Heatbed does not heat

Hello:
I just completed my reprappro Huxley (3rd version with Melzi electronics). Everything seems to function except for the heatbed, which does not heat. I measured 19 vDC at the heatbed electrical terminals. The heatbed coil registers >7 Mohms when the power is off and I measure between the power terminals. Oddly, resistance measures 26 Mohms when I measure at the solder joints on the other side of the heatbed. Anyway, can someone suggest how to troubleshoot the heatbed? I'm not sure if the heat bed is faulty, or whether my board is just telling it to turn off, or some other thing.

Any advice is appreciated.

Thanks and best regards,
Matt

Hi Matt,

Adrian has emailed you about this with some checks to do.

Best wishes
Sally

Hi,

I'm having the same problem (but with the sanguinololu electronics). The heatbead receive the 19v, the temperature reads correctly and the wire tho the transistor have connection. Any ideas? How can I troubleshoot the problem.

Thanks
João

The heated bed is pretty simple to troubleshoot.

Step 1: Using a voltmeter, check that you have voltage coming out of the MOSFET from your electronics and going to the resistor array which will be used to heat your bed. The resistor array could be aluminum resistor blocks or simply a Nichrome wire taped to the bottom of your heated plate.
Step 2: Is your thermistor or temperature probe measuring correctly? This can be checked by placing your finger on the probe and seeing if the temperature read out is fluctuating. If the read out is stable at a certain value then the thermistor is broke. Also if the fluctuation is erradic, fluctuating more then a few degrees at a time, then the probe is not functioning correctly.
Step 3: With your voltage disconnected to the heated bed, take your ohmmeter and measure the resistance of the resistor array. A typical Nichrome wire setup should be in the tens of ohms range. For example heated nozzles would use ~6 Ohm Nichrome strands. Heated beds should not be too much more than this. If you are reading hundreds or more Ohms resistance, then your resistor array is not functioning, and will not draw enough current to heat the bed.

Step 1 Solution: Check your wire connections. Ensure you are making good contact between wire and terminals.
Step 2 Solution: Replace your thermistor or temperature probe.
Step 3 Solution: Again check connections. If connections are good but resistance is large, then replace your heating element.

I hope i explained myself thoroughly. Good luck!

Greetings I have the same problem.

Reprappro huxley
Melzi 2.0 controller
Reprappro PCB heatbed v0.1
pronterface interface

Heatbed is never heating, led is never being powered
removed the aluminium shield to avoid any chance of a false contact/short and it still did not work the slightest.
Tested connections with a voltmeter/ohmmeter,

the innermost pin on heatbed is connected to the right terminal in the Btemp header ( the one closest to E-temp) 0.0 ohms
the second innermost is connected to the left terminal of the Btemp header ( the one closest to z-stop) 0.0 ohms
the second outermost is connected to the melzi's extension header at the right of the USB plug on the first row second to the left 0.0 ohms
resistance between both ends of the heating track is 4.95 ohms
resistance beween both ends of the thermistance is 5.76 Kohms at room temp dropping to 5.65 Kohm with my thumb on it.

when powered
the power comes to the PCB board 19.73 Volts measured with
red probe on the outer terminal of the screw header( that closest to the middle of the board), the terminal connected directly to the heating track and the led's resistor
black/(COM) probe on the middle terminal of the screw header that connects to the MOSFET's drain and the pulldown resistor of the MOSFET's gate
so the heatbed PCB is powered.

The tension between the source and drain of the MOSFET is 19.73 Volts
The tension between source and gate is measured at 17.95 volts
the tension between drain and gate is measured at 0.2 Volts
the tension on both sides of the pulldown resistor is 0.05 Volts

So far it looks okay
now i enter ABS120 on the bed temperature target
I still get the same reading
The tension between the source and drain of the MOSFET is 19.73 Volts (so it is still not applying tension to the heating track)
The tension between source and gate is measured at 17.95 volts
the tension between drain and gate is measured at 0.15 Volts ( that is probably the reason why it doesn't apply any tension on the source pin)

The tension between the MOSFET's gate and the 2nd pin of the first row on the expansion header on the melzi board ( the other side of the cable) is 0.003 V (as expected since the connnection had been tested above with the ohm meter) so the connection is not explaing why the heatbed's mosfet's gate doesn't get any tension from the melzi board.

On the melzi side the tension between both connections of the heatbed terminal ( the one we don't use because we use a heat bed with its own mosfet ) is showing 19.73 Volts

My conclusion is as follows

A - The header pin we use on the Melzi board ( as instructed on the wiring guide) is not the correct one to drive the mosfet
B - there is a firmware problem and the melzi board doesn't drive that pin properly with the commands it receive from pronterface.
C - there is a falty component between the melzi board MCU and the pin driving the gate of the heatbed's Mosfet
D - there is something wrong I haven't detected and the melzi board shuts down the temperature pin voltage to avoid damage.

Until i have found i am going to remove the "gettho" jumper between mosfet's source and heating track and connect the heating track and the 3rd pin terminal with a 5ohm resistor, unplug the board from the main power conector and plug it on the heatbed connector terminals on the melzi board because this one seems to have some tension.

Hi,
I also have the same problem, and I am also using:

Reprappro huxley
Melzi 2.0 controller
Reprappro PCB heatbed v0.1
pronterface interface
Marlin 1.0.0 RC2 firmware

When I connect the MOSFETs' gate to the second pin of the SECOND row of the melzi board (which seems to be always on 5V) the heatbed-LED turns on an heats properly. So I guess the connections and components are OK in principle. There is obviously no temperature control on that output, however.

Is there anything else I could test or a way to get it work?

Hi,

I think I'm having the same error too, using Melzi. Exactly the same as previous post.

Hi, I've experienced the same thing as last 2 pots. I've changed the mosfet to the pin just below the one mentioned on instructions. Seems temperature just keeps rising over the set value.

Any solution?

Thanks

Edited 1 time(s). Last edit at 10/07/2012 11:55AM by Margomael.

Most likely culprit is an incorrect heatbed pin definition in firmware. Try setting HEATER_BED_PIN 10 in pins.h and following instructions for updating the firmware:
[reprap.org]

---

[emaker.io]

[emaker.limited]

Can confirm this works, bed now heats properly. Make sure you get the right line in the pins.h, there are 3 defines for HEATER_BED_PIN for various boards/configs.

Thanks :]

hi

can anybody show me wich line to change?? and into what

thanxs already

I have the same problem. Can someone post a guide on how to fix this please.

For the records:
My huxley's heated bed stopped working recently.
I checked many things and discovered that the MosFET was badly soldered. It worked as soon I pushed on the FET.I had to resolder the two pins and now the bed works again.

Edited 1 time(s). Last edit at 09/26/2014 11:24PM by BenZ.