I was talking with the peoples on #reprap and they brought to my attention that the PCB heaters work better than i had thought, this got me thinking do ii have to go to a HUGE mains powered heater for my new bed, this one is going to be light weight. And that has been the driving design criteria.
I was thinking that with a printbed that is 470x470mm that could be broken up into 9 squares of 156mm, looking at my local electronics shops website i see i can get 10 pieces of copper clad FR4 single sided for $6.95 a piece, and a resist pen for $9 this makes the heaters about $80 as i already have the acid and the spare time :)
there is a website that i sometimes use that will do 5 boards for that $80, so is my time worth $80 to me?? maybe but then there is the risk factor to deal with.. also there is the question is a 3x3 grid the best idea, what about a grid with 5 cells, think quarters then take a square in the center as well, this will mean bigger pcbs i might be able to make a layout that has a optional corner cut-out so i only need to order 5 boards the same and cut the corners from the 4 and leave the center one whole.
this will require me to have more analog inputs on my arduino controller, that will be 3 nozzels and 5 bed segments that's a total of 8, the mega has 16 available but they may be appropriated for other uses and may require re-routing,there are more than enough digital outputs available.
there is another pattern that i did think about using much the same as the 5 segment picture above but the square is rotated 45 degrees to make it easier for the glass cutters if i choose to go with individual tiles but that leads to wasted energy.. and thats what i was after by splitting the bed.
Update : after i made this post another suggestion was put to me for segments, I could use a similar pattern to the 5 section design but the idea was to have two hot zones ie the inside square which is where most prints get started from the middle of the printbed, and the outside square that only gets turned on for lager sized objects, this idea is interesting to me, it may reduce the number of thermistors required to monitor the bed. i am not really chasing efficiency just thinking about whats involved in removing the 240v AC heater from my printer.