time to trip (20+ secs according to the spec sheets), even when it's a direct short.

You may find that you're heating up the bed JUST enough at the start (with the polyfuse tripping all the time), that eventually the current used for the heatbed reduces marginally (the resistance of the heatbed goes up slightly as it's temp goes up). This reduces the current pull so the polyfuse finally stops tripping.

IMO, the polyfuses are not reliable in this application, and in some failure scenarios can be downright dangerous rather than protective. I recommend replacing the 11A polyfuse on a RAMPS board with a 15A fuse.

Really good insight. I tried to sort of "step up" the heat to the bed by heating and switching off, letting the polyfuse cool off between. In the past, once I got beyond about 35C, it would stay cool.

But I tried this last night to no avail. I might solder in a new polyfuse to rule anything else out and if I get the same behaviour, I'll find a way to rig an automotive fuse instead.

Thanks.

To complete this thread, in the end I elected to replace the 11A polyfuse with a 14A polyfuse. This now not only has the heat bed functioning perfectly, but it also runs at a cool temperature.

I can only presume that my heat bed was drawing just over the 11A and overheating the fuse.

Thanks Cefiar for the input and assistance with this issue.

Quote
omphe
To complete this thread, in the end I elected to replace the 11A polyfuse with a 14A polyfuse. This now not only has the heat bed functioning perfectly, but it also runs at a cool temperature.

I can only presume that my heat bed was drawing just over the 11A and overheating the fuse.

Thanks Cefiar for the input and assistance with this issue.

Did you replace the MOSFT? What type does your RAMPS have?

Steve

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My updated Instructable on our Prusa i3 Build
[www.instructables.com]

Quote
omphe
I can only presume that my heat bed was drawing just over the 11A and overheating the fuse.

"Just over" doesn't apply to polyfuses. Just an amp or two might eventually trip it, but probably not for quite a while especially if it got up to temperature and only needed to maintain it intermittently. My heated for instance bed draws 15 amps through the 11 amp rated polyfuse and it's never tripped, even with long prints running at 110 degrees, but I also have a fan blowing over the entire ramps board.

You really should measure the resistance of your heated bed to determine how much current it's drawing if you can't directly measure it with a meter. If your old poly fuse quicky over heated and tripped, you probably weren't a few amps over 11 amps. You might have been very well over 11 amps, which means you're also probably over your 14 amps. You're approaching the limits of the stock RAMPS connectors and if you haven't also already replaced it with a better suited one, your mosfet is probably running very hot as well.