Heated Bed Stopped Working, Help!

I have using my Prusa i3 for a few months now and have made some great successful prints. I now have problem with my Heated Bed which has stopped working.

The last job was an 18 hour print and during that time the heatbed stopped, not sure when exactly.


My printer details :

power supply = 600W ATX
Heated Bed = 'PCB MK2a Heatbed'
Controller = Ramps 1.4 / arduino 2560
Software = Marlin

I have researched this issue for some time now and I still have no success, here is what have done so far.

I replaced the Heatbed mosfet with a IRLB8743PBF, N-channel MOSFET Transistor 150 A 30 V, 3-Pin TO-220AB. It has a heat-sink.

I have replaced the PTC Fuses with Blade fuses 5amp and 15amp versions respectively.

If I connect the heated bed directly to the 12V power supply it heats up fine.

If I connect my Multimeter between the heatbed and D8 it draws 0.25 amps when turned on, also the red light comes on.

When I disconnect the heater from D8 and use the software to turn on D8 the MOSFET red LED comes on. When connected the red light no longer comes on.

the resistance value for the heatbed is 1.1 ohm
the Thermistor is working ok it shows the room temp.

If anyone can help I would very very grateful

Cheers
Tony

Since the bed resistance is 1.1 Ohms, and it's drawing 0.25A when the FET turns on, the voltage across the bed must be about 0.25V, which means the FET isn't actually turning on like it should. Does the FET get warm? The FET is dropping about 11.75V at 0.25A = about 3 watts. That FET should be getting nice and toasty.

Why did you replace all those parts?

Thanks for the speedy reply!

I have found a couple of threads on this subject and the advice given suggested to replace the FET. So i did, That didn't work so I thought maybe the fuse had blown for good but still no joy. At which point I have no idea what to do and so put this shout out here.

The FET remains at room temperature when turned on.

Any Idea where to look for the fault now ?

Thanks
Tony

Measure the voltage on the gate. Should be close to 5V when it is on and 0 when off.

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[www.hydraraptor.blogspot.com]

Some possible explanations:

1. One of the screw terminals feeding 12V to the RAMPS heated bed supply is no longer tight. These need tightening from time to time, especially if you are using stranded wire that you tinned with solder.

2. The 11A PTC fuse has failed and needs replacing. Measure the voltage across the fuse (see also point 4).

3. The RAMPS board has a poor soldered joint to the mosfet, the 11A fuse, or the 12V heated bed input.

4. You have a short across your heated bed, and the 11A fuse is doing its job (and getting very hot). Check whether the fuse is hot.

Edited 1 time(s). Last edit at 02/08/2015 04:25PM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks very much for all your help.

I have an hour before I set off for work..... I find it very difficult to make contact with the Gate pins and my Meter probe when the Ramps is screwed to the wall of my printer. I will try some more tonight when I have a bit more time.

Thanks again

Be careful not to short it to the next pin, which is drain, as it will damage the Arduino.

Check the drain first as that is also the tab and easy to probe. Should be 12V when off and very close to zero when on.

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[www.hydraraptor.blogspot.com]

Heat Bed Mosfet Test results (I had to remove the heatsink to do this)

The Drain is 12V when OFF and a few mV when ON
The Gate is a few mV when OFF and 5.01V when ON
The Source is a few mV when OFF and few mV when ON

The Mosfet appears to be working?

I have double checked the power and the heat bed connections they are secure. There appears a little discolouration around the solder on the 12V+ 11A power terminal on the PCB But seems fine otherwise. I have already changed the 11A PTC Fuse with a 15A blade fuse. Not sure how to check for a short on the heat bed?

Still no joy

I really do appreciate your help

Seems like the connection between the drain and the D8 terminal / LED is faulty, or between +12V and the other D8 terminal. That would explain why the LED goes out with the bed connected.

If you have de-soldered the old MOSFET you might have broken the through hole plating.

Edited 1 time(s). Last edit at 02/09/2015 07:16PM by nophead.

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[www.hydraraptor.blogspot.com]

Thanks Nophead your input is valued.

I have checked the continuity between the D8 neg terminal and the drain and the LED neg contact and all reliably beep, the same goes for the 12V power terminal and the fuse and D8 positive terminal.

One thing i have noticed is when the heat bed is not connected the voltage measured on the D8 Positve terminal is 12V when OFF and 2.7V when ON.... surely when NOT connected to the heat bed the Voltage should remain around 12V?

Am I right

Cheers

Yes D8 positive should be connected to 12V via the fuse, so should always be close to 12V. You have something wrong there.

Rather than just checking for a beep you can measure the resistance, it should be so close to zero that it gives the same reading as the two probes shorted together.

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[www.hydraraptor.blogspot.com]

I would like to thank everyone who has contributed to this thread.

I have to say I am just stumped by this problem. I feel I have spent way too much time this and now its time stop and move on. So I have bought a new Ramp 1.4 for under £10 can't wait to start printing again! I feel I should have done this a while ago but I was determined to fix it myself...err Pride HUH what a fool.

Nophead you're a legend

Well is seems you got close because the MOSFET is working. The rest is just simple connections and a fuse.

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[www.hydraraptor.blogspot.com]

I will get back to this one day and perhaps build a 2nd printer with it, but for now i need to print.

I did check all connections and i even checked the value of R23 resistor which is the only other componant that is connected to the 11A 12V power supply. All seem good, I just cant figure out why the voltage drops when turned on for the heatbed when its not even connected!

Thanks again

Instead of a direct connection to 12V through the fuse you must have a fairly high resistance connection that is barely able to drive the LED. Since you replaced the poly fuse with a blade I assume you might have broken the connection some how. Maybe removed the through hole plating and separated one side of the PCB from the other is my best guess. Or the wire going to the 12V 11A connection from your PSU is faulty. Or if your PSU has two 12V rails perhaps one has failed. Easy to check by measuring the voltage on the 11A connector. If that drops the PSU or the wiring is faulty. If it doesn't drop but the voltage on the D8 plus connector drops then the connection between them via the fuse is faulty.

To get it working you can simply connect the bed positive direct to the 12V feed from the PSU, bypassing the RAMPs board and the fuse. The fuse is pretty pointless anyway. If you get a short circuit on your bed the wiring should be thick enough to shut the PSU down.

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[www.hydraraptor.blogspot.com]

WooHoo. I have a working heat bed. All thanks to you Nophead.

Printing right now.... what a relief!

The solution did seem brutal but brute force is often best, I will fit a fuse and LED in this new arrangement and will feel safe again

Thank You!