Basics calculator
LED resistor
An LED takes its voltage; the resistor takes the rest and sets the current.
Free. It runs in your browser: nothing to install.
The example, worked out
Given
- Supply
- 5 V
- LED
- Red (2 V)
- Current wanted
- 20 mA
- LEDs in series
- 1
- Standard series
- E12 (10 %)
Working
- what is left for the resistor
- already a value of E12
- with the standard value
Results
The formulas
- R = (Vs − n × Vf) / Ifwhat is left after the LEDs, over the current
- P = (Vs − n × Vf) × Ithe heat in the resistor
The usual traps
- Too little voltage left across the resistor: the current swings with every LED.
- Taking the exact value: the next standard value up keeps the current under what you asked.
Questions
What resistor does an LED need?
R = (Vs − n × Vf) / If: what is left after the LEDs, over the current. P = (Vs − n × Vf) × I: the heat in the resistor.
What does it give for typical values?
150 Ω for an LED of 2 V at 20 mA on 5 V. Standard value (E12): 150 Ω.
What are the usual mistakes?
Too little voltage left across the resistor: the current swings with every LED. Taking the exact value: the next standard value up keeps the current under what you asked.
In the app
Your circuit, drawn live
Change a value: the schematic redraws with your numbers on the parts.
The part you can buy
Results rounded to standard values (E12, E24, E96) and power ratings.
The working, shown
Every step written out. Try it first by hand, then check.
Into your notebook
One click puts the circuit on a sheet of your project.
Open the calculatorOpens in the app on 1 December, with Circuit Notebook 1.0
Also in Basics
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- RLC circuit Resonance, Q, bandwidth and damping, in series or in parallel.