At the moment I’m working on a project that needs a bunch of power resistors, and I noticed that all 3D models for power resistors are flat on the PCB, and in “real life” this is uncommon. Power resistors very often have a standoff distance from the PCB. Both to improve airflow, and to reduce heating of the PCB from the resistors.
I’ve been analyzing some datasheets, and I’ve found at least two (Ohmite_50 and Vishay_RS) that have an operating temperature up to 350 deg. celcius, I.e above solder melting temperatures, which makes a decent standoff quite mandatory.
I’ve got no trouble with drawing a 3D model for my own project myself, but I do find it a bit strange that the 3D models for these power resistors are not designed for the “most common” usage of these resistors. But it’s also difficult to decide what a “sensible standoff height” would be for power resistors.
Simply adjusting the Z-height is an option, but then the resistors would float above the PCB. Adding more 3D models just for the legs (so they overlap with the resistors which makes them a bit telescopic) can also be done, but that feels like a kludge. What would be a decent standoff height for these (differently sized) power resistors?
I’m curious about your thoughts. Should I create an issue for this on gitlab?
Back in the old days, I saw white ceramic standoffs, perhaps 3…4 mm diameter and ~3 mm height with conical ends positive/negative for easy stacking. They seem to be out of fashion, as the most normal way of raising a power resistor today is to do special lead forming, where the leads have a “C” bend to prevent deep insertion and thus define a specific height.
I am aware that in some cases people use micanite layers in between, or cut resistor’s body shape from the board (so it can also fit “inside”, or a few holes underneath instead of a complete ‘body shape cut’) and in other cases have them custom mounted out of the PCB and connected with appropriate cable type.
For the 350 Deg C mention, datasheet offers a power derating diagram for that ambient temperature.
I think adjusting only pin length of these models(IIRC these are script generated) will allow a better looking 3D view; combined with the Z height adjustment though.
Also, “in our local library copy”, i find convenient to add one more 3D model with the thermal isolator part and toggle its visibility at will.
As for a decent distance, i do not have a suggestion. This is up to the designer as far as i can say.
Scripts can probably(easily?) get patched to work with a pin length definition in the *.yaml file. (Maybe also add to the output filename)
A gitlab issue is indeed the best way to report this and discuss further with the currently active librarians
(Please note that i am no longer active and my comment is not an “official” reply. I am a single user and sometimes i get to contibute stuff, feel free to ping though…)
(FWIW: For some library related history, Christian, AKA chschlue, was the head librarian after Rene and before cpresser. Still inactive and quiet since a little after Github to Gitlab migration. That was during covid.)
