My heated bed stopped working...

Hi everyone, I've tried searching but am yet to come across anything to solve the issue I'm having (not that I haven't tried all the solutions anyway lol)

I've got a 12V power supply going to a RAMPS 1.4. I had a heated bed hooked up (Mk2) and it worked perfectly for quite a while. Woke up today, tried to preheat it and at about 60 degrees the LED on the pcb turned off and it's never turned back on .

Took it off, re-soldered all the wires, checked for continuity. I get 12.6V at D8 on my ramps, 12.6V from the leads when not plugged to the board, and 0.02V when I plug it in. The board is showing a resistance of 2OHMs unplugged and 3.5ish when plugged in. The LED still doesn't turn on and the board is not heating up.

I have no idea how MOSFETs work and only found out what they do today. They seem to deal more with voltage, and I'm getting 12.6V from the wires going to the heated bed (when they aren't attached to the bed). There's definitely 12.6V going into D8 terminal at all times (plugged and unplugged from heated bed).

Is it possibly that when the heated bed tries to draw current through the circuit the FET is failing, or something like that?

It's had me running in circles all day, hopefully the brain bank here can point me in the right direction.

Thanks.

What happens if you connect the power supply directly to the heated bed?

---

_______________________________________
Waitaki 3D Printer

Ah I read that somewhere but wanted to avoid it just in case it wasn't a safe thing to do. Eventually frustration won and I tried it and, voila, the PCB lights (and heats) up. Of course I have no temp control or anything like that.

I'm assuming this narrows down the likely candidates to something on the ramps board?

There are two self resting fuses on the ramps board. It's those two yellowish things that look like large flat ceramic capacitors. You could have damaged one of those such that is can't reset it's self. There is one for the 5 amp side of the power connector and one for the 11 amp side of the connector. The 11A one is what powers your heat bed. (If I remember right it’s the larger one.) Use a meter to figure out which one has a leg connected to the plus side of the 11A power connector. Once you have identified which one it is check across the leads of this one. It should read almost no resistance. I it has something like 100K then it's probably the issue. If has next to no resistance, then you probably need to check the three legged mosfet transistor. I believe it's the one closest to the power connector. If your lucky your RAMPS builder put a heat sink on it. It's usually the only one with a heat sink. The RAMPS page has a schematic and should help you with your trouble shooting.

Just checked and you're right, it's the larger yellow one that's relevant.
I tested the resistance and I get 0.8 ohms.

I've located the relevant mosfet via the schematic but am not really sure how to test it? The voltage is definitely passing through it, though I have realized my previous measurements of current were conducted incorrectly (in parallel rather than in series).

Given the circumstance though, could the problem really be anything other than the mosfet?

If I need to replace it, is there a particular model that is less prone to failure given the current use case?

Does it have a heat sync on it?

No not at the moment.

Get yourself a heatsink on the heatbed Mosfet, the others should be OK.
Also, get a small fan aimed at the yellow polyfuses - the heat bed fuse commonly shuts down if uncooled.

I assume you have checked to see that your heatbed thermistor circuit is ok? Easy way to check is look at the reading in your host software (pronterface etc). You should see room temp displayed. Without feedback from your heatbed thermistor it cannot switch on.

---

_______________________________________
Waitaki 3D Printer

I had forgot that I noticed that. That's a good diag tip. And on the fan, putting a fan blowing over my ramps solve some of my axis skip issues. Pololus were over heating and shutting down despite the heat sinks. I wish they had a little red led on them to indicate when this is happening. BTW, the pololis cool from the bottom not the top. So position the wires such that your fan will blow air under them. This usually still results in air blowing on the fuses too.

waitaki Wrote:
-------------------------------------------------------
> I assume you have checked to see that your heatbed
> thermistor circuit is ok? Easy way to check is
> look at the reading in your host software
> (pronterface etc). You should see room temp
> displayed. Without feedback from your heatbed
> thermistor it cannot switch on.

Hey guys, The thermistor is definitely working perfectly, first thing I checked since it's happened to me a couple of times due to some dodgy wiring lol. I printed some new X carriages today with the bed plugged straight into the power supply (got to 140degrees at one stage)

Good tip re: the fan, I'll get one on there ASAP. Haven't had any trouble with the pololus yet but don't plan to tempt fate.

The current situation (with the voltage measured at each point with the successive points unplugged)

Power supply (12V) --> RAMPS(12V) --> MOSFET/D8 Output(12V) --> Heated Bed(0.2V)(2ohm)

With the big yellow fuse reading a resistance of 0.8ohm with everything unplugged.

