I picked up five 12 V 40 W cartridge heaters from the usual eBay source for some extruder experiments and did a quick check to make sure they actually worked:
The bench supply is good for 3 A, which isn’t quite enough to light them up all the way, but at 8 V they drew anywhere from 2.67 to 2.20 A, declining by about 0.1 A as they heated over the course of maybe 5 s, which is about as long as you want to run them outside of whatever they’re supposed to be heating.
Those dissipations are a bit lower than I expected; at 8 V you’d expect to see about 27 W = 2/3 * 40 W, not the 18 to 21 W I actually measured. Current & power don’t scale linearly, so I must gimmick up a larger block and make some better measurements when I get the LinuxCNC hardware set up.
The insulating tubes on the wires emerging from the cartridge, inside the main sheath, show the usual attention to detail I’ve come to know and love from eBay suppliers:
Ah, well, it keeps my toy budget under control…
There’s a story behind the dark vertical smudge just to the right of the cartridge. More on that in a bit.
The Smell of Molten Projects in the Morning 
Not saying this is the instance here – but there is a rash of cartridge heaters sold as 12v 40W at the moment that are actually incorrectly labelled 24V 40W… and give the expected lackluster performance as a result.
Thats not really a problem because though because, these cartridges also have a bonus trait of being out-of-round… I’d call them oval, but they have managed to somehow make them not-quite! ;)
So yes – be wary of misc sources next few months – and even then you can’t be certain – I’m only aware of it being wide spread because two separate reputable suppliers discovered the problem early with some ‘urgently sourced’ cartridges during their QA…
Assuming the resistance doesn’t change all that much, a 40 W heater would be 14 ohm at 24 V and 3.6 ohm at 12 V. These run about 3.2 ohm, so they’re probably not mislabeled, just, shall we say, a bit broadly toleranced.
You’ll note I did fire ’em up and make a few quick measurements before doing anything else. [grin]
These are not what you’d call precision items: they’re nominally 0.25 inch diameter and actually run 0.230 through 0.242 or so. Judging from the end of the cans, they were swaged with a hammer and pin punch. Or maybe with just a hammer. Or maybe they were swaged not at all: the cans aren’t all that round, come to look at them more closely.
Given that I’ll be building them into things, I might use a B (0.238 inch) or C (0.242 inch) drill, rather than a 0.250 drill, to get less clearance. Definitely with a through hole so I can pound them out with a pin punch after they fail…
(Incidentally, typo in the above, I don’t think a B drill is 0.380″ and I don’t think it would be quite the fit class you’re looking for if it were. :) )
Good catch… thanks.
They’re not that sloppy a fit!