either be discharged through shunt resistor 24 or used for a secondary load such as leds 50 in a maximum power situation this increases to as much as 19 ma secondary load 38 allows the control of the current through leds 50 to be regulated independently secondary load 38 accepts the primary analog control signal from dac 22 to allow independent control of the current through led 50 s this allows leds 50 to be operated with a controlled intensity for minimizing flickering leds 50 can also be selectively turned on and off based on measured conditions a fault condition available power or a command from a user interface on the process instrument independent control of excess available power offers additional advantages for a two wire process instrument in the past the design philosophy for two wire process instruments was that if a function could be done at minimum power 3 5 ma less the minimum required base current of 1 5 2 7 ma then it would not be done at all this invention allows for the selective control of a secondary subsystem to use available power for increased functionality and processor speed as desired and can be extended to handle many additional non critical loads the described architecture allows a design time decision to route a pre determined portion of the loop current that would otherwise be discharged in shunt resistor 28 to the secondary load for example assume a led current of 1 6 ma is desired for a loop current of 4 20 ma a resistance of 5 ohms is selected for resistor 52 yielding an input voltage range for control amplifier 46 of 5 30 mv the output of dac 22 for a 4 20 ma loop current.