Just to clarify, I'll need to replace the mosfet to get the bed working? (given the fuse appears to be OK)

I need to get one of those electronics hobby kits and learn what the hell is going on haha.

Thanks for your help so far waitaki and appdev007, it's much appreciated.

To test the Mosfet...
Switch the heatbed on in Host software then put the negative meter lead on the negative of the 12v power input plug. Put the other lead on each of the Mosfet legs in turn - you should read 5v, 0v, 0v in that order if the mosfet is switching on ok.
Be careful though where you stick that red meter lead, there's not much room in between the 'Fet and the pololu- insulate the meter prongs (remove pololu if neccesary).

---

_______________________________________
Waitaki 3D Printer

Yep, getting 5V, 0V, 0V.

The little red LED on the ramps turns on when I set the bedtemp in pronterface too, but the heated bed just won't turn on.

5v on one leg should be causing 12v to come out one of the others. One should be 0v because it is ground. If you aren't getting 12v out of one of them then the you need to replace it.

Nas Wrote:
-------------------------------------------------------
> Yep, getting 5V, 0V, 0V.
>
> The little red LED on the ramps turns on when I
> set the bedtemp in pronterface too, but the heated
> bed just won't turn on.

Ordered a replacement, a heatsink and a couple of extra fans. Thanks for the help mate.

Well actually, you should only see 12v when the "fet is switched off. All the mosfet does when its on is to provide a ground path for the return side of the heater. i.e. the source pin is connected to PS ground and the drain pin (heater return) is effectively shorted to it. The +12v is supplied to the positive D8 terminal direct from the polyfuse.
So Nas, can you connect a spare heater resistor direct to the D8 terminal and see if it heats up? Don't burn your finger and don't leave it on for more than 30 secs. You will need your bed thermistor plugged in to the ramps board.

Edited 1 time(s). Last edit at 08/25/2013 07:15AM by waitaki.

---

_______________________________________
Waitaki 3D Printer

Ahh man, I knew that, but forgot. That's kind of a non standard thing to do normally
Guess it could be common in high watt applications. Let's you use a smaller rated component to manage something larger. Thanks waitaki.

waitaki Wrote:
------------------------------------------------------
> Well actually, you should only see 12v when the
> "fet is switched off. All the mosfet does
> when its on is to provide a ground path for
> the return side of the heater. i.e. the source pin
> is connected to PS ground and the drain pin
> (heater return) is effectively shorted to it. The
> +12v is supplied to the positive D8 terminal
> direct from the polyfuse.
> So Nas, can you connect a spare heater resistor
> direct to the D8 terminal and see if it heats up?
> Don't burn your finger and don't leave it on for
> more than 30 secs. You will need your bed
> thermistor plugged in to the ramps board.

I'm beginning to think that the 11A fuse is shutting down immediately its put under load i.e. when the heat bed is connected, indicating a dodgy fuse?

appdev007 Wrote:
-------------------------------------------------------
> There are two self resting fuses on the ramps
> board. It's those two yellowish things that look
> like large flat ceramic capacitors. You could have
> damaged one of those such that is can't reset it's
> self. There is one for the 5 amp side of the power
> connector and one for the 11 amp side of the
> connector. The 11A one is what powers your heat
> bed. (If I remember right it’s the larger one.)
> Use a meter to figure out which one has a leg
> connected to the plus side of the 11A power
> connector. Once you have identified which one it
> is check across the leads of this one. It should
> read almost no resistance. I it has something like
> 100K then it's probably the issue. If has next to
> no resistance, then you probably need to check the
> three legged mosfet transistor. I believe it's the
> one closest to the power connector. If your lucky
> your RAMPS builder put a heat sink on it. It's
> usually the only one with a heat sink. The
> RAMPS
> page has a schematic and should help you
> with your trouble shooting.

---

_______________________________________
Waitaki 3D Printer

Unplug the bed from the controller. What resistance do you read across those two wires? Just for fun, do the wires for the bed thermistor too.

Sorry for the late reply, been a little hectic.

Replaced the mosfets with no luck. When I was replacing them though, I noticed that the D8 terminal looks a little melted on the +ve side.

waitaki: I don't have a spare heater resister ATM, though I can confirm that 12V is definitely at D8 when I turn on the heated bed in pronterface

appdev: the heated bed reads 2Ohms when unplugged and the thermistor reports the correct temperature.

I did test this before, which is embarassing because I'm certain it read 12V at both ends before, but right now when I test at the D8 terminal I get 12.6V, but at the end of the wire (unplugged from the heated bed) I get only 4V (which drops to 0 when I plug it into the bed)

Could it simply be the wire that needs replacing? I'm going to go buy some thicker gauge wire and give it a crack. Not sure how I'm going to get the old one out since it's sort of melted and bonded lol.

