So in other words, you are saying that yes, halving voltage will quarter the wattage; a 240w heater at 24v would become a 60 watt heater at 12v.
Why I want to do it? Because a 3,360 cm2 bed preheating will draw 1344 watts when preheating if using a heater with a power density of 0.4w/cm2. That is not an option for me. Yes, I know that a 15 amp circuit could handle that load by itself with no trouble. Thing is, this printer won’t be by itself, all my other printers are on the same circuit(yes, I know I could shut them down, but I’d rather not have to shut them all down to preheat this one) besides, that isn’t all that is on the circuit. Everything else plugged in in this room and the neighboring room is also on that circuit(it’s the way this house was wired🤷♂️) So, yes, I know this way entails a little more expense and complexity, but if that is what it takes to be workable, so be it.
On the modifying firmware note, why? Will a mosfet not work with regular marlin firmware? Excuse my ignorance, but is PID control in a bed heater context really PWM? I mean, it is sort of in the sense that is turning the power on and off in long “pulses” if you will, but if that qualifies as PWM, then so does an old fashioned thermostat. Or do I have things completely wrong?
With my ender 3s for example, on the screen it shows the power to the bed being on for a couple seconds, then off for a few seconds, and so on. Or does that graphic display on the screen not actually indicate if the power to the bed is on or off? If I have that right, it means the printer is alternating between drawing 240watts plus whatever the hotend, fans, and steppers take, and just drawing whatever the hotend(if on), fans and steppers take. So this custom printer would be drawing 1400w(roughly?) , and 56w respectively if I went with a 0.4w/cm2 heater. Contrast that to a cyclic load of 392w(roughly) to 56w. Granted, with a lower wattage bed heater the higher draw pulses will be roughly 4 times as long(or longer?), but the peak load will still be lower, making this printer workable in my situation.
Please correct me if something I said above is wrong.