Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.

com/tutorials/diodes

learn.sparkfun.com

Diodes - learn.sparkfun.com
Favorited Favorite 13

Introduction

Once you graduate from the simple, passive components that are
resistors, capacitors, and inductors, it’s time to step on up to the
wonderful world of semiconductors. One of the most widely used
semiconductor components is the diode.

In this tutorial we’ll cover:

What is a diode!?

Theory of diode operation

Important diode properties

Different types of diodes

What diodes look like

1 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

Typical diode applications

Suggested Reading

Some of the concepts in this tutorial build on previous electronics

knowledge. Before jumping into this tutorial consider reading (at
least skimming) these first:

What is Electricity

Voltage, Current, Resistance, and Ohm’s Law

What is a Circuit

Series vs. Parallel

How to Use A Multimeter

Ideal Diodes

The key function of an ideal diode is to control the direction of

current-flow. Current passing through a diode can only go in one
direction, called the forward direction. Current trying to flow the
reverse direction is blocked. They’re like the one-way valve of
electronics.

If the voltage across a diode is negative, no current can flow*, and

the ideal diode looks like an open circuit. In such a situation, the
diode is said to be off or reverse biased.

As long as the voltage across the diode isn’t negative, it’ll “turn on”
and conduct current. Ideally* a diode would act like a short circuit
(0V across it) if it was conducting current. When a diode is

2 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

conducting current it’s forward biased (electronics jargon for “on”).

The current-voltage relationship of an ideal diode. Any negative

voltage produces zero current – an open circuit. As long as the
voltage is non-negative the diode looks like a short circuit.

Ideal Diode Characteristics

Operation Mode On (Forward biased) Off (Reverse biased)

Current Through I>0 I=0

Voltage Across V=0 V<0

Diode looks like Short circuit Open circuit

Circuit Symbol

Every diode has two terminals – connections on each end of the

component – and those terminals are polarized, meaning the two
terminals are distinctly different. It’s important not to mix the
connections on a diode up. The positive end of a diode is called the
anode, and the negative end is called the cathode. Current can

3 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

flow from the anode end to the cathode, but not the other direction.
If you forget which way current flows through a diode, try to
remember the mnemonic ACID: “anode current in diode” (also
anode cathode is diode).

The circuit symbol of a standard diode is a triangle butting up

against a line. As we’ll cover in the later in this tutorial, there are a
variety of diode types, but usually their circuit symbol will look
something like this:

The terminal entering the flat edge of the triangle represents the
anode. Current flows in the direction that the triangle/arrow is
pointing, but it can’t go the other way.

4 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

Above are a couple simple diode circuit examples. On the left,

diode D1 is forward biased and allowing current to flow through the
circuit. In essence it looks like a short circuit. On the right, diode D2
is reverse biased. Current cannot flow through the circuit, and it
essentially looks like an open circuit.

*Caveat! Asterisk! Not-entirely-true… Unfortunately, there’s no such

thing as an ideal diode. But don’t worry! Diodes really are real,
they’ve just got a few characteristics which make them operate as a
little less than our ideal model…

Real Diode Characteristics

Ideally, diodes will block any and all current flowing the reverse
direction, or just act like a short-circuit if current flow is forward.
Unfortunately, actual diode behavior isn’t quite ideal. Diodes do
consume some amount of power when conducting forward current,
and they won’t block out all reverse current. Real-world diodes are
a bit more complicated, and they all have unique characteristics
which define how they actually operate.

Current-Voltage Relationship

The most important diode characteristic is its current-voltage (i-v)

relationship. This defines what the current running through a
component is, given what voltage is measured across it. Resistors,
for example, have a simple, linear i-v relationship…Ohm’s Law. The
i-v curve of a diode, though, is entirely non-linear. It looks
something like this:

5 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

The current-voltage relationship of a diode. In order to exaggerate a

few important points on the plot, the scales in both the positive and
negative halves are not equal.

Depending on the voltage applied across it, a diode will operate in

one of three regions:

1. Forward bias: When the voltage across the diode is positive

the diode is “on” and current can run through. The voltage
should be greater than the forward voltage (VF) in order for the
current to be anything significant.

6 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

2. Reverse bias: This is the “off” mode of the diode, where the
voltage is less than VF but greater than -VBR. In this mode
current flow is (mostly) blocked, and the diode is off. A very
small amount of current (on the order of nA) – called reverse
saturation current – is able to flow in reverse through the diode.

3. Breakdown: When the voltage applied across the diode is very

large and negative, lots of current will be able to flow in the
reverse direction, from cathode to anode.

Forward Voltage

In order to “turn on” and conduct current in the forward direction, a

diode requires a certain amount of positive voltage to be applied
across it. The typical voltage required to turn the diode on is called
the forward voltage (VF). It might also be called either the cut-in
voltage or on-voltage.

As we know from the i-v curve, the current through and voltage
across a diode are interdependent. More current means more
voltage, less voltage means less current. Once the voltage gets to
about the forward voltage rating, though, large increases in current
should still only mean a very small increase in voltage. If a diode is
fully conducting, it can usually be assumed that the voltage across
it is the forward voltage rating.

