After thinking a bit about the possibilities, there were only two options: Heating via cartridge or with SMD power resistors on an Alucore PCB. The local metal shop was then kind enough to cut me a steel solid, so all I had to do in experiment one was drill holes for the cartridges and the temperature sensor. So now the TDP simulator looked like this:
The cartridges had too little heat transfer to the steel, so that the lines to the heaters suffered thermal damage even before the steel was warm enough. This possibility therefore had to be eliminated. I was further confirmed by digging through my spreadsheet book, which revealed that steel has a rather poor thermal conductivity value. Ok, learning by doing. Again. I should have known, but when you go off on a bloodlust, it’s far too easy to miss the point.
So with only one option left, I looked at various SMD power resistors. By using these components I also generate the advantage that the total heating power can be controlled much more precisely, because heating cartridges are relatively delayed due to their design. So I was looking for resistors that offered me a high heat dissipation to the aluminum core PCB due to their geometry, but at the same time did not have high resistance values. After sifting through Mouser, the choice fell on power resistors in thick-film design in a DPAK package.
This design allows an optimal soldered heat transfer due to the large shield on the backside, just like a soldered CPU. After calculating the circuit and the total resistance it was then the turn of the PCB layout.
It is actually quite simple at the (heating) core: the resistors are connected in parallel and there is a negative side and a positive side. The current that then flows through the resistors due to the voltage generates enough heat to quickly and safely heat up the aluminum core PCB. The challenge with this layout is that on the one hand the copper areas must be large enough to withstand the current, on the other hand there must not be a second layer (bottom) due to the Alucore construction. Once that problem was ironed out, it went straight into production and I had a bit of time to work on the control board.
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