Power control, (a.k.a. power mode or power loop) is a third control mode that can be added to a variety of Spellman supplies to provide another means to control and regulate the output of the supply. Voltage mode and current mode are the primary controlling modes of most units. Taking the voltage and current monitor signal and inputting them into an analog multiplier circuit, creates a power feedback signal (voltage x current = power). Using this feedback signal with an additional programmable reference signal in conjunction with error amplifier circuitry, a programmable power mode can be created.

Power control is typically used in two types of applications. The less common application is where the power into a load is the needed regulating parameter. A critical heating requirement may have very specific regulated thermal need. Using power mode, voltage and current limit levels can be established, and power mode will provide constant power to the load, immune from any impedance variations from the load itself.

The more popular usage of a power mode is in the area where a power source or load might be rated or capable of more current at reduced voltage levels, but limited to a particular power level. X-ray tubes frequently have this type of capability. If the maximum voltage were multiplied by this "increased current" capability, a power level above the rated power level would result. Power mode can address this problem by limiting the power to the maximum rated (or present) level.