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How Do Diodes and LEDs Conduct Current?

  • Basic
  • Article
  • Ages 13+
  • 4 min read
A black rectifier diode with a polarity band sits beside a glowing red LED on a circuit board.

A diode mainly conducts in the forward direction and restricts reverse current; the ideal-diode model represents this by blocking reverse current completely. In a real diode, forward current rises gradually with voltage; forward voltage is usually specified at a particular current. An LED is a light-emitting diode, so its current must also be kept within limits.

Contents
Current enters at the triangle's base and leaves at the bar.
  • Diode
  • Direction of current

What you will learn

  • Say which way a diode lets current go.
  • Understand what forward voltage is.
  • Explain why an LED needs current limiting.

Before you start

It helps to know what a resistor and current are.

Which way does a diode let current go?

Current through a diode mainly flows in the forward direction. The two terminals are called anode and cathode; under forward bias the anode is at a higher potential than the cathode. In the ideal-diode model, current flows from anode to cathode and is blocked completely in reverse. A real diode can still have a small reverse leakage current.

For an ideal diode, if the voltage across it is negative, no current can flow. In the conducting state of the ideal model, the voltage across the diode is zero; the external circuit determines the forward current.

In the ideal model, the diode conducts forward and blocks reverse current.
  • Diode in forward direction
  • Current passes
  • Diode in reverse direction
  • Ideal model: reverse current blocked

What is forward voltage?

Forward voltage is the voltage from anode to cathode under forward bias; it is usually specified at a particular current. Light-emitting diodes can have a much larger forward voltage than ordinary diodes.

The graph below shows that current rises steeply with forward voltage, without an exact turn-on threshold.

The curve illustrates current rising steeply with forward voltage, without an exact turn-on threshold.
  • Voltage axis
  • Conducting in forward direction
  • Breakdown in reverse direction
  • Illustrative point on the curve

A diode has limits too. A very large reverse voltage can cause breakdown, allowing a lot of current to flow from cathode to anode. If a diode is subject to more voltage or current than it can handle, expect it to heat up, or worse: melt or smoke.

Why does an LED need current limiting?

LEDs are fragile and can only have a certain amount of current flowing through them before they burn out. Like normal diodes, LEDs mainly conduct in the forward direction under normal operating conditions.

The forward voltage of an LED depends on the colour the LED emits. When sizing a current-limiting resistor, look for the LED's typical forward voltage and maximum forward current.

A series resistor can be selected to limit the current through the LED.
  • Battery
  • Current-limiting resistor
  • LED

Real-world example

With an adult, look for a small indicator on a device, such as a power light. It may use an LED; its circuit needs to limit current, often with a series resistor.

Common misunderstandings

A diode conducts both ways. The ideal-diode model allows current from anode to cathode only; a real diode mainly restricts reverse current but can have a small leakage current.

Connecting an LED straight to a battery is fine. An LED has a current limit, so it needs current limiting, often a series resistor.

Frequently asked questions

Which end is the anode and which is the cathode?

Anode and cathode are the names of the diode terminals. Under forward bias the anode is at a higher potential than the cathode.

What is forward voltage?

Forward voltage is the voltage from anode to cathode under forward bias, usually specified at a particular current.

Does an LED have a direction?

Yes. LEDs mainly conduct current in the forward direction under normal operating conditions.

Why does an LED need current limiting?

An LED has a current limit. A suitably chosen series resistor is one way to keep the current within that limit.

Summary

  • In the ideal model, a diode lets current go from anode to cathode and blocks reverse current; a real diode can have small reverse leakage.
  • Forward voltage is usually specified at a particular current; for an LED it also depends on colour.
  • An LED needs current limiting because it has a current limit; a series resistor is one way to provide it.

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

Diode
A semiconductor component that mainly conducts in the forward direction; an ideal diode blocks reverse current completely.
Anode
The diode terminal where forward current enters; under forward bias it is at a higher potential than the cathode.
Cathode
The diode terminal where forward current exits; under forward bias it is at a lower potential than the anode.
Forward voltageĐiện áp thuận
The anode-to-cathode voltage under forward bias, usually specified at a particular current.
Light-emitting diodeLED
A diode that gives off light when current passes through it in the forward direction.

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Sources

These sources are written for older readers. If they feel hard, read them with an adult.