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Picture of How-To: Diodes
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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.
 
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Step 1: Different uses for different Diodes.

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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.

Zener diodes 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.

Schottky diodes 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

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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

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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

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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

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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!

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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.

If I took the number of diodes in the matrix example and put them in series, will I just need one resister and how will the power draw and illumination change of the individual LEDs change?

Tbus1 year ago

My instructor just published an article on how to test
diodes with both an analog and digital multimeter and goes in a little more
detail regarding why the meter might read infinity. http://www.ciebookstore.com/how-to-check-diodes-wi...

But this is a great overview of diodes. Thanks!

jnco1 year ago

Nice information. An analogy I use for diodes is to think the body of it as a hallway with a door (the line) that opens outward allowing bodies (voltage) to pass through. With a large enough body pushing on the other side, it could be made to swing inward and too much weight (current) will permanently break it down.

Nice clear explanations, really helpful!
floxin2 years ago
Regarding Schottky diodes defined as having high switching speed. On a project I was building, the schematic called for a diode type 1N914. I first assumed that by staying within the expected circuit voltage and current characteristics in that part of the circuit, any diode will do. However the project did not work and I consulted a manufacturer's leaflet for 1N914. The diode I needed was described as "Silicon rectifier diode/Ultra fast switch" . Since this description as well as the symbol used on the schematic were unrelated to a Schottky device one should conclude that there are other devices besides the Schottky diode that will provide high switching speeds. Is that a wrong statement? In fact the litterature I consulted used the technical wording "reverse recovery time" (4 nanoseconds for a reverse voltage of 6V)... and my circuit started working.
ve3suy3 years ago
You need a current limiting resistor in series with most LEDs, although some have this resistor built-in. Without a current limiting series resistor, excess current can be drawn when forward biased and 'burn out' the LED. Different voltage sources will require different resistor values. Use ohm's law E=IR, allowing say a 3 volt drop across the LED, and limit the current to perhaps 20 milliamperes and experiment from there.

The LED here probably has a built-in resistor designed for 5 volt operation.
jack8559 ve3suy3 years ago
How do you know how much current or voltage that you can safely put to a LED without harming it, yet get the most light and life from it? Is a 3 volt drop and a 20 mA current a standard for all or is there a tech sheet included with LED's explaining what they will and won't take?
I want to know so if I decide to wire some up to a 12 or 24 volt system, I'll have an idea as to how many I should (or can) put in series and what size and wattage resistor I should use.
Can you help?
ve3suy jack85593 years ago
The approx 3 volt "drop" across the LED depends on its colour - red being minimum and blue being maximum. The current allowable really is determined by the power dissipation capability of the LED, and can be found on the specification sheet for any particular type of LED. 20 ma and 3 volts will keep you in a safe region for most LEDs. You could experiment with one LED and measure its voltage drop, and run it at various currents to see how hot it gets, or at what current it is destroyed.

Sorry I can't really be more specific. You can get an apparent brighter LED by pulsing it at say 50 Hz or higher, and keeping the duty cycle down but while lit, driving it at many times its current, keeping the average heat down.

Hope this helps.

de Bob VE3SUY

vov353 years ago
You make it sound as if schottky diodes are better in every way. Do they have any drawbacks? (if they do, this content should probably be added)
iceng vov353 years ago
Schottky diodes are not high voltage diodes.
dlemke3 years ago
Please tell me there is more to come!.....It would be neat to have a refresher for all basic components.

Dan
sreeci3 years ago
In Science and experiments, description and procedures may contain simple errors and omissions. That is not to be considered as a draw back. Please respect the scientist and try to contribute like him, instead of commenting.
Thank you, AuderyObscura, your contribution is great and I am thankful to you.
We look forward to reading more from you.
russ_hensel3 years ago
Nice, but I need to be a bit picky just to make it a bit better:

1) if any of the leds are reversed it will not light nor will the led in series with it ( unless you break down the reversed led as with a zeener )

2 the graphic has 1 ohm printed should be 1k ohm as the text says, also color code looks a bit like 10k, but I may be wrong on that one.

audreyobscura (author)  russ_hensel3 years ago
Thanks for the tip - I went through and made the diagram a bit more clear.
Great instructable! I would like to make a night vision using invisible LED's and an IR detector. I would also want to have the ability to run the lights full time for my camera, that can see in the dark with these diodes. I want to put it on the dash to see better at night, like deer, SKUNKS, and other wildlife... I want to be able to see someone jacking up a car when there are headlights behind him blinding me. The IR will let me see everything within range. I have 17 invisible LED's, an old VHS camera with zoom ability.

Right now, I don't have time to put such a device together. I still have to buy a battery and charger at Batteries Plus here in the local area of KCMO.
I went through engineering school, but I just don't have the time. I am working on an all electric car that will beat all production cars. I am only building one. I just got the electric motor to run my high powered hydrostatic drive system. This car is so advanced compared to most vehicles, but now GM and others are catching up with parts like magnetic suspension. I am also using a magnetic clutch that acts like a full time regenerative system. There is so much more to this project. I can't go into details right now, I am sure some special interest groups won't let me divulge either.
bpfh3 years ago
Just a quick reminder to ALWAYS use a resistor when wiring up LED's to a power source - the circuit resistance of the diode is so low that directly attached to a power source, it acts as a dead short circuit!

I have ruined several adjustable 12 volt car power adapters after miscalculating the resistance of several LED's in series for a lighting project.

Remember from then that you may have to recalculate your power needs, especially if you have either a very low current power supply - or a lot of LED's on a higher power supply!

In any project, always have a multimeter to hand to check your values and do not hesistate to have Ohm's law written down somewhere so you can check that you are calculating values the right way!

Cheers,
Daniel
At LOW voltages, using schottky barriers may save a lot of power, but if you have a 12V supply, running at 1A, the diode drop is losing you .6W, or 0.3W if you go Schottky. 5%, to 2.5%. At lower currents, or higher voltage, it often doesn't matter.

A ZENER is a voltage reference device, not used as a "switch" !
jimology3 years ago
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?
rleemans3 years ago
Thanks for the informative How-to.
I have a small solar panel that I want to charge a battery with. I was given a diode to keep the battery from discharging.
Can you tell me by the marking on the diode which direction is the permited flow and which direction the flow is stopped?
Thanks.
It should have a band around one end. I am 90% sure that the end with the band goes to the negative terminal to forward bias.
zomfibame3 years ago
thanks for the education, I'd wondered on the science behind how a diode worked.
tqwerty3 years ago
In the picture in Step 1, generic is misspelled.
Otherwise, great 'ible! Very clear.
tqwerty tqwerty3 years ago
Also, typo in the first picture in Step 4, there are 3 r's in current.
audreyobscura (author)  tqwerty3 years ago
hey thanks! one day i'll get it! I made the corrections to both. :)
fozzy133 years ago
Thanks for the well-written descriptive yet simple guide to diodes!
mrmerino3 years ago
Sweet! Ive always wanted to learn how to diode!