I just made a mod that speeded up the heating of my bed, and greatly increased the maximum temperature. I've see some posts here about turning up the pot on the power supply to 13.4 volts, but mine, at least, will go to 14.85. However, there are warnings online about turning it up that high due to over-driving the Vin to the mega. New versions of the Mega 2560 are supposed to be able to handle up to 20 Volts but I don't think ours are genuine Arduinos, so I was hesitant to do that, and I imagine that is also why folks on here only put it at 13.4 or 13.5 volts.
I read up on this and found the web page of the creator of the RAMPS board. He spoke of these issues and also recommended against turning it up over 12 volts. However he also explains that the only issue there is the Vin to the 5 Volt circuitry of the Mega 2560. There is a diode on the RAMPS board, D1, that supplies the Vin to the Mega. If that is removed, you can safely supply much higher voltages to the RAMPS. But, you'll no longer have the 5 Volt supply, so you need to use USB power for the Mega. But there's another way...
Now to the point! I removed the 1N4004 diode D1 and replaced it with three diodes. I used 1N4001's because that's what I had on hand. Any of the 1N4xxx series from 1N4001 to 1N4007 will work fine. Each Diode will theoretically drop approximately 0.6 to 1.1 volts depending on the current through the diode. In practice, the 1N4001's each dropped 0.77 volts.
So, with the three diodes replacing the one diode, and the power supply turned up all the way, I have 14.85 volts going to the RAMPS and 12.54 volts going to the Mega's Vin Pin. Totallly safe. The there are two 12 volt systems on the RAMPS. One supplies only the bed and hotend heaters. The other supplies the Vin to the Mega's 5V regulator and the Steppers and the RAMPS board cooling fan and the hotend cooling fan. Both of those fans I think should be fine with a 2.85 volt overdrive. You can hear them speed up when you increase the voltage with the power supply adjustment pot. Another advantage is that the steppers will now have higher voltage available. This is fine since the drives are current-controlled. With this type of control, the higher the voltage available, the better. My cnc desktop mill has 90 volt drivers driving 3.4 volt 2 amp steppers, for instance. I won't go into that further here unless someone wants an explanation. It's out there, just google it.
So now for the results! My bed now heats up to 80 C in 10 to 12 minutes. 100 degrees C in 22 minutes. 105 C in 28 minutes. I used to wait 35 to 40 minutes for 80 C. Here's the math: I know many (most) of you will know all this but for those that would like an explanation, here it is: )
The bed resistance is 1.5 ohms. At 12 volts, this gives I (current) = 12 V/1.5 ohms = 8 AMPS. The power is then V * I = 12 * 8 = 96 Watts. Coincidentally, The surface area of the board, 8 * 12 inches, is 96 square inches, giving exactly one watt per square inch. Actually I doubt that is a coincidence, but rather by design.
But, contrary to what intuition might suggest, Power is not linearly dependent on V and I. It is exponential. At 14.85 volts, the current is 14.85 / 1.5 = 9.9 AMPS. Power is now V * I = 14.85 * 9.9 = 147.015 Watts. The increase in voltage is 14.5/12 * 100 = 120.33%, but the increase in power is 147.015/96 * 100 = 153.125%. So a small increase in voltage can give a larger increase in Power. It's because Power is also equal to I^2 R (Amps * Amps * Resistance) a non-linear relationship. You can work that out from the most basic Ohm's law formulas. I'll leave that to you.
Anyway, if the Arduino's Vin max is really 20 V as the RAMPS creator states, you should be able to just max out your 12 V power supply with no changes. I decided to play it safe and make this mod. It's not difficult, and it really sped up the heating curve.
Here's a link to the web pages that I used, that show D1 and discuss the power issues.
Scroll down about half way to find the section on Power Supply, and read down from there about power supply with and without D1 installed. Then scroll down some more to find the picture of the ramps board to find D1's placement on the board. (It's a 1.4.2 version so it's a little different, but it's in about the same place as the on V1.4 boards. I don't know if the PICA has dealty with this issue or not, that's beyond the scope of this post. Maybe @mjrice will weigh in here on that.
Bottom line: The diodes are 11 cents or 13 cents from digikey, and they can ship USPS first class. It's maybe a half hour job. Well worth the time spent. It will save many hours of finger tapping waiting for heat.
Oh, get some heat shrink tubing that will fit over the diodes. solder them together end-to-end with about 6 mm leads laid side-to-side, and solder a 40 - 50 mm 22 gauge wire to the far end and bring it back to the front of the assembly, after sliding the insulation over the diodes. You need the insulation to prevent any possibility of shorting against the smt resistors and the cans of the caps on the board.
I'll probably do a video when I do the next one.