Diodes & LEDs
One-way valves for current: forward voltage, load lines, rectifiers and Zeners.
Lesson progress
- Source Vs
- 5 V
- Across diode Vd
- 1.88 V
- Current I
- 14.2 mA
- Diode power Pd
- 26.7 mW
- resistor 44.2 mW
I–V curve and load line
The circuit sits where they crossLive scope
Switch the source to AC sineLED resistor calculator
The most common question in hobby electronics: what resistor does my LED need?Supply
LED
Most 5 mm LEDs are rated for 20 mA. 5–10 mA is plenty for an indicator.
R = (Vs − Vf) / I
(5 − 1.89) / 15 mA = 208 Ω
Use 220 Ω
Next standard value up → 14.2 mA actual, resistor 44.2 mW (⅛ W is fine)
A one-way valve for current
A diode lets current flow easily from its anode to its cathode (the end with the band), and almost not at all the other way. Inside, it is a PN junction: two kinds of silicon pressed together, with a thin barrier between them that takes a certain voltage to push current across.
That voltage is the forward voltage, Vf: about 0.7 V for silicon, 0.3 V for Schottky, and 1.8–3.3 V for LEDs, depending on colour. Below Vf almost nothing flows. Above it the current rises steeply, so steeply that a diode connected straight across a battery will draw as much current as the battery can supply. That is why diodes and LEDs always need a series resistor.
Try this