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Heat Bed no longer functions RAMPS 1.4

Posted by congaree 
Heat Bed no longer functions RAMPS 1.4
February 10, 2014 11:33AM
I think my RAMPS board just bit the dust, with respect to the heat bead. It just stopped working. I took a multimeter to the connection posts on the RAMPS board to measure voltage when the bed is turned on, and get something like 0.3 VDC. Obviously dead. My extruder posts measure 11.4 VDC when the extruder is turned on. So it seems to be a component on the board that failed. Everything else seems to be functioning fine. Anybody know what to look at next or should I be looking for a new ramps board?
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 12:37PM
Extruder is powered off of a different power input than the heated bed. IIRC the heated bed is the only thing that is connected to the 2nd power input. Check to make sure that you have power with the 2nd input. Presuming you have power there, check the fuses. You may also have fried your MOSFET transistor, especially if you didn't have a heat sink and/or fan cooling it.
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 02:40PM
cdru,

Thanks for the quick response.

I checked both of the power inputs and both have 11+ VDC. I didn't realize these little boards had fuses on them. Are they those yellow square things? I don't see anything odd about them, but I've never seen fuses like that. 1) How do you tell if they are blown and 2) how do you change them if they are?

Thank you again.
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 04:38PM
Congaree: They're PTC resettable fuses. They "should" automatically reset once the power is removed.

That said, a lot of people have had issues with them.

They work internally by heat, so make sure that you have some airflow (doesn't need to be a lot - spill-over from a fan pointing at the stepper driver heatsinks is enough), and that they're not touching each other (the heat in one can trigger the other).

While the bed is turned on, measure the voltage from both pins on D8 compared to ground using a voltmeter (mutimeter on volts range - 20V max is fine). One pin should ALWAYS be 11+ VDC. If not, your fuse has most likely tripped, and something is drawing too much current.

If there is voltage there, then it's possible that the MOSFET is dead and needs replacing. The MOSFET for the bed can generate a lot of heat and needs to be actively cooled (heatsink on the MOSFET with a fan is best). If the MOSFET is dead, replace the stock MOSFET with an IRLB8743PBF (it's better than the stock component, and generates less heat). If you have trouble finding them, let me know where you're located and I'll see if I can find something similar from a local supplier for you.

Note: If the thermistor for the bed is disconnected, then the bed will NOT heat. This protection is in most firmwares, so you need to solve that first.
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 05:21PM
Cefiar,

Thank you so much for that thorough reply. A lot of very good information there.

I had nothing on my ramps actively cooled, so that is probably what ultimately did it in. I get nothing (well, 0.3 V, which is effectively zero) on D8.

So, I have some things to look at here...the fuse should have reset. BTW, this just happened out of the blue...has been working fine for about 8 months until this happened.

Thank you again!
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 05:30PM
What pin would one check under similar circumstances, but the problem being the extruder not the bed? Additionaly, what is the best way of checking the MOSFETS, short of removing them.

Thanks.
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 05:47PM
For the fuse:
During operation, you could check to see what voltage is on either side of the fuse, compared to Ground. It should be very close to the same. just be careful to only touch the lead of the fuse with the multimeter probe, and nothing else.
With the unit completely off and disconnected from the PSU, you could check the resistance of the fuse (from one side to the other). Make sure the PSU is disconnected, else you may get a wrong reading, or if it's still got some voltage, fry your meter.

With regards to the MOSFET, you really can't easily test it in-circuit. Eliminate all the other possibilities first. Once that's done, with RAMPS removed from the Arduino and no power, you "could" check the resistance between pairs of the 3 pins on the MOSFET. The main one to test is between the pin that goes to D8 from the MOSFET and Ground. That will only prove if the MOSFET has shorted out, but not if it's died open circuit.
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 06:56PM
Thank you Cefiar! I'll try it out.
Re: Heat Bed no longer functions RAMPS 1.4
February 10, 2014 07:17PM
I had a similar problem, mine was caused by the power connecter on the ramps. It melted itself to the point of no contact, but looked ok on the outside. So i would also check that there is power on the back side of the connector.

Edited 1 time(s). Last edit at 02/10/2014 07:18PM by cat.farmer.
Re: Heat Bed no longer functions RAMPS 1.4
February 11, 2014 01:38PM
Quote
cat.farmer
I had a similar problem, mine was caused by the power connecter on the ramps. It melted itself to the point of no contact, but looked ok on the outside. So i would also check that there is power on the back side of the connector.

That's exactly what it looks like on mine...I looked very carefully at the power connector, and the side of it seems to have a blob of melted plastic. I wonder if this is a common problem?
Re: Heat Bed no longer functions RAMPS 1.4
February 12, 2014 09:45AM
I think that it is a common problem. I replaced mine with wire and an external power plug. It was a 4 prong PC power plug ( the 12v 5v used to connect drives).
Re: Heat Bed no longer functions RAMPS 1.4
February 14, 2014 02:52AM
Have you tried the RAMPS 1.4 test code? When I upload that the bed heats up, just not with marlin :/
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