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Negative bed temperature?

Posted by kev 
Negative bed temperature?
December 14, 2012 12:27PM
This is my first post, so hello! I'm Kev smiling smiley

I've just assembled my RepRapPro Huxley and am having some trouble with the electronics.

I'm running an Ubuntu virtual machine on my Mac and now that I've finally been able to connect to my Melzi board (I did have a permissions error on accessing the USB port), I can't get a sensible reading out of the bed.

When I Check the Temp in Pronterface, I get a reading of -273.1 Degrees C, yet I'm getting 21.8 for the hot end (about room temp).

If I disconnect the headbed from the controller and check the temp, it reports a temperature of -43 degrees.

Can anyone suggest what this might indicate?

All the best,
Re: Negative bed temperature?
December 14, 2012 02:06PM
You need to be a member of the dialout group if you want to access usb-serial ports in linux!

WIth your heated bed, disconnect and measure resistance. I Guess you have a short or a broken cable. Both give extreme temperature readings. Normal resistance is around 100000 at 25°C.

Other reason could be misconfigured analog input pin. I mean you set the wrong pin number for the connections used, but then disconnecting it wouldn't change the output.

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Re: Negative bed temperature?
December 16, 2012 01:54PM
That's a tough one, but not impossible to figure out.

First see what happens if you swap the bed and hot end thermistor connections. If the problem moves, your issue is in wiring, or the thermistor itself. If the problem stays put, you could have misconfigured pins, or worse, a damaged Atmega.
Re: Negative bed temperature?
December 16, 2012 02:19PM
Thank you for your responses folks, I've had the bed apart, and found a solder bridge shorting out the thermistor.

I've corrected that and now the bed reports back at -14degrees which is a big improvement! However, when the power is in, the bed gets hot but the temperature doesn't change from -14. It's as though the controller is trying to bring the temperature up from -14 to 0 so it's wacking the heater to full power.

I'm starting to wonder if the thermistor is dead...

T:23.9 /-0.0 B:-14.3 /0.0 @:0


Edited 1 time(s). Last edit at 12/16/2012 03:08PM by kev.
Re: Negative bed temperature?
December 16, 2012 03:56PM
Measure the resistance of the thermistor using the leads that you use to connect it to your controller. At room temperature it should be near 100 k (assuming it is a 100 k thermistor). If it doesn't, you either still have a wiring problem or your thermistor is broken. if that is ok, I would expect a problem on your controller board.
Re: Negative bed temperature?
December 16, 2012 04:40PM
Thanks again everyone, after much checking and double checking, it would appear it is a software issue.

I've been trying to run pronterface from a virtual linux installation (as eventually it'll be used with a linux laptop). I've just switched back to Mac OSX Mountain Lion and everything is hunky dory (well, apart from the steppers, but that's a different post!).

Bottom line, Pronterface doesn't like virtual machines, at least not the way I have my computer configured. Case closed.

Many thanks again wonderful people smiling smiley

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