Circuit calculators

Ohm’s law — type any two of V, I, R, P; the other two are filled in (shaded). V = I·R and P = V·I = I²R = V²/R. The suggested resistor rating is the next standard rating above twice the dissipation.

Series / parallel — write a network such as (1k + 2k2) || 4k7: ` is series, `||` (or `//`) is parallel, and `||` binds tighter than `. Choose capacitors or inductors to combine those correctly (capacitors in series add as reciprocals). Or list values one per line for the plain series and parallel totals.

Voltage divider — Vout = Vin·R2′/(R1 + R2′), where R2′ is R2 in parallel with the load. The source impedance the next stage sees is R1 ∥ R2. The design table searches a standard series for the R1/R2 pairs closest to a target voltage within a total-resistance range.

LED resistor — R = (Vs − n·Vf)/If, rounded up to a standard value so the current stays at or below the target, with the resistor’s dissipation and a rating with a 2× margin. Forward voltages by colour are typical figures; use the datasheet.

Bridge & current divider — the Wheatstone output Va − Vb and the Rx that balances it (Rx = R2·R3/R1); and how a total current splits between parallel branches (I·G/ΣG).

ADC — LSB size (Vref/2ⁿ), voltage ↔ counts, presets for common microcontrollers, and divider pairs that bring a higher voltage into range while keeping the source impedance within the ADC’s limit.