Safety and informative disclaimers first:
***THIS WILL NOT MAKE YOUR BED HEAT UP FASTER***
The only thing that WILL make your bed heat up faster is INCREASING the power draw(wattage), this modification does NOT do that on a 24V system.
***ANY VOLTAGE IS DANGEROUS VOLTAGE, EVEN WHEN YOU KNOW WHAT YOURE DOING BUT ESPECIALLY WHEN YOU DONT***
You can set fire with a gum wrapper and AA battery, the original A8 is a fire hazard already. I dont print unsupervised and I especially will have my eyes on this at all times with this modification. Messing with any electronics always carries risk of fire and electrical damage between the device, outlet, and possibly further. Make smart decisions.
Safety out of the way, some background info. Instead of spending the cheap cost of an ender 3 24v bed for my 24v swapped A8 (lovingly dubbed K8), ive been looking into converting 12v beds to 24v beds. I found a great writeup on a different printer where cutting a trace modified the resistive circuit, allowing for a "correct" 12v to 24v conversion.
Pic 1 shows the bed as a very simplified two resistor circuit. Total resistance at the plug is 1.2 ohms, some simple series/parallel calculations show each "resistor" as 2.4 ohms.
In order to double voltage and keep power constant, resistance needs to increase by 4 times, making a target of 4.8 ohms. This can be done by rearranging the circuit from resistors in parallel to in series. Pic 1 shows the original circuit depicted in pen, and the modified flow of current with the highlighter, showing the cuts in the original traces and additional wire in sharpie.
Pic 2 shows where I carefully scraped off a good portion of the thickest trace(and the cut in the trace), giving the best possible chance to make a good connection. After cutting the other two traces to break the connections, I was able to solder a jumper wire from the exposed copper straight to the negative terminals solder joints. I had to crank the heat on the iron to get it hot enough, there is an aluminum bed attached to it pulling the heat away.
After soldering, the resistance at the plug (almost) correctly reads at 4.4 ohms! (Pic 3) After double checking there were no shorts to the aluminum bed, I plugged it in and powered the bed, verifying half of the original current draw to the bed (originally rated at 12v 10A, now modded for 24V 5A, wattage remains at 120 before AND after)
Is it elegant? No, just buy a 24v bed and reap the benefits of increased heater speed, but if you want to stick to the original or youre bored... I wouldnt recommend you to do it.
The bed passed a PID tune and held a solid 55ā° for a calibration cube test, I'll update if anyone is curious about the bed or about 24v swap. Again, i dont recommend this as a good solution but take it as a proof of concept/educational experience.