As with transistors, diodes are fabricated out of semiconducting materials. So, the first letter in their mark could be an A (germanium diode) or B (silicon diode). They can be encased inside of a glass, metal or plastic housing. They have two leads: a cathode (K) and an anode (A). The most important property of all diodes is that their resistance is very small in one direction, 6O for example, and very large in the opposite, i.e. 600 kO. What this means is that when a diode is in an electrical circuit, voltage on the anode is higher than the voltage on the cathode, and it acts like a low value resistor (6O). If it is connected in the opposite direction it acts like a large value resistor (600 kO). In the first case, it is referred to that diode as conducting polarized, and as nonconducting polarized in the second case.
Picture 5.1 depict several different diodes, and picture 5.2 shows their symbols.

Fig. 5.1: Several different types of diodes

Transforming diodes are, as their name states, used in transformers, whether as single components or as four diodes inside of a housing. They are called Gretz (or bridge) rectifier.
On the other hand, there are diodes whose primary characteristic of having passing and non-passing direction is of no importance. They have other capabilities, and are used in other circuits than transformers.
Symbol in 5.2a is standing for regular transforming diode (some of them are 1N4001, BY238, AY260, etc.). They are designed in such a way to withstand relatively high current in conducting direction, and voltage in non conducting direction. These are their main characteristics.
HF, or detector, diodes are represented on schematics using the same symbol as in rectifying doides (5.2a), but in reality these two types are very different. These diodes are used for very low currents, in circuits like the modulated signal detector in radio receivers, voltage limiters, etc. They are mainly made of germanium, so their marks are usualy starting with a letter A, AA121, for example. Second A is used to specify that this is a HF diode. Most common package for them is a glass tube tinted in some dark color (black or gray) from which are coming two wires.

Fig. 5.2: Diode symbols: a - regulating and HF diode, b - LED,
c, d - Zener, e - photo, f,g - tunnel, h - Schottky, i - breakdown,
j - capacitative

LEDs (Light Emitting Diodes) are constructed in form of small red, yellow, green (or more rarely blue or transparent) light bulbs, and they are used as light indicators. When they are connected to a DC circuit, their polarity should always be considered. Anode must go to the point where voltage is higher. On the other hand if circuit operates on AC, LED's polarity is not important. Whenever LED is used in a circuit, there should be a protective resistor connected in series with it, because LED burns out without it. Several kilo-ohm resistor should be used for operating on voltages under 20V. If the light is too dim value of that resistor should be lowered, if on the contrary, light is too bright value of the resistor should be higher.

Zener diodes (5.2c and 5.2d) are stabilizing diodes in transformers and they are used, as we applied, to stabilize the voltage. Second letter in their mark is Z (BZ6, for example). Number shows operating voltage of that diode. If there is an Y behind the Z, that is high-power zener diode (BZY12, for example). Mark could be formatted in some other way, but it always states the zener voltage. There are diodes which are marked as ZPD5.6V or ZPY15V whose operating voltages are 5.6V and 15V. Zener diodes are always non-pass polarized, which means that DC voltage on the cathode is always positively polarized comparing to the voltage on the anode.

Photo diode (5.2e) is constructed in such a way that it allows light to fall on it's P-N connection. When there is no light, Photo diode acts as a regular diode, when current flows through it, it has high resistance in one, and low resistance in opposite direction. When there is light both resistances are low. In practice, this means that when there is no light voltage on the anode is lower than the voltage on the cathode so diode is polarized negatively and acts as a large resistance resistor. But when the light is on, it's resistance is lowered which makes this diode appropriate for different alarm and signal devices. Photo diode and a LED are main parts of optocouplers (who will be discussed in more detail in chapter 9).

Tunnel diode (5.2f and 5.2g) is commonly used in oscillators with very high frequencies. It is conducting polarized in operating conditions. When DC voltage is set to a needed value, diode, for AC current, acts like a negative resistance resistor.

Schottky diode (5.2h) is used on extremely high frequency rates as well as with high power transforming devices (because of it's low voltage drop in pass direction) on frequencies of 100kHz order.

Breakdown diode (5.2i) is actually a Zener diode used in various different devices for protection and voltage regulation. It passes current only when voltage rises above some diode's predefined value. European standard symbol is on 5.2c, and symbols on 5.2d and 5.2g are american standard symbolic representations of this diode.

