Voltage Regulator Dropout
Linear regulator
Enter input, output, dropout voltage and load current; minimum input, headroom, dissipation and efficiency appear live.
Regulation
How to use this calculator
A linear regulator holds its output constant only while the input stays above the output by at least the dropout voltage: Vin_min = Vout + Vdrop. The calculator shows the minimum input, the headroom you actually have (Vin − Vin_min), and, if you enter a load current, the worst-case power dissipated in the regulator (P = (Vin − Vout)·I) and its linear efficiency (η = Vout/Vin).
What it means in a real circuit
For a 5 V regulator with 0.3 V dropout, the input must be ≥ 5.3 V — a 12 V supply gives 6.7 V of headroom and a 7 W dissipation at 500 mA (a 7805 from a 12 V rail at high current will run hot; a 78L05 from the same rail is fine at lower current). If headroom goes negative the output "drops out" and tracks the input — the calculator warns you. Lowering Vin to just above Vout reduces dissipation and heat at the cost of less margin against input sag. Use this to size the regulator, pick a heatsink, and decide whether a switching regulator would be more efficient.