Question 1

How do I calculate the resistor for an LED?

Accepted Answer

Subtract the total LED forward voltage from the supply voltage, then divide by the LED current in amps: R = (Vs - n x Vf) / I. For example, a 5 V supply, one LED with Vf = 2 V, at 20 mA needs (5 - 2) / 0.02 = 150 ohms.

Question 2

What is the forward voltage (Vf) of an LED?

Accepted Answer

Forward voltage is the voltage dropped across the LED when it conducts. It depends on color and chemistry: roughly 1.8-2.2 V for red, 2.0-2.4 V for yellow, 2.9-3.4 V for white, blue and green. Always check the LED datasheet, because Vf also rises slightly with current.

Question 3

Why does an LED need a series resistor?

Accepted Answer

An LED has a very steep current-versus-voltage curve, so a tiny voltage increase causes a large current increase. Without a current-limiting resistor the LED would draw too much current and burn out. The resistor sets a safe, stable operating current.

Question 4

What power rating should the resistor have?

Accepted Answer

Compute the power dissipated in the resistor, P = (Vs - n x Vf) x I, then choose a resistor rated well above it. A common rule of thumb is to pick a rating at least double the calculated dissipation; a 1/4 W resistor is fine for most small indicator LEDs.

Question 5

What is the E24 standard resistor series?

Accepted Answer

E24 is a set of 24 preferred resistor values per decade (10, 11, 12, 13, 15, ... 91) with 5% tolerance. Because your exact calculated value is rarely manufactured, this tool snaps the result to the nearest E24 value you can actually buy.

LED SeriesResistor Calculator

LED Series
Resistor Calculator