The one-way roads of electonics, diodes are powerful and useful devices.
In the Classroom this month, we take a look at our namesake component – the diode. These somewhat simple components are used for a variety of essential roles in electronics, many of which are unseen to the casual observer; but without them, modern electronics would not exist.
While many complex devices rely on diodes, makers who just discovered DIY electronics last week will also encounter them. Diodes are part of a broad family of electronic components called “semiconductors”, which means that under certain conditions, the materials the component is made from, conduct electricity. Under other conditions however, they do not. To know what a diode is and how it works, we will begin by taking a step back to look at what many electronic components are made from.
A MATTER OF MINERALS
Semiconductors are most commonly made from elemental silicon. “Silicon” is a pure element, but should not be confused with “silicone”, which is a synthetic, manufactured polymer that contains silicon. The element, silicon, is the second most common mineral in the Earth’s crust by mass, but rarely occurs in pure form. Silicon is usually combined with two oxygen atoms to form silicon dioxide. We all encounter this material daily, as silicon dioxide is actually the main constituent of glass. Much of the sand, clay, and rock in the Earth’s crust is also composed of silicon dioxide, along with other minerals. Without these other minerals, pure silicon dioxide forms quartz. The fine grains in beach sand are usually decomposed quartz.
Keen readers may have realised at this point that glass is an insulator. This is because the silicon dioxide form is inert, having no electrical activity left in the shells of electrons that surround each atom – the set of three atoms balance each other.
Pure silicon is a different story. It was first isolated and purified in 1823 by Swedish chemist Jakob Berzelius, who discovered that in its pure form, silicon is hard, brittle, and has a shiny grey/blue appearance. The crystalline solid looks, in fact, a lot like dark glass with a metallic sheen. In its pure form, silicon is also a semiconductor. In the electronics manufacturing industry, purified silicon is grown into a crystalline structure, and this product is naturally a semiconductor, and more conductive at higher temperatures. However, the conductivity is still too high in its resistance to be practical, and so through various chemical and physical means, other elements such as phosphorus, boron, arsenic, and antimony are used to “dope” the silicon crystal.
As an aside, these sometimes-toxic chemicals, along with lead in traditional solder, are the main source of environmental contamination from electronics disposal.
DOPED UP
Silicon that has been doped with elements with three outer electrons, become what is called a “P” type material; it has a collective deficiency of electrons, and wants to attract them. Silicon that has been doped with elements with five outer electrons become what is called an “N” type – it has too many electrons.
If a piece of P-type silicon is bonded to a piece of N-type silicon, electrons from the N-type material cross over into the P-type material, removing free electrons from the N-type and filling the missing electron “holes” in the P-type, thus creating a PN junction. The PN junction is a narrow joint, which is now an insulator, called the “barrier region”. This is the basis of an electronic solid-state diode, that we simply call a “diode”. It is worth noting that silicon diodes are not the only material that diodes can be made in, and the PN junction is not the only technique.
Additionally, in semiconductors, it is important to remember the difference between current and electron flow. Current flows from positive to negative by convention, and is therefore called “conventional current”, but electrons actually pass from negative to positive.
If a diode is connected with the P more positive than the N, by a voltage known as the “barrier voltage”, the diode will conduct. However, if a diode is connected with the N more positive than the P, then the diode will not conduct, and the barrier will be made wider.
This is a very simplified description of the PN junction and how they are made, but deeper understanding would require the whole publication, and is really just academic to most readers. By the end of this article you should be able to use diodes effectively.
The terminals of a diode are called the “anode”, which is commonly abbreviated to “A”, and the “cathode”, which is abbreviated to “K” (derived from the German word “Kathode”). Anode traditionally means the positive terminal of a chemical bath, and cathode traditionally means the negative terminal.
The circuit symbol for a diode is an arrow pointing in the direction of current flow (NOT electron flow), and a blocking bar against current flow [1]. Notice that if the diode is reversed, the cathode forms a nice K to remind you that current won’t flow into the cathode.
