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Heat Bed help

Posted by scubi 
Heat Bed help
April 18, 2015 11:03AM
Hi Please can you help with my heated bed.

1. I can not see which side is postive or negative on my heated bed. ( Mk 2 ). Does it matter which wire goes to the positive or negative, Ramps board 1.4.

2. I want to test if the heated bed is working, can I connect it directly up to the power supply 12v and see if it gets warm , then switch off ?

Re: Heat Bed help
April 18, 2015 01:19PM

The heated bed is not polarity sensitive unless you have added the LED in which case you need to observe its polarity requirement.

You can test the heated bed on a 12V power supply but be aware that the heated bed draws significant current. If the supply you are testing with is not the one to be used on the printer you need to consider its suitability.

Neil Darlow

I try to write with consideration for all nationalities. Please let me know if something is unclear.
Printing with Mendel90 from fedora 25 using Cura, FreeCAD, MeshLab, OpenSCAD, Skeinforge and Slic3r tools.
Re: Heat Bed help
April 18, 2015 01:23PM
Polarity only matters to the heat bed if you have a single LED on it and you want it to light.

The wiring scheme polarity in the ribbon cable is chosen to give least thermistor noise. Neither the thermistor or the heater mind which way they are wired but the negative bed lead has the most switching noise, so it is best next to the thermistor ground to minimise capacitive coupling. Can't do much about the current creating inductive coupling though.

Yes you can wire it direct to a 12V PSU indefinitely because as it warms its resistance rises, so it wont go much above 130C @ 12V.

Re: Heat Bed help
April 19, 2015 03:05PM
Hi, thanks for the reply. OK I have abit of problem. The heated bed has as a Led on it. If I connect it the ribbon cable the wrong way round, will the only problem be is the led will not light up?
Re: Heat Bed help
April 19, 2015 04:48PM
Probably, but to be safe I would use a multimeter on its diode setting across the LED to find out which is the anode. Or consult the company that sold you the bed.

Re: Heat Bed help
April 20, 2015 02:34PM
Hi Guys

Tried using a Multimeter to test polarity but can not get the Leds to light up. Tried also on the Leds legs themselves.

1. The Heated Bed has two Leds and a resistor. If it has two Leds does it make a difference ?
2. If I get the polarity wrong what will happen if i connect the bed up, to 12v or my Ramps Board ?

Sorry I got the board second hand and removed the wires without taking not of which was which.

Re: Heat Bed help
April 20, 2015 03:18PM
1. Beds with two leds usually have them orientated opposite of each other. That way it's guaranteed that one will be orientated correctly. One will be lit and the other will not. If you make sure it's orientated correctly, only one is necessary.

2. With the polarity wrong, the bed should still heat but the LED will not light up. No harm should happen.

In your case, it could be the LED is bad. If it was operated at higher voltage than the resistor was selected for, it might have released it's magic smoke. Honestly, I'd hook the bed up one way. If that doesn't light it, flip it around. If it still doesn't, replace the LED (and maybe the resistor too) when you get a chance. But the bed will continue to operate even without the LED.
Re: Heat Bed help
April 21, 2015 01:31AM
Thanks for the reply guys, much appreciate it. Just don't want to damage my Ramps board. Thanks again.
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