I am trying to repurpose an old hotend of my 3D printer (Link) as an improvised temporary "space heater" to raise the temperature in a small enclousure by 5-10°. I happen to have a 24V 48W power supply lying around, so that is a great fit for the 24V 40W heating tube.
For some simple testing I directly connected the two ends of the heating tube to the 24V PSU. While it worked, after a few seconds I noticed the dust on the tube burning up. Since I was testing in a safe environment I let it continue heating up and after 10 or 20 seconds the stainless steel started to glow red, so I disconnected it. While apparently in terms of ratings everything is fine (fuse of the PSU did not blow), obviously having a red hot glowing and smoking steel tube is not what I intend to have.
I remember from setting up my 3d printer that various parameters have to be configured to properly adjust the PWM control for the hotend, so I took an appropriately rated DC motor controller (VNH5019) and set it to 20% duty cycle. While it didnt heat up so fast it still started smoking after roughly 20 seconds.
I am wondering, how are those ceramic heating tubes normally wired? Heating my 3d printer up to 240° takes 2-3 minutes, the steel tube glowing means it went to like 400° in just a few seconds. Is there some current limiting normally involved? Are they running at super low duty cycle? For both of them: Why not use a heating tube that does not require current limiting/small duty cycle for the working temperatures?