Power Budgets

Useful Versus Wasted

close the power budget

This useful-versus-wasted lesson closes a source-power ledger by splitting 24 W into 6 W of useful load power and 18 W of dissipated waste. The 1/4 useful fraction is exact only for the stated ideal power entries in the training budget. Real systems have component tolerances, heat flow, conversion losses, source/load limits, calibration limits, finite precision, and measurement uncertainty.

A circuit budget must count useful power and dissipated power from the same source power. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Count the source power

The source power for this budget is 24 watts.

Psource=24 WP_{\text{source}} = 24\ \text{W}
Useful versus wastedThe loaded divider is the circuit being budgeted.12 V3 ohm6 ohm6 ohm2 A1 A1 A+-6 V+-6 V+-6 Voutput

Useful plus wasted closes the budget

The useful load power is 6 watts and the wasted resistor power is 18 watts.

Puseful+Pwasted=6 W+18 W=24 WP_{\text{useful}} + P_{\text{wasted}} = 6\ \text{W} + 18\ \text{W} = 24\ \text{W}
Useful versus wastedThe load branch and divider resistors share power.12 V3 ohm6 ohm6 ohm2 A1 A1 A+-6 V+-6 V+-6 Voutput

The useful fraction is exact

The useful fraction is one fourth of the source power.

PusefulPsource=6 W24 W=1/4\frac{P_{\text{useful}}}{P_{\text{source}}} = \frac{6\ \text{W}}{24\ \text{W}} = 1/4
Useful versus wastedThe fraction is computed after the budget closes.12 V3 ohm6 ohm6 ohm2 A1 A1 A+-6 V+-6 V+-6 Voutput