LED series resistor
A resistor for each LED string, with supply/forward-voltage spread and preferred values.
Development preview. Check the assumptions below before relying on a result. Calculations run in your browser.
How it calculates
Model: R=(Vs−nVf)/I; Imin=max(0,(Vs,min−nVf,max)/R); Imax=max(0,(Vs,max−nVf,min)/R)
Assumptions
- Every parallel string has an independent resistor. LEDs are represented by fixed voltage bounds; temperature-dependent I–V curves and resistor tolerance are not modelled.
- Electrical share is LED voltage divided by supply voltage, not optical efficiency.
Worked example
These are the calculator's default inputs. Change any input above to recalculate.
Inputs
- Nominal supply
- 12 V
- Minimum supply
- 11.5 V
- Maximum supply
- 12.5 V
- Nominal forward voltage per LED
- 2 V
- Minimum forward voltage
- 1.8 V
- Maximum forward voltage
- 2.2 V
- LEDs per series string
- 3
- Parallel strings, each with its own resistor
- 1
- Nominal string current target
- 0.01 A
- Resistor selection
- E24 rounded up
Results
- Selected resistance per string
- 620 Ω
- Maximum resistor power per string
- 0.0813065 W
- Ideal resistance per string
- 600 Ω
- Nominal string current
- 0.00967742 A
- Minimum string current
- 0.00790323 A
- Maximum string current
- 0.0114516 A
- Nominal total current
- 0.00967742 A
- Nominal LED electrical share
- 50 %