The relationship behind every circuit
Ohm's law says the voltage across a resistor equals the current through it times its resistance. That single equation only has three variables, so knowing any two always pins down the third — and multiplying voltage by current gives the power the resistor dissipates as heat.
- V — voltage across the resistor (volts)
- I — current through it (amps)
- R — resistance (ohms)
- P — power dissipated (watts)
More detail
Why the mode selector matters
V = I·R has three unknowns and one equation, so it can't be solved from a single number — you always need two of the three to find the rest. Pick which two you're holding above: a power supply and a target current usually give you V and I (solving for the resistor you need); a resistor and a known supply give you V and R (solving for the current it'll draw); a datasheet current limit and a chosen resistor give you I and R (solving for what voltage produces that current).
Sizing a resistor. Driving something from a fixed supply at a target current — an LED, a sensor bias, a pull-up — is a V+I problem: pick your supply voltage and target current, and the resistance shown is what to put in series. Just confirm the power figure fits the resistor's wattage rating (¼ W handles most small-signal work).
Frequently asked questions
5V supply, 20mA target current — what resistor do I need?
Select "Voltage + current", enter 5 V and 20 mA. R = 5 ÷ 0.02 = 250 Ω, dissipating P = 5 × 0.02 = 0.1 W — well within a ¼ W resistor.
How much current flows through a 100Ω resistor on 12V?
Select "Voltage + resistance", enter 12 V and 100 Ω. I = 12 ÷ 100 = 0.12 A (120 mA), and P = 12 × 0.12 = 1.44 W — that needs at least a 2 W resistor, not a standard ¼ W part.
I know the current and resistance — how do I find the voltage?
Select "Current + resistance" and enter both. V = I × R: for 2 A through 3 Ω, V = 2 × 3 = 6 V, and P = 6 × 2 = 12 W.
Why is 0Ω resistance sometimes valid and sometimes not?
It depends which two values you supply. In current + resistance mode, R = 0 is an ideal wire — a short circuit — so V = 0 and P = 0 are the correct answers. (An open circuit is the opposite case: resistance is effectively infinite and no current flows.) But if you supply voltage + resistance with R = 0, current would be infinite — that's rejected as out of range instead of showing a nonsense number.