Ramps 1.4 overheating or shorting?

I've got a ramps 1.4 board, and I'm using a heated bed. The bed was working fine for the past 2 weeks since I started this, but today I get bad things happening.

The heatsink next to the D8 where the heated bed is connect grows immediately hot as soon as I apply power to the machine. I don't even have the computer plugged in anymore, just the power, and this heat sink will immediately reach untouchable temperatures. If I leave the power plugged in for 5 or 10 seconds the plastic mofset terminals start to smoke. Yikes. Also, the LED on the heated bed is now always on when there's power. It doesn't matter if there's no computer giving it instructions. I tried re-connecting the computer and sending commands to turn off the bed, but it just won't turn off.

I'm not very experienced with electronics, but sounds like a short? Is there any thing that can be done, or does it sound like I need a new ramps?

Update, Okay, so I used a multimeter on the D8 connectors (the heated bed connectors) , and they are putting out 12V even when the wires to the heat bed are not attached. No other connector puts out power when not in use, so I'm going to assume that's bad.

Err, yeah, that's not good
Anyway, disconnect the 12v power to the inner connector (next to D8) and just plug the outer 12v connector in. Does the printer operate normally?

Note that waitaki is talking about it printing normally without a heated bed.

My guess would be that the FET is damaged, but it would be good to also check your Arduino Mega (specifically the state of the D8 pin when it is powered up).

The FET on most RAMPS boards is a STP55NF06L, though I would personally replace it with something different like an FDP8870 or (and the one I like most at the moment) the IRLB8743.

The advantages of both of these FETs is that they have a MUCH lower Rds(on) than the original (this means they dissipate less heat when switched "on"), are pin compatible, and are about the same cost. The IRLB8743 has the lowest Rds(on) value, but both it and the FDP8870's work well for driving the heated bed.

Note: The IRLB8743 only supports up to 30V, however this should not be an issue if you're only running 12V or 24V. I've not seen anyone use >24V for the heated bed through the FET, so I generally consider this to be a non-issue, but I know some people are looking at 36V for their motors, so I thought it best to mention it in passing.

PS: If you're going to remove the FET, I recommend cutting the existing FET off (just under the plastic body) then desoldering the pins individually with a decent temp controllable iron that can put out a decent power and a pair of needle-nose pliers. Once you've removed that, you can use solder wick and a solder sucker to clean up the pads/holes. One of the pins on the FET (the source) is connected to ground. As the ground plane on the PCB is quite large, it requires a bit of heat and power to properly remove the FET pin and then clean the pad/hole. Trying to heat all 3 pads on the PCB at once (so you can remove the FET without cutting it's leads) is not easy, especially when one of them requires more power/heat to remove.

+1 on what Cefiar said - sounds like a blown FET.

Just curious, where did you get your RAMPS from and which bed heater do you have (how many watts)? What voltage are you running at?

Waitaki : Yes, the everything else on the printer operates normally when I disconnect the 11a power input. The D8 stops putting out power too

Cefiar: Hmm, I'm afraid that's all a little advaced for me. My ramps came pre-assembled as I don't have a great familarity with soldering electronics. I only went as far as soldering the stepper drivers and various wires. I don't even know what part is the FET. I can connect communicate to the Arduino, how do I pull the state of the D8 pin?

Mitchese: I got the electronics from [www.a2aprinter.com], specifcally Ramps 1.4 and heated bed

To check the state of the D8 pin, do the following:

1. Unplug the RAMPS from the Arduino Mega.
2. Plug in the Arduino by itself (USB is fine). You don't need to run Pronterface or any printing app.
3. Check for voltage on the D8 pin (it's labelled on the Arduino) with respect to Ground/any GND pin.

The voltage should be 0V (the output should be "OFF" by default, as you've not told the heated bed to turn on). This indicates it's the RAMPS board is totally at fault (most likely the FET as above).

If it's 5V, or some weird value in between, then you most likely have a problem with the Arduino.

Note: Even if a problem shows up with the Arduino, there may still be a fault with the RAMPS board (and this damaged the Arduino, producing the fault). Best thing is to perform the test and let us know the result before going any further.

Re: What the FET is: The thing you refer to as the heatsink most likely, though it may have an actual heatsink on it. On RAMPS, a heatsink is usually necessary for the heated bed FET (the one near D8).

The FET looks like this:

A heatsink for the FET will look like this:

Pics are what was to hand, and not to scale.

The heatsink is usually attached to the FET using a small bolt/nut. There is usually heatsink compound between the heatsink and the FET (helps transfer heat from the FET to the heatsink). The "better" FETs I referred to don't always need a heatsink, tho I usually recommend one anyway (or at least a bolt/nut, as this acts like a mini heatsink).

PS: If you are lucky enough to have a HackerSpace or TechSpace nearby, you can probably take it down to them and ask nicely if they'd help you figure out what is wrong and fix it. They're bound to have some 3D printers about. There's also a number of 3D printer groups appearing around, so that might be a good lead to chase too.

AH! That's the FET. I mistakenly referred to it as the heatsink. That FET next to the D8 is what is overheating and melting the mosfet. This ramps does not have a heatsink on the FET at all. If it comes to getting a new ramps (most likely) I'll look for one with a heatsink.

Thanks for the info on checking if it's the arduino or the ramps, I'll check that tonight. Oh I hope it's not the arduino. It's my second one, as I couldn't flash the first one. Some business with resetting it just before the data upload and some secret time interval... I just wasn't getting it.

BTW: If you hadn't guessed, FET is short for MOSFET.

Went on the irc! Replaced the 1n400x diode and the thing is working!

I had a similar problem with my RAMPS 1.4 board. It ran great for about five minutes after I installed it. Then D2 - a 1N4004 diode - failed and clamped the 12V rail at 0.3 volts, causing my power supply to shut down. I never applied power backwards so I'm suspecting there was a bad batch of diodes.

D2 is supposed to clamp when reverse voltage is applied and kick off the fuse but it doesn't work as planned. The diode is rated at 1 amp while the 5 amp fuse can easily handle 15 amps for a couple of seconds. If your power supply is big enough, the diode simply fries and shorts if polarity is reversed. I guess that's still better than frying all of the electronics,

There is a simple solution to this. Since I had to replace the diode anyway, I used a 5 amp / 100V rated part. The only downside is, it's both larger and more expensive. But if I ever connect power backwards, D2 will protect the electronics from damage.

just would like to say thank's all on this Topic as it has help me big time

hi can someone Help me

does anyone know some were in the u.k where i can find this relay ? someone must make them here !! [www.makerfarm.com]

You can use a general purpose 12 volt 30 amp automotive accessory relay which should be available at your local auto parts store. Just ask for one with a suppression diode built in. They are also available with a suppression resistor but the diode is more effective at protecting the controller board from spikes.