Oh Dear! I Just Fried My Whole Truck!!!

[BrianG]

If he's gonna get a 5v or 6v fan, I'd just hook it to the AUX output of the Rx or use a Y harness off the Rx pack - no need for any diodes or resistors then. This would be by far the easiest solution. It's the dual speed thing that is making things more complex. Also, if he had a fan and had it running at its rated voltage, it would spin at a reasonable rate and wouldn't need a hi/lo speed switch.

BTW: Fans don't need resistors (are you thinking of LEDs?) The diodes will simply drop enough voltage so the fan doesn't spin so fast since he's running 16.8v to a 12v fan. At any rate, the fans internal circuitry only pulls what current is required, so no resistor is needed to limit it.