Varicap diode (5.2j) is used instead of a variable capacitor in high frequency devices. It's polarized using a DC voltage not to conduct current (cathode has higher voltage than anode). When this voltage's value is changed, capacitance between cathode and anode is changed. This diode is commonly used in radio receivers, transceivers, oscillators, eg. every place that has a demand for variable capacitor with relatively narrow range between it's minimum and maximum value.

Low power diode's cathode is marked with a ring painted on the housing of the component, but it is worth noting that some manufacturers label anode this way, so it is best to test it with a multimeter (you'll commonly buy more than one diode, and they come in strips, so it is only needed to test one, others will be the same as that one).

Powerful diodes are marked with a symbol engraved on the housing. If diode's internals reside in a metal package, cathode is (not always) connected to it, and anode is a lead that goes through a plastic cap in the housing.

5.1 Diode marks

European diodes are marked using two or three letters and a number. Possible variation of this is a letter behind the number. First letter is used to note the material used in manufacturing of the component (A - germanium, B - silicon), or, in case of letter Z, a Zener diode. The second and third letter specify the sort and usage of that diode. Some of the possibilities are:
A - a very low power diode, like the AA111, AA113, AA121, etc. - they are used in the detector unit of a radio receiver; BA124, BA125 : varicap diodes used instead of variable resistors in different receiving devices, oscillators, etc., BAY80, BAY93, etc. - switching diodes used in devices which operate using logic circuits. BA157, BA158, etc. - these are switching diodes with short relapse time.
B - two capacitive (varicap) diodes in the same housing, like BB104, BB105, etc.
Y - regulation diodes, like BY240, BY243, BY244, etc. - these regulation diodes come in a plastic packaging , and operate on maximum current of 0.8A. If there is another Y behind this one, diodes specifics remain the same, except that they are intended for higher currents. For example, BYY44 is a diode whose absolute maximum current rating is 1A. When Y is the second letter in a Zener diode mark (ZY10, ZY30, etc.) that means that it is intended for higher power usage.
G, G, PD - different tolerance marks for Zener diodes. Some of these are ZF12 (5% tolerance), ZG18 (10% tolerance), ZPD9.1 (5% tolerance).
Third letter is used to specify the branch of certain two-letters model type with some specific property (designed for higher currents, for example).
American markings are beginning with 1N followed by a number, 1N4001, for example (regulating diode), 1N4449 (switching diode), etc.
Japanese style is similar to american, the main difference is in that instead of N there is S, 1S241 being one of them.
Russian diode marks are consisted of two numbers (GD - germanium, KD - silicon) and a number.
As with transistors, number does not have some deeper meaning, it is there only to help users find that specific model in a catalog and see it's specifications. Only difference to that, as already mentioned, are Zener diodes, whose number shows operating voltage of the certain Zener diode.

5.2 Diode characteristics

The most important characteristics when using power diodes used in transformers and similar devices are maximum current rating in conductive direction (IFmax), and maximum voltage they could withstand in non-conductive direction(URmax).
One should bear in mind that characteristics read on schematics are effective values. Maximum values, which are important for selection of certain diode are calculated when their effective value is multiplied by 1.41. For example, if the schematic of certain transformer states that secondary voltage of some transformer connected to the wall plug is 12V, maximum voltage of this voltage is 17V, so the diode should have URmax>17V.

Important characteristics for Zener diodes are Zener voltage (UZ) and Zener current (IZ) and maximum dissipation power (PD).

When working with capacitative diodes it is important to know their minimal and maximal capacitance, as well as values of DC voltage during which these capacitances occur.

With LEDs it is important to know the value of current nd voltage which pass through the diode when the light of the component is brightest. Voltage comes from 1.6V to several volts., and current goes from several mA to several tens of mA. It is a common thing to connect a protective resistor in series with a LED, whose values is easily acquired through experiment.

