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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?

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  • \$\begingroup\$ if steel was glowing, in air, there is no plastic to heatup, so it will be much too fast without air flow . you need >1m/s going thru the tube. I made a picnic box into an environmental heat box by suspending a 25W soldering iron with a fan inside to reach +50’C rise \$\endgroup\$ Dec 28 '20 at 17:28
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The power supply for the hotend in your printer may not have been able to supply as much power as your power supply. It won't try to heat up too quickly because this can cause overshoot in the temperature. It has no active cooling, so overshoot is very bad. It may also be limited for safety reasons, or to prolong the life of the heater.

Once the hotend is at or even near the set temperature, the duty cycle is very low. This is because you want your idling power to be a small fraction of your total ability to drive. This is so that the control system can quickly and accurately force the unit to stay at the correct temperature.

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you need high speed air flow with thermal feedback to regulate the exhaust temp.

A muffin fan may achieve this with care with a thermistor and partial derivative feedback to anticipate overshoot and eliminate it.

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