Current

Current through the valve's coil creates the magnetic force that moves the spool in a proportional valve. The coil current is equal to the voltage across the coil divided by the coil resistance. It is important to talk about current control of the spool, not voltage control, because the coil resistance of most valves increases as the valves heat up in use. More voltage is required to achieve the same spool shift as the coil resistance increases. High wattage coils can increase their resistance by more than 50% due only to the heating effects of running the valve at full shift. This can result in insufficient voltage to cause the valve to fully open when hot, if the manufacturer has not left enough headroom. Heat from the hydraulic fluid can also increase coil resistance.
Coils for proportional valves are often specified by their wattage. Coil wattage divided by the coil voltage gives the rated current. This current value may be used in sizing a valve driver and power supply if there are no graphs specifying spool shift vs. current. Some proportional valves are rated for maximum voltages much less than the commonly available power supply voltages, and require a current regulated valve driver for safe operation.

Watts	Volts	Current	Resistance
W = V * I	V = I * R	I = V / R	R = V / I
W = I ² * R	V = W / I	I = W / V	R = V ² / W
W = V ² / R	V = √ (W * R)	I = √ (W / R)	R = W / I ²

W = watts, V = volts, I = amps, R = ohms