HEAT BED PROBLEMS

Hello! I have been round and round with this heat bed and can not seem to figure it out. I have a MK@a 12v bed connected to ramps 1.4. The thermistor is working I have tested it. When i try the M303 auto temp config, the bed turns on for a few sec and then turns off. The big yellow fuse gets very hot so hot can not touch. I have tested ohm and am getting around 0.9 to 1.0.. I have soldered and unsoldered and soldered again and a gin, trying to make sure that I have a good solid joint. If someone can try to lead me the right way, that would be great.


Thanks Demon

When you tested the thermistor, did you measure the ohmic value? What did it read?
Does it show room temp when the bed is cold?
Have you got a heat sink on the Mosfet for the bed?
Do you have thick enough wiring going to the heat bed from D8?
You should have a fan blowing constantly over the Ramps board. The fuse can get hot if its not cooled and can shut the heat bed down.
If you switch the bed on in your host software, what happens?
Note:
Some beds I've seen have a solid copper layer on the back which envelopes the solder terminals on the front causing a short circuit. Cure is to drill out the through hole plating.

---

_______________________________________
Waitaki 3D Printer

While testing with Multimeters have you checked the resistance of Probe, Some probes will have resistance around 0.6 ohms to 1.5 ohms some times we miscalculate this as original reading, for safety check the resistance between the probes by touching with each other while the Multimeter knob in 200ohms

---

Enjoy life

Thanks for the replay's.

Waitaki,

Themistor does read room temps. When I tested it, it is reading 94.2 ohm, ot sure if that is good.
I do not have a heat sink on the Mosfet should I have one on it. It has not been getting hot just the fuse.

As for wire I am running 14 AWG from ramps to the heat bed. Also when I test the power on ramps D8 I'm getting 11.90 volts to a solid 12 volts. I have also checked the volts on D8 with the hot end and motors running to make sure I was not losing power, and everything was good.

I do have a fan running on the ramps, I have two fans blowing on it in two places so that the ramps is getting air across the whole board.

It does not matter aow I trun the bed on it does the same thing runs for a few sec and then shuts down, and that big fuse is very hot.


GowthamJGR


I have checked the meter and it doesn't read any ohm's when probes are together, so I'm good there.

I just tested the bed using just the power supply and it heats up, the LED comes on with no problem. Is it possible that the ramps can't handle the current? Is there a way to use the power supply to heat the bed with out the ramps? I have read that these MK2a heat beds can be a little bit tricky. Should I just buy a standard MK board or?

Thank , Demon

Edited 1 time(s). Last edit at 07/07/2014 02:45PM by demongimp.

That heatbed is the one I use and its trouble free so no reason to abandon it. The Ramps should handle the heatbed although its sinking a lot of current!

Q. If you are using an XT power supply, have you got a dummy load on the 5v line?

You still need a heatsink on the Mosfet.
Did you check the back of the heatbed as I commented on previously?

---

_______________________________________
Waitaki 3D Printer

Deleted double posting..

Edited 1 time(s). Last edit at 07/08/2014 04:11AM by waitaki.

---

_______________________________________
Waitaki 3D Printer

Waitaki,

I am using a 1000watt atx psu and yes it has a load on the 5v line. When I turn on the D8 line , I also ran the motors and the hot end just to see if there was a power drop, and it was good. The problem is just when the board is connected to the ramps. I did today connect the heat bed to the psu by its self and it heated up fast and seemed to be fine. I did not leave it on there for a long time though as I could not control the temp.

I did flip the board to see what you ahd said and I did not see nothing. To me it just looks like one solid piece but on the frount side you can see two main lines running up each side and then a ton of small lines running across the middle.

Edited 1 time(s). Last edit at 07/08/2014 08:50PM by demongimp.

Quote
demongimp
Waitaki,

To me it just looks like one solid piece but on the frount side you can see two main lines running up each side and then a ton of small lines running across the middle.

That's what I was talking about previously - the back of the board is a solid copper layer is it not?. If so, and its also connected to the solder tabs it will mean a lower resistance and too much current drawn.

---

_______________________________________
Waitaki 3D Printer

I got the same Problem with the RAMPS 1.4a which is manufactured and assembled completely by myself
When I use first, the fuse (big yellow one MFR1100) blown away and doesn't work for a while and after few hours it started working (checked with Multimeter) it seems it uses more current, (I always use couple or more wires to solve current related issues!)

Later I worked with that but it cannot push more than 70 deg C @ 12v DC for Heated bed

Now I am Using 24v DC, but the fuse is rated of only 16V !
Let me check how long it can run, well I am thinking to bypass the resettable fuse to the glass fuse rated @ 15amps and so we can supply more current for big heated platform!

---

Enjoy life

Forgot to say I am using MK2b heated bed platform which uses 24v DC too!
It is good you know, faster heating ! I love 24V rather than 12V nowadays!

---

Enjoy life

Waitaki,

Yes it is solid copper on the bottom. I have tried to connect a ssd relay to it and that just moves the heat problem from the board to the relay it takes about a minute or two and then its to hot to touch. Is there a way to solve the bottom plate and the top plate from touching at the tap point?

Have a look at the attached pic of the back (solid copper side). You will see 2 holes which are plated through to the other side (the copper tracks side). If you look closely at the holes you will see they are electrically isolated from the solid copper - that's how it should be. If your board has no such isolation then the tracks are being shorted to the solid copper side. To remedy that, just drill through the holes to remove the through hole plating. Make sure you solder your wires on the track side of the board. Also check the 4 corners of the board where the screws and springs go - there should be similar isolation there too.

---

_______________________________________
Waitaki 3D Printer

Ok, Thanks I see what you were talking about now. I am going to drill out the holes, cause this board does not look like that, but the conners look good though...

Thanks for all the help. I will let you know what happens after drilling out the holes.

Demon

@Waitaki, heated bed MK2a, which I had once, have the same problem...

At that time when I measure the resistance between the two lines it is 0.02 ohm, then I was advised to remove the copper layer by reprap members via IRC, and so I milled the backside copper and checked the resistance between the two lines, it is measured as 1.2 ohm, then I connected the power supply it worked fine

"The problem is just when the board is connected to the ramps. I did today connect the heat bed to the psu by its self and it heated up fast and seemed to be fine. I did not leave it on there for a long time though as I could not control the temp. "

I suspect Here he is getting 0.9 ohm it should be working fine, it may be due to some other faults (may be the fuse, Mosfet, wires?)!

To solve this use Double wires so current will be separated and the wires will get less hot, and more current you get
Mosfet, you need to heat sink it! (normal TO-220 heatsink will be available in electronic store, you can attach the heatsink upside down)
Attach a cooling fan to cool down the Mosfet and stepper drivers

if it is okay with you, use a Glass fuse, with glass fuse holder by bypassing the yellow fuse!

Edited 1 time(s). Last edit at 07/12/2014 10:22AM by GowthamJGR.

---

Enjoy life

the poly fuse on some ramps are cheap made...and blow quickly all will eventually start blowing as most heatbeds pull from 11-12 amps when they first come on.
the best way to fix this is what i did that is remove both poly fuses and replace them with mini blade fuse holders and put a 15 amp in place of the 11amp poly and a 5amp in the other then if one blows it is easy to replace...
tho you will want to look for your short first as that is the only reason one of these will blow.

Edited 1 time(s). Last edit at 01/05/2015 01:38AM by hillbilly.