Ramps drawing to much current?

Hey all,
Ive been running my prusa i3 for afew months now without much issue, that was until today, i set a print going which required the bed to heat upto 115 degrees C (which this machine has done many many times before), the bed heated to about 60 degrees and stopped heating, i noticed the light on the bed was no longer on and the temperature was starting to drop, after afew failed restarts i found that after the bed stopped heating the yellow fuses on the ramps 1.4 board were incredibly hot (as in they nearly took my skin off when i touched them).

The power supply im using is a 12v 30amp psu, thinking that perhaps the power supply was faulty i switched it for a 203w xbox 360 psu and tried again, this time it got to over 75 degrees before stopping and again the fuses were to hot to touch.

Im guessing that either something on the ramps board isnt working correctly or perhaps the heatbed is faulty and drawing to much current, if one of these are drawing to much current then it could overheat / trip out the fuses on the ramps board

Has anyone got any ideas what could be causing this?

My setup is:
Ramps 1.4 / Mega 2560
mk2a heatbed
12v 30a psu / xbox 360 psu
marlin firmware

Make sure you don't have any shorts in the wires for the heat bed secondly do you have a fan blowing on the board I mean the ramps board you definitely probably need this

Hello cnc,
i dont have a fan attached, havent had a fan at all and have had no issues, i did try to fit a fan however i couldnt find one that would fit my ramps board as i also have the gadgets3d lcd ramp attached onto and all the wires would obstruct the airflow.

I will have to remove the heatbed to check for breaks in the wiring, if there were a break wouldnt it just not heat up at all?

Okay, so i disconnected the heatbed wires from the ramps and one was very burnt (see attached pic), not sure whats caused it tho

find some more damage, one of the connectors for the heatbed has melted to the point it appears to have actually melted the solder on that pin (ive attached acouple of pics to show the damage), sadly as the damage is already present im now unsure whether or not it was the power supply or the ramps or the heatbed, ive checked the wiring on the heatbed and besides the melted part where it connects to the ramps it seems okay

Anything that do with the heat bed on the ramps are many other boards are very marginal as far as current goes that is why I always use solid-state relay. You definitely need a fan blowing on the board. Once the heat gets to a connection like the wire in your photo it compounds it creates more resistance and gets hotter it gets worse and worse and worse make sure you start off with a very tight connection not to say that will cure everything because as soon as it heats up the wire expands and when it is off it contracts and gets loose which starts the whole process of toasting

so you think the ramps board and the heatbed are ok? providing i replace the wiring with some high quality wire, perhaps some 14awg silicon wire, i believe they can handle upto 600v, 32amp and 200 degrees C. Obviously the idea of my electronics burning and possbily setting fire to itself are extremely alarming, im curious about the solid-state relay you mentioned, does that make running a heatbed safer? I will check thingiverse to see if i can find a ramps fan that fits, the problem will be finding one that can cool the fuses / connections, the wires will obstruct the airflow and if i go with a bigger fan it will blow air over the heatbed due to where the ramps board fits onto the i3.

I'm building a Reprap too and this is one of my concerns, maybe longer wires could help to cool down temperature along the wire.

Hey Rappers,

Had the same problem with mine a while ago that stopped me from printing for a while. I replaced my board when the pin on the supply to my hotbed melted and fell off.

I bought another board and the same thing happened again. I have just now replaced the wires from my board to my hotbed with much thicker wires. The old wires were the power supply wires from an old computer supply to the hard drive and I think these were too thin. They heated up to the point of melting the insulation off the wires and melting the pin from the board.

My wires from my old computer power supply to my power in on my board are also melting but only when I use the hotbed, which is most of the time but I can get away with a print as long as I turn off the hotbed after the furst layer.

I am hoping the thicker wires to my hotbed will solve this problem. I will try a print tonight and see how it goes. I might fit a fan as well. Don't know why I didn't think of that.

I have attached a pic of my old board. You can see where the right pin is missing and the old plug on the top that melted.

The other pic is of my thicker wires on my second board.

Thicker wires are always a good idea, I use 14ga zip cord, like on a table lamp. The problem is, it is the weakest point that causes the issue. Contacts on the board to the heatbed tend to vibrate loose,and/ or build up resistance as they get used. The marginal ratings of the connectors also play, they melt and cause issues. Best advice is to run a relay like cnc dick mentioned. It can be solid state or an automotive relay ( radio shack carrys them, right next to all their cell phones).
I replaced the standard connecter on the the ramps with a better connector, but I am goint to put in a relay, it just makes sense, as the mosfet will eventuality cook itself, and you will spend the money on a new mosfet, if not a new board, most likely with in a years time. Wiring in a relay is simple, the board heatbed conections go to the relay coil, 12volts from the power supply to one side of the relay(normally open side) and the other side goes to you positive heatbed input( if you have such a thing) the other lead from the heatbed goes to the power supply ground.. done and done...

Might have solved my problem.

I changed the old psu wires from the supply to a cd rom that used to run from the circuit board to the hotbed with much thicker wires and also placed the exhaust air from the computer power supply to cool the board and the wires don't even get warm to the touch.

I like the idea of a relay and now you have explained the connections so well, I might try that soon also.

As I understand your instructions, the relay will adjust the current going to the hotbed thereby adjusting the temp depending on the current supplied from ramps?

I don't see how this could work as I understand a relay, when power is supplied to pin 86, this turns 86 to 85 into an electromagnet which connects supply voltage ( pin 30 ) and current to the output ( pin 87 ). There is no connection between ramps positive, ( pin 86 ) to output ( 87) so how does the relay adjust output current.

Edit... Just found a post about a homemade bed that used a relay. The ramps turns the relay on or off to adjust the temp as per the input from the thermistor. Now I understand...

Thanks again all for the help. If we keep helping each other, we just might survive.. I have included a pic of a relay setup as I understand.

Greg

Edited 2 time(s). Last edit at 05/31/2014 04:36AM by gregted.

The pic you sent looks like a mechanical relay. It might work OK with bang-bang control, but you would need a solid state relay for reliable, long lasting PID control.

RepRap's PID control uses a Pulse Width Modulation (PWM) process to rapidly switch the power completely on and off for an amount of time proportional to the control. Mechanical relays aren't set up for cycling at the many-Hertz scales that some PWM schemes use.

Bang bang usually works perfectly fine with the bed since it's longer heating times doesn't make it overshoot much

Edited 1 time(s). Last edit at 06/02/2014 09:05AM by cnc dick.

I will have to get a solid state relay as you suggest. I hooked up a relay as the pic and the hot bed wouldn't stop heating at 110. Got to 150 before I cut the power and cracked my glass. No problem. I've got lots of glass. I doubled up the wires from ramps to about 6 inches down and then into 10 amp wires to the hotbed. Seems to be fine now.

Just built a heated compartment for my printer to stop warping so we'll see how that goes.

Really starting to get into some long overdue upgrades.

Led lighting next, maybe!!

Hopefully your MOSFET is okay on the board and go in and change or make sure that is set the bang bang not PID in configuration H and make sure it's set the Max 255 and minimum 255 on the output get a solid-state relay their is no worries no mechanical parts to wear out

Edited 2 time(s). Last edit at 06/02/2014 12:13PM by cnc dick.