Quote Originally Posted by DrJeff View Post
Any chance you got a photo of it. It can't be a rheostat, it must be a PWM or something similar. Dave (bitsyncmaster) may know, but I think there is a less common PWM circuit that drives through the 12v side rather than the ground side. IIRC driving through the ground side is considered the prefered method by 9 out of 10 cats.
FYI:
Driving the ground side requires less electronics because you forward bias the transistor or N-channel MOSFET. To forwared bias driving the 12 volt side requires a voltage above the 12 volts. Transistors 1 volt above, MOSFET 5 to 10 volts above. When driving the ground you only need 1 volt with transistors and 5 to 10 volts with MOSFETs. So my relays need a 24 volt internal supply.