The change to the resistors is part of the requirements for the DIY power booster addon (if you were to buy it from them, they'd mail you the heatsink they use, and I guess the resistors, and instructions on what to change on the PCB).
The only thing I am doing is removing the DPAK transistor, and using the same transistor in a higher power package (TO-220), and the change to the resistors. The DPAK's are only really rated for 20W, but I guess adding a heatsink, they can squeeze a few more watts out of it. The TO-220 is rated for 70W. With my big pwm heatsink, it probably can dispate a lot more heat safely - not that I need to
If you click the links above, for both the DPAK and TO-220, you'll see 'safe operating area' graphs that show Vce (which is basically the voltage across your fan or pump) vs. Ic, which would be the current the pump draws. for a D5, that would be 2 amps @ 12V.
for the TO-220, at Vce of 10volts it can handle 3amps, at 15 volts it can handle 2amps, and that doesn't include power dissipation from a nice heatsink... just from the package.
I'm guessing it's gonna work nicely, if not, i just wasted like $115, but had some fun

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