How-To: Diodes




Introduction: How-To: Diodes

About: I'm an Instructables success story! After relying on the site to DIY my way through art school, I was able to join the Instructables Design Studio in 2012. It's the best! Whe...
If you have embarked upon electronics projects in the past, there is a good chance you have already encountered this common component and soldered into your circuit without second thought. Diodes are valuable in electronics and serve a variety of purposes, which will be highlighted in upcoming steps.

First, what is a diode?

A diode is a semiconducting device,  that allows current to flow in one direction but not the other.  

A semiconductor is a kind of material, in this case silicon or germanium, whose electrical properties lie between those of conductors (metals) and insulators (glass, rubber). Consider conduction: its is a measure of the relative ease of which electrons move through a material. For example, electrons move easily through a piece of metal wire. You can change the behavior of a pure material, like silicon, and turn it into a semiconductor by doping. In doping, you mix a small amount of an impurity into the pure crystalline structure.

The kinds of impurities added to pure silicon can be classified as N-type or P-type.
  • N-type:  With N-type doping, phosphorus or arsenic is added, in parts per billion, to the silicon in small quantities. Phosphorus and arsenic both have five outer electrons, so they are displaced when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. Electrons have a negative charge, hence the name N-type.
  • P-type - In P-type doping, boron or gallium is  added to the pure silicon. Those elements each have three outer electrons. When mixed into the silicon structure, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. 

Diodes are made from two differently doped layers of semiconductor material that form a PN junction. The P-type material has a surplus of positive charge carriers (holes) and the N type, a surplus of electrons. Between these layers, where the P-type and N-type materials meet, holes and electrons combine, with excees electrons combining with excess holes to cancel each other out, so a thin layer is created that has neither positive nor negative charge carriers present. This is called the depletion layer.

There are no charge carriers in this depletion layer and no current can flow across it.  But when a voltage is applied across the junction however, so that the P-type anode is made positive and the N-type cathode negative, the positive holes are attracted across the depletion layer towards the negative cathode, also the negative electrons are attracted towards the positive anode and current flows.

Think of a diode as a one-way street for electricity. When the diode is in forward bias, the diode allows traffic, or current, to flow from the anode, towards the cathode leg. In a reverse bias current is blocked so there is no flow of electricity through the circuit. When current is flowing through a diode, the voltage on the positive leg is higher than on the negative leg, this is called the diode's forward voltage drop. The severity of the voltage drop is a function of  the semiconductor material that the diode is made from. When the voltage across the diode is positive, a lot of current can flow once the voltage becomes large enough. When the voltage across the diode is negative, virtually no current flows.

Step 1: Different Uses for Different Diodes.

There are many different kinds of diodes, and each one serves a different purpose as an electronic component.

A Light Emitting Diode or LED is probably the most well known and most easily identified. The LED emits visible light when electrons jump across the PN junction.The resulting light is referred to as electroluminescence.

Photodiodes conduct only when they are exposed to light. These can be useful in making projects with a light activated switch, so that a circuit in only active in the presence of light.

Zenerdiodes are designed to conduct in the reverse direction, only when something called the breakdown voltage is reached will the circuit conduct. These are dialed to precise tolerances, see the section on Zener Diodes in step 3.

Rectifier diodes are designed to stop electricity from flowing in the wrong direction. Diodes are sometimes known as rectifiers for their use to rectify alternating current electricity into direct current, by removing the negative portion of the current.

Schottkydiodes are designed to turn on and off very rapidly when the breakdown voltage is reached, responding quickly in digital circuits. When current flows through a diode there is a very small voltage drop across the terminals.  Silicon diodes have a voltage drop, or loss; a Schottky diode voltage drop is significantly less. This lower voltage drop enables higher switching speed and better system efficiency.

Diodes can be used in a number of ways, like to protect a current-sensitive circuit. A device that uses batteries will likely contain a diode that protects it when battery is inserted improperly. The diode will stop the reversed current from traveling from the battery to the rest of the circuit-- thus, the diode protects the sensitive electronics inside the your circuit.

In the next few steps, you will find information about some of the most commonly used kinds of diodes.

Step 2: Light Emitting Diode

A light-emitting diode or LED lights up when electrically biased in the forward direction. This effect is a form of electroluminescence. 

A LED is a special type of semiconductor diode. Charge-carriers are created by an electric current passing through the pn-junction, and release energy in the form of photons as they recombine. The wavelength of the light, and therefore its color, is dictated by the materials forming the pn junction, which elements doped the pure material. A normal diode, emits invisible far-infrared light, but the materials used for a LED have bandgap energies corresponding to near-infrared, visible or near-ultraviolet light.

Unlike incandescent bulbs, which can operate with either AC or DC, LEDs require a DC supply of the correct polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward biased. The voltage across the LED in this case is fixed for a given LED and is proportional to the energy of the emitted photons. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted.

The semiconducting diode is encased in a solid plastic lens. Sometimes the plastic is colored, and you can find LEDs in almost every hue. Aside from the current rating on your LED, the size and shape of the plastic enclosure will dictate how, and how much, light the LED is able to throw. 