Component diode packages always have some marking for the correct orientation, but we’ll touch more on this later.
Silicon is not the only mineral that semiconductors are made from; however, it is the most common today. While germanium can also be used, it is less common today than even 10 years ago, and certainly less so than 50 years ago.
The germanium crystal diode was commonly used to detect AM radio signals in a small radio receiver. AM radios now however, use an integrated circuit for all the electronics, so the simple detector diode is almost forgotten. Despite the IC technology, AM is becoming increasingly hard to receive, both because of electronic interference, and from a declining range of available transmissions.
See the sidebar on the precursors to the diode. It is important to know, however, that certain materials can give particular properties. Most diodes are made of silicon, but others are made of different combinations of materials, such as gallium and arsenic, forming a gallium-arsenide diode, with it’s own “super powers”.
DIODES AND DC
When the anode of a diode is more positive than the cathode by more than the barrier voltage, it is said to be “forward biased” and it will conduct current. For a silicon diode, the barrier voltage is only 0.6V to 0.7V, but increases with current flow. However, if the polarity of the potential difference is reversed, the diode becomes “reverse biased” and will not conduct.
This very useful property allows an AC power source to be “rectified” into DC. A single diode is used to pass the positive half of an AC sine wave [2], and is therefore called “half-wave rectification”. If a centre tapped transformer is used [3], two half-wave rectifiers can be used to become a full-wave centre tap rectifier circuit. Notice that the transformer inverts the bottom half-wave, but the bottom diode allows the current to flow in a positive direction in the circuit, so both half-waves are working. However, if you don’t have a centre-tapped transformer, all is not lost. A circuit that uses four diodes in what is called a “diode bridge” [4] to direct the current from the transformer to always flow in the same direction in the load. We call the circuit bridge rectification.
A half-wave rectifier [2] uses a single diode and exploits the ability of a diode to “block” current flow in an undesired direction. When the AC power is rising 0.6V above the zero line, the current flows through the diode, as it becomes forward biased. However, when the current crosses below 0.6V conduction stops, and below the zero line the diode is becomes reverse biased, and current cannot flow. This means that current flows, between 0.6V up to the peak voltage and back down to 0.6V, is fed to the circuit.
As can be seen from the diagram, this results in very “rough” or “lumpy” power, having “half-wave-ripple” with the DC power being “off” for slightly more than half the time. Most electronics circuits would not appreciate working with “dirty power” and audio amplifiers would have a loud buzzing or humming sound – not that the younger generation would notice! The dirt can be filtered out, or more correctly, smoothed out using a filter capacitor [2], as described in the last issue. How well the capacitor works depends on the size of the capacitance, the current in the load, and how sensitive the electronics are in the load. However, this is still less than ideal for modern electronics, although it may suit simple applications such as controlling a relay.
The full-wave centre tap rectifier [3] is better, as it effectively turns the negative half of the cycle upside down; however it demands a centre-tapped transformer. This is a topic on its own; however, the diagram shows that the winding of a centre-tapped transformer has two ends and a centre point halfway along its windings where a common connection can be made.
If the common connection is made to ground, usually the negative terminal, the two phases of the transformer connected to the two diodes will give a wave form as in the diagram. The addition of a sufficiently sized filter capacitor gives quite a reasonable power supply for many applications.
TECH NOTE: If a 12V transformer and a suitable size capacitor is used, the output voltage will be approximately √2 times 12V less the forward voltage of two diodes (i.e., 12 x 1.414 – 2 x 0.65 = ~15.7V).
Please note, the square root symbol was accidentally omitted from our print edition of this tip
Provided the lowest point on the section of the wave form marked at “A” is still 2.5V higher than the desired output, this circuit can be fed into a voltage regulator, such as an LM7812 or LM317 and the electronics inside the regulator will actively correct and control the output voltage. This is not always practical. Centre tapped transformers are not always easy to come by, although the major retailers sell them in a range of values, but you may be working on an existing power supply that doesn’t have a centre tap.
