Proportional Fan Circuitry The Proportional Fan Circuitry of the DLS provides an extra measure of protection. During times of heavy load demands, the automatic circuitry will activate the fan before thermal stress can occur on any internal components. The Proportional Fan Circuitry maintains fan speed in direct proportion to the internal ambient temperature of the DLS, enabling the fan to turn on and off slowly and quietly, eliminating unwanted and sudden fan noise.