An LED needs something to limit its current; connected straight to a supply, it draws far too much and fails. The simplest limiter is a series resistor. Laptop boards use this arrangement for power, charging, caps-lock and status LEDs. This calculator gives the exact resistor, the nearest standard value above it and the current that value actually produces.
How to use it
- Enter the supply voltage feeding the LED and resistor.
- Choose the LED colour, or choose "Enter forward voltage" and type the value from the datasheet.
- Enter the current you want — indicator LEDs on laptops usually run at a few milliamps.
- Read the resistor value, the nearest E24 value at or above it, and the power in each part.
The formula
R = (Vsupply − Vf) ÷ I
The resistor takes the voltage the LED does not use. The forward voltage depends on colour and on current, so the colour presets are typical values; a datasheet value is better when you have one. Choosing the next standard value above the exact result keeps the current at or below your target.
Worked example: a white status LED
A white LED on a 5 V rail should run at 5 mA. Its forward voltage is around 3.1 V, so R = (5 − 3.1) ÷ 0.005 = 380 Ω. The nearest E24 value at or above is 390 Ω, giving 4.9 mA. The resistor dissipates under 10 mW, so any small package is fine. On a 3.3 V rail the same LED would leave only 0.2 V for the resistor, and the current would vary a lot with temperature and part-to-part differences — one reason designers drive white LEDs from 5 V or with a dedicated driver.
Where it matters on a laptop board
- Replacing a missing resistor next to a status LED.
- Understanding why an LED is dim (resistor too high, supply low) or burnt (resistor shorted or bypassed).
- Checking an LED in diode mode: a meter's diode test often cannot reach a white or blue LED's forward voltage, as explained in How to Test a Diode with a Multimeter: Schottky, Zener, TVS and LED.
Many laptop LEDs are switched by the EC through a small transistor or open-drain output rather than driven from a rail directly; the resistor calculation is the same. The LED itself is described under led.
FAQ
Why choose the value above rather than the closest one?
The higher value keeps the current at or below your target, which protects the LED and keeps brightness predictable.
Can I use the same resistor for several LEDs in parallel?
Not reliably. Small differences in forward voltage make one LED take most of the current. Give each LED its own resistor.
Why is my replacement LED much brighter than the others?
Modern LEDs are more efficient than older ones. Increase its resistor until the brightness matches.