C.28.MR:5.2 - Change a display or a supply command

Consider an ideal regulated supply and a resistive load. C is the command in volts, V the delivered voltage, D the displayed voltage, R a fixed resistance, I the load current and W its power. In the operating region being modeled,

C=12 V; V=C; D=V; R=6 ohm; I=V/R; W=V I.

The natural result is I=2 A and W=24 W.

Replacing the display equation by D=6 V leaves V=12 V; current and power remain 2 A and 24 W. Replacing the command equation by C=6 V instead gives V=6 V, I=1 A and W=6 W. The two changes share a numeral but replace different mechanisms.

Suppose the supply’s current limit is now relevant. Use the revised positive-command model V=min(C, I_max R) with I_max=1.5 A. The 12 V command now gives 9 V at the load and 13.5 W. The 6 V command still gives 6 W. Relative to natural behavior, the calculated power reduction is now 7.5 W, instead of 18 W in the earlier operating-region model.

This is a model comparison. Using this result for a physical supply requires an account of its regulation, load and limiting behavior. Altering a graphical interface value is a physical intervention only when the interface actually controls the modeled command.