New Build. Hotend Overheating. Not Printing Yet My build with the Hypercube Evolution continues with a replacement BIQI KFB2.0 board. I am not at the printing stage yet. This is due to my latest problem.
The kit contains a “no-name” hotend, complete with round finned heatsink and nozzle. Looks a bit like a V5 J Head Hot End.
Everything is wired up. A 24 V power supply is being used. The printer is connected to Pronterface via USB. Using Pronterface, I set the hotend temperature to 70 °C. The KFB2.0 board lights a bright red LED when power is being sent to the hotend (and/or the heated bed). On initial heating, the red LED is on until the hotend temperature reaches around 65 °C and then turns off. The temperature continues to climb to around 120 to 125 °C, then starts falling. At 69/70 °C the red LED comes on again for about two seconds, the temperature continues to climb after the red LED is off to around 86 °C. It then cycles around the 69/70 to 86 °C position.
I have removed the thermistor to check its resistance. Reads greater than 100 kΩ. When in situ in the hotend, it was reading around 85 k&amp;ohm, but a bit of Kapton tape sorted that. I was now reading 100 k&amp;ohm with the thermistor in place. I then used Pronterface to set the temperature again. This time with a multimeter hooked up to the hotend terminals on the board. It confirmed that the bright red LED was sending 24 V and nothing when not on, so the hotend is recording a temperature increase from around 65 to 125 °C with no power. But, I now got something different when the temperature fell back to 69/70 °C. The red LED flickered for about two seconds and the multimeter showed readings fluctuating between 0.1 and 4.2 V. From this point the hotend kept the correct temperature, even after I had removed the multimeter.
Has anybody any idea why the temperature is going so high and what can I do to fix it?
3.9 ohms is a 12V heater.  It won't work on 24V.