Current Limiter
Series limiter
Enter input voltage, load voltage and target current; the limiter resistor appears live.
Limiter resistor
How to use this calculator
A series current-limiting resistor drops the difference between the supply and the load voltage at the target current: R = (Vin − Vload) / I. Enter the input voltage, the voltage the load needs, and the current, and the resistor value (and the power it must dissipate) appears live.
What it means in a real circuit
For a 9 V supply feeding a 5 V module at 50 mA, the resistor is (9−5)/0.050 = 80 Ω dissipating 2.5 V × 50 mA = 125 mW. This is a simple, cheap way to limit current, but it wastes power as heat and is not a constant-current source — the current varies if the supply or load voltage changes. Use it for fixed loads where the headroom is stable; for precise current control, use a proper current regulator. The calculator warns if Vin ≤ Vload, where no resistor can limit current.