Heated bed doesn't receive power

Hi, my nephew bought a Huxley some time ago, but blew up the stepper drivers on the first try, and never got the printer working. So now he gave it to me, to fix it. I bought a RAMPS V1.3 and everything works fine, except the heated bed.
When I click the "set" button in pronterface to set the heated bed temperature, nothing happens. I checked with my voltmeter, but there is no power coming out of the D8 connection. What can be the problem?

The most common issue is failing to provide power on the heatbed circuit. The heatbed runs from the 11A terminals, everything else runs from the 5A.

Both the 5A and the 11A power inputs are powered by a 12V/20A computer power supply... Imho it is a software problem.

Does your bedtemp read normal at room temp?

Same as the extruder thermistor...

Time to troubleshoot wiring/components then. Unless something is funny in your firmware. What's your config.h look like? wanna post it?

Edited 1 time(s). Last edit at 02/27/2013 02:10PM by xclusive585.

Sure, I'll post anything you like
I don't have a config.h in my sprinter directory, so Configuration.h will have to do ???

Abou the bed temp in pronterface: the graph looks the same as the graph for the extruder, but it is kind of smallish... Is there a G-code command to get this temp, then I can check for sure...

Edited 1 time(s). Last edit at 02/27/2013 02:33PM by sokopok.

M105 = read current temps

M105
response:
T:23 @:0 B:0

does this mean anything to you?


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edit
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I guess B:0 means bed: 0°C -> the thermistor is not giving any reading. So maybe I connected the hotbed wrong...

Question: does the hotbed work when the thermistor doesn't?

Connections:
On the RAMPS: 2 pins right next to the extruder thermistor
On the hotbed: see attachments
- : I put a jumper on the backside to bypass the mosfet that is on there (is this OK?)
- : I connected the 2 red pins to RAMPS

Edited 1 time(s). Last edit at 02/27/2013 03:26PM by sokopok.

The Ramps board will do nothing without the thermistor input. No 12v will be outputted

The heated bed won't work when there is no thermistor. This is by design to stop the PCB being driven at 100% power trying to warm it up, and having no way to tell what temp it's at.

If you've got a thermistor, check it's connected correctly. It should have a resistance somewhere between 100k and 50k (depending on what temp and assuming a 100K thermistor).

For testing, you could put a normal resistor on the inputs for the heated bed thermistor. Somewhere around 82K should give it a reading that the machine will consider "somewhat normal" if you've configured the firmware for a 100K thermistor. I think you need to have a temp over 5 deg C registering before the firmware will allow you to switch on the bed.

If your thermistor now works, but your output still fails, try the following:

Check the voltages on either side of F2 compared to ground.

This is a PTC fuse (reset-able) which is responsible just for the heated bed. If it trips or is broken, you won't have power to the FET, which then provides power to D8.

If the fuse seems to work, check the voltages on both sides of D8 compared to ground.

One of them should be +12V, and the other (if the heated bed is switched on) should be 0 volts. If your heated bed is wired up to the D8 output, you will see that when the heated bed is not switched on, the voltmeter should read +12V (getting voltage through the heated bed). If both sides stay at +12V, then you may have a bad solder joint, the FET may be bad, or the D8 connector may be flaky.

Hope this helps.

FWIW: Do not bypass the mosfet! I'm not sure what pins you're crossing there but it looks like you're connecting the Arduino output directly to the heated bed.

Edit: Where is that Mosfet? I just realised that doesn't seem to be on the RAMPS board.

If you do that, you could connect +12V to the Arduino output for the heated bed, which may fry the Arduino, or at least the output for a heated bed meaning it will most likely never work with that Arduino.

Additionally, the Arduino outputs are only rated for a very low current (in the order of 20-50mA at most) and simply will not be able to supply the necessary current.

Edited 1 time(s). Last edit at 02/27/2013 06:49PM by Cefiar.

the heated bed is the one that came with the huxley, the transistor I'm speaking about is in the picture (hb1.jpg) this bed needs to be connected in a different way, I don't think ramps supports this kind of stuff. If I put 12V on it connected like in the picture it heats up (slowly), that's why I jumpered it.

I don't know 3D printing, but I have been working with arduino before. The arduino is connected only to the RAMPS. The D8 output of the RAMPS is connected to the heatbed (2 yellow/green wires). All the other wires in the pictures are just lying there (excess wires from the power supply). I know it's a bit confusing.

When I check the 4 pins on the heatbed with my ohmmeter, the 2 pins in red on hb1.jpg give a reading of 123 Kohm, so those 2 go to the thermistor...

If I connect a 82K resistor to the ramps on T1, the heatbed works! The 2 red pins have a decreasing resistance while the bed heats up. So: can I change the source code to allow for the offset in resistance of the thermistor?

---------------- edit ----------------

I changed #define THERMISTORBED 2
in Configuration.h: now it works, but is this correct?

Edited 1 time(s). Last edit at 02/28/2013 02:25AM by sokopok.

You need to find out what thermistor you have and get/generate thermistor tables suitable for use with it. These are then easily added to Sprinter/Marlin.

It would be good if you had the docs for the heated bed, as that would most likely tell you the type of thermistor.

I though as much already. I found a webpage about the heatbed, and sent a mail to the author, we'll see.

[www.reprap.org]