Okay so it works. It turns out the D8 terminal bit wasn't connecting properly to the wire, which is why I got the 12V reading when I tried before. If I pulled on the wire, it would regain contact and read 12V at the end of the wire. I put some more solder onto the end of the wire and really jammed it into the terminal, then removed the connector that I used to use to plug the heated bed into the wire coming out of D8, and the combination seems to have solved the problem!


The reason I'd get 12V when it wasn't plugged in and zero when it was was because when I plugged it in I wasn't pulling on the wire to get it to my multimeter, I was just testing at the bed.

I've been laughcrying for the last 10 minutes. Thanks for the help.

I know this is a little old but I recently suffered from my heat bed only reaching 60 degrees and then shutting off occasionally and then the printer would freeze altogether and the display would be solid white. So I found this fuse blown and I'm hoping that is the only cause of my issues. Also, what would cause this fuse to blow? Could it be a cheap generic power supply or extruder/bed getting too hot?
Thanks!

Starting a new thread with a new problem is better than tacking on to one that's been dead for a while.

Those fuses blow like that when there is a short. The only time I have seen them pop like that is when the voltage was above their max ratings or they had been cycled a lot. My guess is that you have a wiring short downstream of that fuse. It's been tripping a lot and it's finally worn out. Find and fix the short. It could be a defective heater, it could be a wiring error. It could even be a short on the electronics board.

Quote
uncle_bob
Starting a new thread with a new problem is better than tacking on to one that's been dead for a while.

Those fuses blow like that when there is a short. The only time I have seen them pop like that is when the voltage was above their max ratings or they had been cycled a lot. My guess is that you have a wiring short downstream of that fuse. It's been tripping a lot and it's finally worn out. Find and fix the short. It could be a defective heater, it could be a wiring error. It could even be a short on the electronics board.

Thanks uncle. Sorry I didn't start a new thread here. I thought i had similar issues as the OP. Can you tell me if either of the following could cause that fuse blow... I recently unclogged my nozzle and turned around the heating element on the extruder to where it was close to my fan and this caused the nozzle to take about 30 minutes to heat up and then when it reached its temp, I'd see it very rapidly drop and instantly lose about 20 degrees and again take a long time to get back up to 130. This cycle happened repeatedly until I turned around the nozzle heater away from the fan. The other thing, as I mentioned, was a cheap 480w power supply used.. I measured voltage on it and immediately after plugging it in, it reads about 12.1v and then it very slowly climbs until after 15 minutes it reaches 12.6v and stops there. Could this voltage change or nozzle temp rapidly fluctuating cause it? I checked and double checked my wires well to make sure no shorts and properly connected. I really hope it's not a bad Ramps board. I've only had it and used it for one week.
Thanks again,
Brandon

I would bet that you have a broken heating element on the hot end. You might also have a thermistor wiring issue. If the heating element is pulling a lot more than it should, you will cause that fuse to trip. If you are running off of a computer not a whole lot alarming will happen when it does unless you try to move a motor. If you are running without a computer plugged into the Mega, the whole thing will shut down when the fuse cuts out. After a while the fuse will cool back down again and there will be power to the hot end and the motors.

The fuse is only rated for 5 amps +/- some tolerance. With a slightly high power supply and a slightly low resistance 40W heater cartridge you might get close to 5A. The fuse will trip in something like a half hour at 10A. It will hold for hours and hours at 6 amps or so, unless the temperature goes up quite a bit. My guess is that you either have an intermittent short, or your element is way over spec. A quick check with an ohm meter should tell you a lot about the element. A 40W element should be around 3.6 ohms. If it's much below 3 ohms, that's a problem. If it's a heater resistor, they usually are in the 5 to 6 ohm range. While you have the ohm meter out, check from the heater wires to the hot end tip and to the thermistor wires. They all should read open circuit to the heater leads.

I'm not real savy on resistance but I measured the hot end element and got a reading of 6.5 ohms between element leads and then measured the thermistor wires and got 108k ohms between leads. My printer is a Makerfarm Prusa i3 so I'm not sure which current rated element is included in the kit. And I measured with the unit off, but I did not get open circuits on the heater leads to the nozzle tip. I did not get any resistance reading.
So the fan cooling the element too fast couldn't cause the heating element to work too hard and blow the fuse?