Step 3: Zener Diodes

Zener diodes are doped with a higher concentration of impurities to give them a very thin depletion layer. In use they are reverse biased. This means that current cannot move across a zener diode until the breakdown voltage is reached. In any diode, there comes a point where, if sufficient reverse voltage is applied, reverse current will flow from cathode to anode. The tightly bound electrons in the depletion layer are torn away from their atoms and there is an abrupt increase in current. If this current is allowed to build up to too high a value, damage can occur. However, if the reverse current is limited to a safe value, the diode will not be harmed and once the reverse voltage is reduced the diode stops conducting again.

Choose a zener diode if you need to have a voltage sensitive switch in your circuit. The available voltage breakdown ranges from about 2 volts to 200 volts. 

Step 4: Schottky Diodes

Unlike a PN-junction diode, a Schottky Diode has a metal–semiconductor (M–S) junction is a type of junction in which a metal comes in close contact with a semiconductor material. They are semiconductor diodes with a low forward voltage drop and a very fast switching action.

For the junction, molybdenum, platinum, chromium or tungsten are used; and a semiconductive an N-type silicon. The metal side acts as the anode and N-type semiconductor acts as the cathode. This is called the Schottky barrier. There are advantages in speed because Schottky diodes do not rely on holes or electrons recombining when they enter the opposite type of region as in the case of a conventional diode. These kinds of diodes, by design, have a very precise breakdown voltage, and are able to respond, or switch, rapidly due to having a partially metal junction.

When current flows through a diode there is a very small voltage drop across the terminals.  This lower voltage drop is conducive of faster switching speed and better system efficiency. It reduces the power losses normally incurred in the rectifier and other diodes used within the power supply. With standard silicon diodes offering the main alternative, their turn on voltage is around 0.6 to 0.7 volts. With Schottky diode rectifiers having a turn on voltage of around 0.2 to 0.3 volts, there is a significant power saving to be gained.

Step 5: Rectifier Circuit

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction.

The most popular application of the diode is used for current rectification. This involves a device that only allows one-way flow of electrons. This is exactly what a semiconductor diode does.

There is a design called a called a full-wave bridge rectifier, it is built around a four-diode bridge configuration. (see image) Alternating current is fed to the bottom and top of the bridge rectifier, which the diodes filter into direct current by directing the current to the correct positive and negative points.

This circuit produces a DC output from an AC input, as well as reverse polarity protection. That is, it permits normal functioning of DC-powered equipment when batteries have been installed backwards, or when the wires from a DC power source have been reversed, and protects your circuit from damage caused by reverse polarity.

Step 6: Make an LED Grid!

A really simple way to get some experience with diodes is via LED circuits. To make an LED matrix, I used a 9V battery, a breadboard, 3V LEDs, and some 1K resistors.

I wired them with the positive on the right, moving to ground on the left. I created six distinct rows, and two columns of LEDs. Wiring in series, it goes from V(+) to the positive lead of the LED, and then another LED, then a 1K resistor to ground. Take a look at the schematic in this step.

Current moves from the anode to the cathode of each LED, and if any of the LEDs terminals are reversed - it will not illuminate.



    • Metalworking Contest

      Metalworking Contest
    • Creative Misuse Contest

      Creative Misuse Contest
    • Water Contest

      Water Contest

    49 Discussions

    how will i know the value of a diode? it has a 1 beige band in its cathode...


    1 year ago

    Sorry, these are 6v batteries. Want to tap 6v and 12v off a battery bank with even load on bank for both voltages.

    Will this work?


    1 year ago

    Will this work?


    i have a set of leds that im tring to power from 2 different sourses. 1st sources is a automobile dome light and the 2nd is a head light switch. what kind of diode can i install on each source. i want the LED lights to turn on when the head lights or the dome light are on without cross powering the other source. (ex, so when the head lights are on, it wont power the dome light.)

    if i have a 5v solar panel, which i use to charge my mobile phone. Do i need to put any kind of diode to stop the current flow back to solar cell from mobile battery..?? plz let me know if i need to use any.

    7 replies

    no solar panel itself is not consumer (if you put diode voltage will fall to 4.8-4.92)

    No, you can thank the usb standard and battery protection circuitry. As long as it's an unmodified usb cable, you are fine. You can add a diode if you want, but I'm sure there already in there.

    This would be very helpful, my science fair project is testing the efficiency of renewable energy sources.

    Take a look at this guide. The voltage is a little smaller, but the princible should still be the same

    Your phone probably has some kind of protection so that won't happen,

    BUT, if it doesn't, you cant add a diode because it will lower the voltage to 4.3 volts, or 4.6 volts if you use a schottky diode

    I want to use a 45 watt solar panel from HarborFreight to charge a 36 volt Golf Cart. Any Ideas on the type of diode I need?

    1 reply

    did you found which diode to use?)))

    I have a 9v dc motor I tried to charge a battery with it but the electriccity return to the motor what type of diode i have to use please tell with photos

    1 reply

    Schottky or rectification diode for ex 1N4007


    1 year ago

    Hey, I am Siray. I need to know how the diode blocks the current flowing in an incorrect way. I mean it acts like a resistor or it only block it by reversing it in the other direction or what exactly.

    1 reply

    read how semiconductors acts

    I must say this sight is a learning center an props to those who keep it runing


    2 years ago

    Wow! The best overview + explanation + application guide for diodes I've found on the web. Wonderfully informative & just technical enough, for me, to get my layman's mind wrapped around the concepts without making my head hurt. Thank-you audreyobscura & well done :)