Source-Load Power Scan - Electronic Circuits III: Comparators and Interfaces Step by Step

A loaded source has one current, one voltage split, and one power budget; all three must cite the same row. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Source and load resistance set one interface current

The source rail is fixed at 12 volts. Changing the source and load resistances changes both the voltage split and where power is dissipated.

I=\frac{V_s}{R_s+R_L}

Three rows close the voltage split

Each row has one current through the source resistance and load. The two voltage drops must add back to the same 12 volt source.

\begin{array}{c|c|c|c|c}R_s&R_L&I&V_s\text{ drop}&V_L\\2\ \text{ohm}&4\ \text{ohm}&2\ \text{A}&4\ \text{V}&8\ \text{V}\\4\ \text{ohm}&2\ \text{ohm}&2\ \text{A}&8\ \text{V}&4\ \text{V}\\3\ \text{ohm}&3\ \text{ohm}&2\ \text{A}&6\ \text{V}&6\ \text{V}\\\end{array}

The power budget follows the same current

The power table is not a second story. It reuses the checked current and voltage drops, then verifies useful load power plus source-resistor power.

\begin{array}{c|c|c}P_L&P_{R_s}&P_{\text{source}}\\16\ \text{W}&8\ \text{W}&24\ \text{W}\\8\ \text{W}&16\ \text{W}&24\ \text{W}\\12\ \text{W}&12\ \text{W}&24\ \text{W}\\\end{array}

Equal resistances split both voltage and power

The final row has equal source and load resistances. That makes the drops equal and the dissipated powers equal in this resistive model.

6\ \text{V}=6\ \text{V},\quad 12\ \text{W}=12\ \text{W}

Power closure keeps useful and lost power together

A loaded output is trusted only when the rail power, load power, and source-resistor loss close in one table. Frequency response and active-device limits are deferred.

8\ \text{W}+16\ \text{W}=24\ \text{W}