Unless your fan puts out hurricane force winds, it's not cooling the heater element to much. You may have trouble with too much fan if you are trying to get to 300C, but that will not blow the fuse. The resistor heater (as opposed to the 40W heater cartridge) pulls about 25 watts. That should not be a big strain on the fuse no matter what is going on.

I'm not real clear on:

"but I did not get open circuits on the heater leads to the nozzle tip. I did not get any resistance reading. "

If the thermistor is unplugged from the board it should read open circuit to the tip. If the heater is unplugged from the board, the same should be true of it. What sort of reading did you get when you checked the leads? I'm assuming it's not a problem.

The risk is a short between the hot side of the heater resistor and the ground side of the thermistor. That would put current straight from the supply to ground. Even doing that briefly will pop the fuse. If the short is the

The heater element is not the only thing that could be wrong. It's just the most likely thing.

Here's a bit more complete list. It includes most of the stuff that could / might / may create the issue.

If:

1)you have no hot end wiring shorts (either when cold or when hot)
2) the element is always about 6 ohms. (= the resistor isn't defective or shorting to the hot end body)
3) there is no connection between the element or the thermistor and the tip of the extruder.
4) Your power supply is always close to 12V (11-13V)
5) Your motor drivers do not short out.
6) Your motor wires do not short out.
7) Your motor driver currents aren't set way to high. (very unlikely if you have MakerFarm motors).
8) Your ramps board does not have a short on it
9) Your mega board does not have a short on it (you would likely see smoke and a failed board)
10) The fuse is not defective (I've never seen a defective one)

Your fuse will not trip out, let alone explode.

I'd do a quick check on the motor stuff next.

So checking the leads on the thermistors and heater, you were right in that it was an open circuit between heater leads and showed exactly 5.1 ohms when disconnected from the board, but the thermistors both read 108k ohms and were not open between leads when disconnected from board. And there was no open circuits between the nozzle and thermistor and heater element.

I noticed one of your causes in the list was a voltage range up to 13. So if voltage exceeds 13, it could blow the fuse? What about a power surge of some sort? We did have a brownout approximately around the same time period this occurred and stupid on my behalf, but my power supply was plugged directly into the outlet. Although the PSU and printer was not turned on when the brownout occurred. Could that have been my cause? I didn't suffer any other electrical equipment problems from the brownout.

Otherwise, next I'll be checking the motors and board for shorts but I'm not real sure how to check the motors for shorts. Would I just check the continuity between wires in the motor?

I went ahead and ordered 2 replacement fuses in case the next one blows on me right off the bat.
Thank you

If your computer power supply is running correctly it should never go over 13V. I happen to dislike using computer supplies for printers, but that's another topic (I'd use a 12V 30A industrial supply that costs $45 or so). PC supplies (especially surplus ones) can have all sorts of odd issues.

If you have a normal setup and your supply goes over 35V on the 12V line you likely will blow the fuse quickly. You will also blow up a bunch of stuff on the board as well. I doubt this has happened in your case.

If you are running and the supply drifts up to over 15V you may blow the regulator on your Mega board. It’s only happy below about 13V. Brief transients to 20V are not an issue, just running voltage. If the regulator goes, your Mega is dead.

If you go over 13 or 14V with the hot end powered up, you will pull a bit more current. If you have a 40W heater element (not your 5 ohm resistor) and some high power motors all running at once this *might* create issues. The only case I can think of where it would is a 6 hour long print where the hot end is running at full power because you are moving so much filament.

Motor checks:

1) Unplug each motor and ohm out each lead to the motor case. They all should read open circuit.
2) Look at all the wires and make sure the insulation is intact.
3) Pull the motor drivers off the board (keep track of which one is which):
a) check the caps under them on the Ramps for damage
b) check to be sure the heat sink tape (and heat sinks) are not hitting parts
c) check for solder blobs running around
d) check for missing parts / exploded parts
e) check to be sure the jumpers under them are seated and not touching the underside of the driver board.
4) Pull the ramps board off of the Mega, check both sides for solder blobs and blown parts

Wire checks:

1) Pull the cable ties off of your wires. Look over each of them. Make sure the insulation isn't torn somewhere.
2) Un tape your solder joints and make sure the tape hasn't slipped so it's not insulating the two wires from each other.
3) Give everything a (gentle) tug. Make sure the connections are sound.
4) Confirm that the wires go where they are supposed to.
5) If you crimped any connections, (rather than solder) check the crimps to be sure they are tight.
6) Re-tape everything and carefully re-cable tie it. Make sure there is plenty of slack

When you do find the source of the problem, charred black stuff in the vicinity would be one really good indication that it’s the site of the short