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Help with heat reading problem with custom heated bed

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I bought this kit from eBay:


(There is a "primitive" wiring description on the bottom of the page. And yes, I know it uses a relay instead of MosFET. Will change that later! :wink: )

Now to my problem. Whenever my Ultimaker needs more heat on the bed and starts it, the temperature on the nozzle plunges about 6 degrees.

This is REALLY annoying and I can't really figure out where the power thief is that is.

Is there anyone out there that has any idéas?




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Yes. I have solder the 4.7kOhm resistor there. I tried measure it (though while solderd) and got 4.7kOhm through it also, so I think that part should be ok.

I power it with a switched power supply (this one: http://www.aliexpress.com/item/250W-24V-Small-Volume-Single-Output-Switching-power-supply-for-LED-Strip-light/628583985.html ). Though I had to take it down to 20v instead of 24v or otherwise it hit the limit of 10A.


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Now I have tried building a mosfet-controller for the on/off of the bed and I still get a big dip in the heat reading when the bed activates and a big jump up when the bed stops loading.

Is there anyone that has an idea on what I can do to make this not happen?

Or any idea of where I should search for the error in temperature reading..

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