7 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

A multimeter with a diode setting can be used to measure (the

minimum of) a diode’s forward voltage drop.

A specific diode’s VF depends on what semiconductor material it’s

made out of. Typically, a silicon diode will have a VF around 0.6-1V.
A germanium-based diode might be lower, around 0.3V. The type of
diode also has some importance in defining the forward voltage
drop; light-emitting diodes can have a much larger VF, while
Schottky diodes are designed specifically to have a much lower-
than-usual forward voltage.

Breakdown Voltage

If a large enough negative voltage is applied to the diode, it will give

in and allow current to flow in the reverse direction. This large
negative voltage is called the breakdown voltage. Some diodes
are actually designed to operate in the breakdown region, but for
most normal diodes it’s not very healthy for them to be subjected to
large negative voltages.

For normal diodes this breakdown voltage is around -50V to -100V,

or even more negative.

Diode Datasheets

All of the above characteristics should be detailed in the datasheet

8 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

for every diode. For example, this datasheet for a 1N4148 diode
lists the maximum forward voltage (1V) and the breakdown voltage
(100V) (among a lot of other information):

A datasheet might even present you with a very familiar looking

current-voltage graph, to further detail how the diode behaves. This
graph from the diode’s datasheet enlarges the curvy, forward-region
part of the i-v curve. Notice how more current requires more
voltage:

9 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

That chart points out another important diode characteristic – the

maximum forward current. Just like any component, diodes can
only dissipate so much power before they blow. All diodes should
list maximum current, reverse voltage, and power dissipation. If a
diode is subject to more voltage or current than it can handle,
expect it to heat up (or worse; melt, smoke,…).

Some diodes are well-suited to high currents – 1A or more – others

like the 1N4148 small-signal diode shown above may only be
suited for around 200mA.

That 1N4148 is just a tiny sampling of all the different kinds of

diodes there are out there. Next we’ll explore what an amazing
variety of diodes there are and what purpose each type serves.

Types of Diodes

Normal Diodes

Standard signal diodes are among the most basic, average,

no-frills members of the diode family. They usually have a
medium-high forward voltage drop and a low maximum current
rating. A common example of a signal diode is the 1N4148. Very
general purpose, it’s got a typical forward voltage drop of 0.72V and
a 300mA maximum forward current rating.

10 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

A small-signal diode, the 1N4148. Notice the black circle around

the diode, that marks which of the terminals is the cathode.

A rectifier or power diode is a standard diode with a much higher

maximum current rating. This higher current rating usually comes at
the cost of a larger forward voltage. The 1N4001, for example, has
a current rating of 1A and a forward voltage of 1.1V.

A 1N4001 PTH diode. This time a gray band indicates which pin is
the cathode.

And, of course, most diode types come in surface-mount varieties

as well. You’ll notice that every diode has some way (no matter how
tiny or hard to see) to indicate which of the two pins is the cathode.

Light-Emitting Diodes (LEDs!)

The flashiest member of the diode family must be the light-emitting

diode (LED). These diodes quite literally light up when a positive
voltage is applied.

11 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

A handful of through-hole LEDs. From left to right: a yellow 3mm,

blue 5mm, green 10mm, super-bright red 5mm, an RGB 5mm and
a blue 7-segment LED.

Like normal diodes, LEDs only allow current through one direction.
They also have a forward voltage rating, which is the voltage
required for them to light up. The VF rating of an LED is usually
larger than that of a normal diode (1.2~3V), and it depends on the
color the LED emits. For example, the rated forward voltage of a
Super Bright Blue LED is around 3.3V, while that of the equal size
Super Bright Red LED is only 2.2V.

You’ll obviously most-often find LEDs in lighting applications.

They’re blinky and fun! But more than that, their high-efficiency has
lead to widespread use in street lights, displays, backlighting, and
much more. Other LEDs emit a light that is not visible to the human
eye, like infrared LEDs, which are the backbone of most remote
controls. Another common use of LEDs is in optically isolating a

12 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

dangerous high-voltage system from a lower-voltage circuit.

Opto-isolators pair an infrared LED with a photosensor, which
allows current to flow when it detects light from the LED. Below is
an example circuit of an opto-isolator. Note how the schematic
symbol for the diode varies from the normal diode. LED symbols
add a couple arrows extending out from the symbol.

Schottky Diodes

Another very common diode is the Schottky diode. The

semiconductor composition of a Schottky diode is slightly different
from a normal diode, and this results in a much smaller forward
voltage drop, which is usually between 0.15V and 0.45V. They’ll
still have a very large breakdown voltage though.

Schottky diodes are especially useful in limiting losses, when every

13 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

last bit of voltage must be spared. They’re unique enough to get a

circuit symbol of their own, with a couple bends on the end of the
cathode-line.

Zener Diodes

Zener diodes are the weird outcast of the diode family. They’re
usually used to intentionally conduct reverse current. Zener’s are
designed to have a very precise breakdown voltage, called the
zener breakdown or zener voltage. When enough current runs in
reverse through the zener, the voltage drop across it will hold
steady at the breakdown voltage.

