Heater Bed not turning off

Hi

Machine - Mendel Prusa
Electronics - Sanguinolulu
Firmware pre-installed - Sprinter
Heated Bed - PCB MK2
Software - Pronterface

I am a Reprap newbie so please be gentle!
I have just finished building a Mendel Prusa complete with Linear bearings.
I have come to get the machine up and running and I am having a problem with the heated bed.
As soon as I turn the power supply on the heated bed starts to warm up on its own without any input from the computer!
The thermister is working fine and Pronterface shows the temperature steadily rising.
I have tried setting the Bed temperature to zero but the bed keeps heating up.
The hotend seems to be working fine
The Sanguinolulu electronics came ready configued because electrics are my weak point.

Any ideas?

Thanks

Andy

Edited 1 time(s). Last edit at 01/27/2012 03:08PM by Normsthename.

Just done some investigating.
The X Motor was running in reverse so I swapped all the 4 leads over (e.g 1234 to 4321) it now runs the correct way.
Now the Heater Bed will not turn on at all
The extruder is working perfectly holding 210°
The heated bed will now not heat up at all

Andy

Sounds like a solder bridge or maybe a cold solder joint. Look closely at all the solder joints connected to the HBP FET. And check for solder bridging between pads and traces. You might desolder and resolder the related pads just on principle. And double check that the FET is installed in the correct orientation.

Thanks for the reply, but I think I might have blown the part of the electronics that control the Heater Bed.
Now I cannot get anything
See my other post in the Electronics section showing how I got around it!

Andy

could be one of three things

1. The MOSFET is faulty or has a solder bridge keeping it on. A: replace the part or de-solder the bridge

2. The output pin of the Sanguinolulu is blown. A: pick another one, add a new wire and cut the trace on the board.

3. The pin that is controlling your heated bed has been chosen incorrectly in the firmware. A: set it to the correct one in firmware.

I have a similar problem.

When I turn on the power, the heat bed led is off.

When I set the heat bed temp, the heat bed led turns on and the heat bed temperature starts rising.

When I try to turn off the heat bed , its' led does not turn off.

If I click disconnect and then connect on the pronterface the led turns off.

Do you think it is a transistor problem or MCU problem?

SDIM: Unlikely an electronics problem, however if there is some leakage in current to the gate of the FET, it would do this. I would check the Arduino separately to the RAMPS (put a voltmeter on D8), and see if it helps.

Note: If you're using Marlin, you will need to override the temperature handling that shuts down the extruder. You can do this with an M-code, or you can "fake" the voltage into the Thermistor input (A13 for T0, which is used for the nozzle heater, and A14 fort T1, which is used for the heated bed). Closer to 5V = lower temp. I use a 100k pot for testing, with the centre pin connected to the input you want to test, and the outer pins connected to +5V and Ground respectively. You can then adjust the pot and see a temp reading (note: It won't be a linear adjustment! - Turning 5 degrees at one end and at the other of the pots movement won't equal the same temp difference).

The temp reading is correct, so I'm sure that the thermistor works fine.

I have Gen 7 1.4 electronics, it has IRFZ 44N MOFSET.

I just noticed that its' heatsink colour is gone, so i guess that it was overheated (the fan over it was not enough for cooling) and the mofset was damaged.

I ordered MOSFET IRLB 8743 to upgrade it to Gen 7 1.4.1, I hope it will solve the problem.

By the way, do you know that the heat bed may need more than 10Amps? A typical heat bed has resistance near to 1Ω. 12V/1Ω=12A !!!

The idea of using a pot was so that you could alter the temp and see if the bed was turning on and off as you manually forced the temp change (ie: without anything attached to the bed outputs).

Yes, a heated bed can draw a lot of current. 12A is quite typical.

I run 24V throughout so my current draw is significantly less, and losses through voltage drops in cabling are lower. It's not a simple change though, so I'm not specifically recommending you should go to 24V here, just stating how I've done it.

Quote

I just noticed that its' heatsink colour is gone, so i guess that it was overheated (the fan over it was not enough for cooling) and the mofset was damaged.

Make sure FAST_PWM is commented out in Teacup's config.h. Also a version recent enough to support FAST_PWM is required, as older versions had it turned on hardcoded. See [reprap.org]

Quote

I run 24V throughout so my current draw is significantly less, and losses through voltage drops in cabling are lower.

This also requires a different heated bed, one with significantly higher resistance.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

Traumflug wrote in response to Cefiar:
-------------------------------------------------------
>> I run 24V throughout so my current draw is
>> significantly less, and losses through voltage
>> drops in cabling are lower.
>
> This also requires a different heated bed, one
> with significantly higher resistance.

Yes definitely. I use one of the MendelMax heated beds which are made for 24V operation. Note that I'm not running a MendelMax (it's sort of a hybrid Sells/Prusa atm), just the heater for the bed off one.

DO NOT use 24V on the standard Mk1 or Mk2 PCB heated bed, as you'll be drawing a HUGE amount of current (20+ Amps), which will most likely cause all sorts of issues. The large heatbed PTC fuse is not rated for 24V either, so you need to replace this with a wire link or a proper fuse.

FWIW: I am not a fan of PTC fuses, as near the limit of their operation, they can be quite iffy and easily tripped.