It is more likely to find the voltage you need in a transformer with a single secondary winding. In this case, a full-wave bridge rectifier – usually just referred to as a bridge rectifier – is the answer. The secret to bridge rectifiers is to remember that there are two half-waves, just as in the centre tap circuit, but they overlap in the single winding in the “fourth dimension” (i.e., time). There are two time periods we are interested in: the positive half-wave, and the negative half-wave. During both half-waves the positive is to be at the top of the load resistor, so it is the job of the diodes to make sure that the positive of the load remains positive.
In the first half-cycle, a diode should have its anode connected to the top of the transformer, and it’s cathode to the top of the load. To complete the circuit, a second diode is connected with its anode to the bottom of the load and its cathode to the bottom of the transformer.
In the second half-cycle, a diode should have its anode connected to the bottom of the transformer and its cathode to the top of the load. To complete the circuit, a second diode is connected with its anode to the top of the load and its cathode to the bottom of the transformer.
Now to prove it to yourself, run your finger down the load, and choose either diode to follow and continue back to the load. Next time you get to the bottom of the load choose the other diode and make another loop. Do this until you are convinced and clear on how this circuit works. It is simply a matter of following the arrows.
The same capacitor, as used in the full-wave rectifier, will give the same relatively smooth power supply, and can be used with the same set of voltage regulators. Lucky for us, complete bridge rectifiers are available in neat packages already containing four diodes correctly connected and encased as one unit. Bridge rectifiers, such as the WO4, are well marked with AC inputs (~) and DC outputs (+) and (-).
Remember that PN junctions have a voltage drop across the barrier region, called the “barrier voltage”, or in data sheets, the “Vf” value. If there is a current flow and a voltage drop, energy is converted into heat, and heat is the enemy of electronics.
Heat means increased temperature, which means that materials begin to flow, and in a diode, the barrier region becomes less predictable. It may all melt together making a very large insulator in the centre, or crack into two making an insulator in the gap, or fuse into a silicon copper alloy making a permanent conductor. None of these results are what the designer wants!
In silicon diodes [5], the voltage drop, Vf, is about 0.6V. Some diodes of other materials such as germanium are only 0.2V–0.3V.
Manufacturer’s data sheets are a good source of this information if you need to get specific; however, in most cases the drop across signal diodes can be ignored, and the standard value of 0.6V is an acceptable estimate. Voltage drops in series add up; in a bridge rectifier the two diodes give a total voltage drop of 1.2V.
IDENTIFYING AND ORIENTING
There are many package types available in diodes [6]. The small orange glass examples are signal diodes, while others are presented in varying forms of resin package. You can see that some very high current types are in the TO220 package, like some power transistors. The physical area of a PN junction has a bearing on how much current it can handle. There are names for all of these packages, such as the TO220 mentioned, and standard physical dimensions for each, so they can be known to fit in a PCB pattern.
Here is the tricky part. As described earlier, the terminals on a diode are labelled according to current flow. What may be referred to as the anode is the positive terminal, and the arrowhead of the symbol. The negative terminal is the cathode, labelled K on diagrams and flat packaged diodes.
Looking back to the circuit symbol [1], you’ll notice that the diagram represents what is going on in the circuit. The triangular arrow is pointing in the direction of forward current flow. Current coming to the (K)athode first encounters a solid bar – much like a steel bar preventing entry, or like a brick wall marking the cathode.
Look at a circuit diagram and you will soon learn what orientation the component is, in respect to the circuit, without worrying about labels such as “A” or “K”. Importantly, most packages also include a marking reflecting this. Diodes packaged in a cylindrical black resin case usually have a silver line around one end (if cylindrical), and across (if block types). This represents the line in the circuit diagram, or the “K”. On glass packages, the line is black, but it means the same.
DIODE PARAMETERS
When choosing a diode, catalogue listings usually include two figures. In the case of the ubiquitous 1N4004 device, the numbers listed are 400V, 1A.