Taking advantage of their breakdown property, Zener diodes are

often used to create a known reference voltage at exactly their
Zener voltage. They can be used as a voltage regulator for small
loads, but they’re not really made to regulate voltage to circuits that
will pull significant amounts of current.

Zeners are special enough to get their own circuit symbol, with
wavy ends on the cathode-line. The symbol might even define
what, exactly, the diode’s zener voltage is. Here’s a 3.3V zener
diode acting to create a solid 3.3V voltage reference:

14 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

Photodiodes

Photodiodes are specially constructed diodes, which capture

energy from photons of light (see Physics, quantum) to generate
electrical current. Kind of operating as an anti-LED.

A BPW34 photodiode (not the quarter, the little thing on top of that).
Get it under the sun and it can generate about few µW’s of power!.

Solar cells are the main benefactor of photodiode technology. But

these diodes can also be used to detect light, or even communicate
optically.

Diode Applications

15 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

For such a simple component, diodes have a huge range of uses.

You’ll find a diode of some type in just about every circuit. They
could be featured in anything from a small-signal digital logic to a
high voltage power conversion circuit. Let’s explore some of these
applications.

Rectifiers

A rectifier is a circuit that converts alternating current (AC) to direct

current (DC). This conversion is critical for all sorts of household
electronics. AC signals come out of your house’s wall outlets, but
DC is what powers most computers and other microelectronics.

Current in AC circuits literally alternates – quickly switches between

running in the positive and negative directions – but current in a DC
signal only runs in one direction. So to convert from AC to DC you
just need to make sure current can’t run in the negative direction.
Sounds like a job for DIODES!

A half-wave rectifier can be made out of just a single diode. If an

AC signal, like a sine wave for example, is sent through a diode any
negative component to the signal is clipped out.

Input (red/left) and output (blue/right) voltage waveforms, after

passing through the half-wave rectifier circuit (middle).

16 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

A full-wave bridge rectifier uses four diodes to convert those

negative humps in the AC signal into positive humps.

The bridge rectifier circuit (middle), and the output wave form it
creates (blue/right).

These circuits are a critical component in AC-to-DC power supplies,

which turn the wall outlet’s 120/240VAC signal into 3.3V, 5V, 12V,
etc. DC signals. If you tore apart a wall-wart, you’d most likely see a
handful of diodes in there, rectifying it up.

17 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

Can you spot the four diodes making a bridge rectifier in this
wall-wart?

Reverse Current Protection

Ever stick a battery in the wrong way? Or switch up the red and
black power wires? If so, a diode might be to thank for your circuit
still being alive. A diode placed in series with the positive side of the
power supply is called a reverse protection diode. It ensures that
current can only flow in the positive direction, and the power supply
only applies a positive voltage to your circuit.

This diode application is useful when a power supply connector

isn’t polarized, making it easy to mess up and accidentally connect
the negative supply to the positive of the input circuit.

The drawback of a reverse protection diode is that it’ll induce some

voltage loss because of the forward voltage drop. This makes
Schottky diodes an excellent choice for reverse protection diodes.

Logic Gates

Forget transistors! Simple digital logic gates, like the AND or the
OR, can be built out of diodes.

18 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

For example, a diode two-input OR gate can be constructed out of

two diodes with shared cathode nodes. The output of the logic
circuit is also located at that node. Whenever either input (or both)
is a logic 1 (high/5V) the output becomes a logic 1 as well. When
both inputs are a logic 0 (low/0V), the output is pulled low through
the resistor.

An AND gate is constructed in a similar manner. The anodes of

both diodes are connected together, which is where the output of
the circuit is located. Both inputs must be logic 1 forcing current to
run towards the output pin and pull it high also. If either of the
inputs are low, current from the 5V supply runs through the diode.

19 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

For both logic gates, more inputs can be added by adding just a
single diode.

Flyback Diodes and Voltage Spike Suppression

Diodes are very often used to limit potential damage from

unexpected large spikes in voltage. Transient-voltage-suppression
(TVS) diodes are specialty diodes, kind of like zener diodes –
lowish breakdown voltages (often around 20V) – but with very large
power ratings (often in the range of kilowatts). They’re designed to
shunt currents and absorb energy when voltages exceed their
breakdown voltage.

Flyback diodes do a similar job of suppressing voltage spikes,

specifically those induced by an inductive component, like a motor.
When current through an inductor suddenly changes, a voltage
spike is created, possibly a very large, negative spike. A flyback
diode placed across the inductive load, will give that negative
voltage signal a safe path to discharge, actually looping
over-and-over through the inductor and diode until it eventually dies
out.

20 of 21 12/05/2016 21:42
Diodes - learn.sparkfun.com about:reader?url=https://learn.sparkfun.com/tutorials/diodes

That’s just a handful of applications for this amazing little

semiconductor component.

Resources and Going Further

Now that you’ve gotten a handle on diodes, maybe you’d like to

further explore more semiconductors:

Transistors

LEDs

Or learn about integrated circuits, like:

555 Timers

Operational Amplifiers

Shift Registers

Or discover some of the other common electronic components:

Resistors

Capacitors

Inductors

Voltage Regulators

21 of 21 12/05/2016 21:42