5.
Diodes
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.
|