The forward current rating (called “IF”), refers to the current that the diode can pass safely without overheating under normal operating conditions. The 1N400x series are rated at 1A comfortably without damage. A 100V diode and a 1000V diode in the same package, can have the same current rating because the heat generated is the same. The heat is a result of the forward voltage (0.6V) multiplied by the forward current (1A) and time – and not the reverse voltage rating.
Reverse current (called “Ir”) is the amount of current “leaking” when in reverse biased mode, and is in the order of microamperes, or negligible for most circuits. The reverse voltage rating means the voltage that the diode can withstand when connected across a reverse polarity voltage. The better term is the “peak inverse voltage”, and data sheets will refer to Vpiv. This refers to the maximum voltage the diode can handle, and not the average voltage or the voltage measured on a typical voltmeter.
The peak voltage of any AC sine wave is √ 2, or 1,414 times the RMS value as read on a voltmeter. For 250VAC, the PV (peak voltage) will be 1.414 x 250 = 354V, so a 1N4004 should be the minimum you would choose.
Full wave centre tapped rectifier diodes must withstand the PIV of two windings in series, and therefore must have twice the PIV of a bridge circuit diode. Therefore a 1N4007, 1000V diode is suitable for centre tapped 220 to 250V.
Finally, the forward voltage drop, Vf, increases across a diode, as it approaches maximum current due to series resistance of the materials. But in the case of a nominally 0.6V SSR, this will only rise to around 1V. The manufacturer’s data sheets normally include a graph that shows this.
All of this information can still be gained if you have the diode in your hands – the part numbers are somewhat standard, and are printed around the body. A quick internet search on the part number will usually find a manufacturer’s data sheet, or for specific details search both the part number and the supplier (assuming you know it). Most diodes of the same part number are very close, but not guaranteed with the trend towards cheap cloned parts from overseas.
If in doubt, data sheets have diagrams and dimensions (toward the end), so you can make sure you have the right device, although be aware that standard packages are used; you will quickly know if you are holding a signal diode and have the data sheet for a power diode with a similar number!
There are other types of diodes, in fact many special types exist. Diodes may be Zener types that have a specific voltage at which they conduct in reverse without damage, as long as the current is limited; or Varactor diodes that use the barrier region as a capacitor and can be controlled by the value of the reverse voltage. There are also Schottky or Tunnel diodes, Pin diodes, Point Contact diodes, and many others which makes for interesting reading after you have learned about what a standard diode is.
The next most familiar diode, which is worth a note here, is the “Light Emitting Diode” or “LED”. LEDs are worth mentioning now, because although they are a PN junction, they’re constructed in a very special way. While the technical details are substantial, the important point is that although LEDs are diodes, construction considerations for light generation make the junction particularly weak to reverse bias and over current. When using an LED, it is important to be careful when feeding it power.
Using LEDs on AC without special IC circuits is possible, but doing so will shorten the lifespan of the LED. In simple, in low brightness, diffused LEDs, this may mean a drop from a 100,000-hour rated lifespan, to 30,000 hours, which is hardly an issue for most of us. However, in higher brightness devices, this may be a reduction to only a few hours’ life. I have even seen some of the earlier generations of high brightness 5mm LEDs (think 2002 vintage) fail in minutes, when reverse biased.
Although many cheap commercial decorative string lights – like Christmas or fairy lights – run on AC, this is considered bad practice for the components themselves. More modern or expensive types tend to run on 36V DC via dedicated current controllers.
This Classroom “lesson” should allow you to effectively identify, orientate, and use diodes in your projects, particularly if you are a maker who has less confidence in component-level electronics, and have come by the need via an interest in programming. However, even experienced tradespeople will forget the basics at times, and I know that on the days when I am working in schools, students often point out when I have forgotten something basic – it happens most days. There is, however, more to the diode than we can discuss here, so we will continue to explore further types and applications in later issues, such as Schottky and Zener diodes, as well as Silicon Controlled Rectifiers (SCRs). Please let us know what you want to learn about, as future editions will be based on reader requests.