Beside universal transistors TUN and TUP (mentioned in Chapter 4.4), there are universal diodes as well. They are marked with DUS (for universal silicon diode) and DUG (for germanium one). These diodes have following characteristics:

5.3 Practical examples

The schematic of a stabilized transformer (3.8) has several diodes. The first four of them are in a single package with mark B40C1500. This is the well known Gretz (or bridge) rectifier which is a two way rectifier for 24V AC.
LED is used to optically indicate that transformer is working. The resistor R1 is used to protect the diode, diode's brightness is changed with the change of it's value.
Diodes marked as 1N4002 are protecting integrated the circuit if a consuming device (which is connected between points + and -) has a large electrolythic capacitor.
There are several other examples of the usage of diodes on picture 5.3. Light bulb's lifespan could be prolonged using the device on 5.3a. By simply connecting a diode to a light bulb in series current passing through a bulb is halved and it last a lot longer. Of course, there is a downside to this method: brightness of the bulb is lowered and the light becomes yellow, so this solution s optimal for use in building corridors and other places where there is a need to have a long lasting light source but don't need it to be very bright. Diode should have an inverse voltage of over 400V, and a current higher than the light bulb's. Some of them (for a 200W bulb) are 1N4004 and a BY244.
While we are discussing building corridor lights, 5.3b shows a way to connect a LED to the switch, so that it lights only when the light bulb is off for easier finding in the dark. Both the resistor and diode are placed in a switch housing, with LED peeking through a hole on the switch. (Of course, this is commercially available for a long time, so this is only to show how are those circuits which you'd normally buy function)
Very simple DC voltage stabilizer for low currents could be made using the schematic 5.3d as a reference.

Fig. 5.3: a - using a diode to prolong the light bulb's life span, b - stairlight LED indicator, c - voltage stabilizer,
d - voltage rise indicator, e - backup supply, d - rain noise synthetizer

Unstabilized voltage is marked with an U, and stabilized with UST. Voltage over the Zener diode is equal to UST, so if we wanted to achieve stabilized 9V, we would have used ZPD9.1 diode. Although this stabilizer has limited usage it is the base design found in all power supplies today.
We could also devise a voltage overload detector, 5.3d has a LED connected to the circuit as a signal that voltage is over some predefined value. While voltage is lower than the operating voltage of the Zener, diode acts as a high value resistor, so DC voltage on the base of the transistor is very low, which means it is not conducting electricity. When the voltage rises to equal the Zener voltage, it's resistance is lowered, and transistor receives enough electricity on it's base to start conducting electricity, which lights the LED. This example has 6V Zener diode, which means that LED is lit when voltage reaches that value. For other voltage values, appropriate Zener diode should be used. Brightness and the exact moment of lighting the LED could be set with the right value of Rx resistor (in several kO range).
To modify this circuit in the way that it signals voltage drop below some predefined level, all one should do is swap places of the Zener diode and Rx resistor. For example, by using 12V Zener diode in this manner, we could make an car battery level indicator. So, when voltage drops below 12V, battery taken out from the car and recharged.
A bit odd usage for a diode is shown on 5.3e. It is the noise synthesizer, which produces rain like sound. DC current flowing through the conducting polarized diode AA121 isn't absolutely constant, but changes over some middle value (which would be shown using ampermeter connected in series with the diode). This variable component which creates the noise is amplified using transistor (any NPN transistor) and passed over a filter (resistor-capacitor circuit vuth values 33nF and 100kOhms) is brought to an audio amplifier and reproduced on a speaker.
One day, author of this book ran really late to work, which by the way cause enormous amount of joy among his students for loosing first three classes. This all happened because of the power failure in the electrical grid, which led to his electrical alarm clock reset (this is the other-side-of-the-fence equivalent to the "dog ate my homework"). In these situations, when some critical device looses it's main power supply, back-up power from the battery should come into picture and remain normal functionality of said device. Schematic 5.3f shows how two diodes, which are able to operate on voltages needed by the device, and a battery are added to the stabilized transformer (this can be any off the shelf transformer you already have for your home appliances). For this to function properly, UIZ voltage should be a bit higher than the voltage over the battery. That makes D2 diode nonconducting, so battery doesn't supply. When network voltage drops, UIZ is zero so D2 conducts electricity, and battery supplies needed electricity.
D1 is there to prevent battery to power the transformer, which is needed to prolong the battery life, and protect transformer from damage. For devices up to 1A diode 1N4001 is sufficient, and 1N5400 if amperage is up to 3A


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