Wondering: Why heat your bed with 12V or 24V instead of a mains-powered SSR? Upsides of SSR (much more efficient use of power, 12V/24V PSU can be lower current->less expensive) seem to outweigh downsides (SSR bulky, non-zero cost). But I may be missing something…
And while I’m wondering…Is there any reason to use 12V instead of 24V for new designs? With a $1.20 12V 3A LM2596S module for old gear or anything that has to be 12V? Steppers (particularly when driven by the quieter drivers) run so much better on 24V…
I’m surprised that 12V and low-voltage heaters still seem so popular.
@Josh_Rhodes 24V can drive a lot more current into the inductive (and resistive) load of stepper motors. So more torque & more speed & more headroom for the “quiet” stepper drivers to do their thing.
@Dont_Miyashita You’re right - you have to be careful, and you reminded me of another plus for mains voltage: more powerful heaters for faster bed warmup.
@Fred_Hamilton
You’ll not drive mote current into your steppers if you use 24v instead of 12v. But what you’ll do instead is building up the magnetic field sooner because you’ll build it up with a higher voltage in the pwm signal of the motor. But the current will stay the same.
Mains definitely require stricter safety precautions. SSRs tend to fail shorted so it’s best to have a safety cutoff when the bed exceeds the normal operating range.
The big, hairy, frightening downside to mains voltage AC bed heaters is wire flex. Particularly on X-bed or Y-bed printers where you’re racking up millions of bend cycles on the wiring.
Hobbyists simply don’t know how to properly implement high-flex wiring. Mains voltage will kill you if the wire insulation frets away from rubbing, or a wire breaks off its terminal from bend cycling.
So, my recommendation is to use AC heaters on Deltas, but not moving-bed Cartesians. 24v bed heaters are safer.
AC heaters also have a lot more potential oomph behind them if something goes wrong and the heater sticks on. You MUST have a good thermal fuse if your bed heater power is more than about 0.5 watts per cm^2.
Switched to diy 120v bed, best upgrade I’ve made yet. Bed hits 60 just as the hotend reaches 200. Bed is a sandwich of 3/8" Mic6 disc/kapton/kanthal wire in a spiral/kapton/ceramic insulation/1/6 inch aluminum sheet. Thermal fuse on the bed, slow blow fuse on the switch before the SSR. Highly recommended… Atleast if u have a Delta/non moving bed
I hadn’t thought about bed movement! It’s not really an issue on my delta or corexy (with appropriate consideration for the slow vertical movement), but yeah, I’d be nervous if the bed was the Y-axis. Good point!
And I really need to add a thermal cutout to my bed.
I just realized one other potential drawback of mains+SSR architecture. While it might heat up much faster than a 12/24V heater, that becomes a big liability if the SSR gets stuck ON. My RepRapFirmware calculated my bed temp would rise to something like 350C if my SSR was frozen ON. Slower heating beds won’t reach nearly as high a temp in a fault condition.
A thermal fuse like the KSD301 should prevent the bed from ever getting that hot, but I didn’t realize how important that was when I built mine, so I need to add it ASAP. Thumbs up to RepRapFirmware for reminding me/scaring the the crap out of me during bed calibration…
