Start with the Current, End with a Resistor
The design sequence almost never goes voltage-first. It goes: this LED should not see more than 20 mA, or this sensor loop is speced at 4–20 mA, or this motor must not pull more than 3 A. The current is the constraint written in the datasheet; the voltage is whatever rail happens to be there; the resistor is the part you get to choose. Convert the current to a resistance and the standard value you pick off the shelf — 150 Ω, 220 Ω, 470 Ω — is the number this page produces.
ohms = volts ÷ amps
R = V/I
Exact, by Ohm's law — pick the current, the resistance follows.
Design Values That Come Up Constantly
| target current | supply volts | ohms | Context |
|---|---|---|---|
| 20 mA | 5 V | 250 Ω | LED current limit (before subtracting the LED's ~2 V drop). |
| 20 mA | 12 V | 600 Ω | LED on automotive power (with drop: ~500 Ω). |
| 4–20 mA | 24 V | 1.2 kΩ | Industrial sensor loop — 250 Ω shunt turns the loop into a 1–5 V signal. |
| 3 A | 12 V | 4 Ω | Car accessory load — the equivalent resistance of the whole circuit. |
| 0.5 A | 120 V | 240 Ω | 60 W bulb at operating temperature, solved backwards from its current. |
The 4–20 mA row is the one that pays industrial electricians' bills. A 250 Ω precision shunt across a 4–20 mA transmitter output produces 1 V at 4 mA and 5 V at 20 mA — the classic 1–5 V signal that PLC analog inputs expect. The "conversion" is nothing more than R = V/I with the current range fixed and the resistance chosen to land the voltage range. Instrumentation catalogs sell the 250 Ω resistor as a standard part for exactly this reason.
Engineering Context
The forward direction of this calculation is the ohms to amps converter — resistance known, current found. The same R = V/I relation viewed from the supply side is volts to ohms. When the power budget matters rather than the current, the sibling formulas live in volts to watts (P = V·I) and watts to amps. Everything above traces back to the electric hub, and the resistor's power rating — how hot the part you just sized will run — is a watts question: P = I²·R, worth checking before you solder in a 1/8 W part where a 1/2 W belongs.
More: Ω to A · V to Ω · V to W · Electric Hub
Related Unit Converters
Frequently Asked Questions
How do I convert amps to ohms?
Divide the voltage by the current: resistance (Ω) = voltage (V) ÷ current (A). A 5 V supply pushing 20 mA needs 5 ÷ 0.02 = 250 Ω. Amps alone cannot give ohms — you must also know the voltage that is pushing the current.
What resistor does an LED need?
Use R = (Vsupply − Vled) ÷ I. A typical red LED drops about 2 V and is happy at 20 mA: on a 5 V supply, R = (5 − 2) ÷ 0.02 = 150 Ω. On 12 V it becomes (12 − 2) ÷ 0.02 = 500 Ω. The LED's own forward drop is subtracted first — that is why the resistor is called current-limiting.
Why does this conversion matter outside electronics?
Audio and industrial design use it constantly. A speaker amplifier that must deliver 3 A into its output stage at 12 V sees a 4 Ω equivalent load, and impedance matching questions are the same R = V/I calculation. Shunt resistors for current sensing are sized the same way: pick the resistance that turns your target current into a readable voltage at the meter input.