5.3 Practical examples

The diagram of a power supply in figure (3.8) uses several diodes. The first four are in a single package, identified by B40C1500. This is a bridge rectifier.The LED in the circuit indicates the transformer is working. Resistor R1 is used to limit the current through the LED and the brightness of the LED indicates the approximate voltage.Diodes marked 1N4002 protect the integrated circuit.Figure 5.3 below shows some other examples of diodes. The life of a globe can be increased by adding a diode as shown in 5.3a. By simply connecting it in series, the current passing through the globe is halved and it lasts a lot longer. However the brightness is reduced and the light becomes yellow. The Diode should have a reverse voltage of over 400V, and a current higher than the globe. A 1N4004 or BY244 is suitable.A very simple DC voltage stabilizer for low currents can be made using 5.3c as a reference.

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

Unstabilized voltage is marked "U", and stabilized with "UST." Voltage on the Zener diode is equal to UST, so if we want to achieve a stabilized 9V, we would use a ZPD9.1 diode. Although this stabilizer has limited use it is the basis of all designs found in power supplies.We can also devise a voltage overload detector as sown in figure 5.3d. A LED indicates when a voltage is over a predefined value. When the voltage is lower than the operating voltage of the Zener, the zener acts as a high value resistor, so DC voltage on the base of the transistor is very low, and the transistor does not "turn on." When the voltage rises to equal the Zener voltage, its resistance is lowered, and transistor receives current on its base and it turns on to illuminate the LED. This example uses a 6V Zener diode, which means that the LED is illuminated when the voltage reaches that value. For other voltage values, different Zener diodes should be used. Brightness and the exact moment of illuminating the LED can be set with the value of Rx.To modify this circuit so that it signals when a voltage drops below some predefined level, the Zener diode and Rx are swapped. For example, by using a 12V Zener diode, we can make a car battery level indicator. So, when the voltage drops below 12V, the battery is ready for recharge.Figure 5.3e shows a noise-producing circuit, which produces a rain-like sound. DC current flowing through diode AA121 isn't absolutely constant and this creates the noise which is amplified by the transistor (any NPN transistor) and passed to a filter (resistor-capacitor circuit with values 33nF and 100k).Figure 5.3f shows a battery back-up circuit. When the "supply" fails, the battery takes over.

5.2 Diode characteristics

The most important characteristics when using power diodes is the maximum current in the forward direction (IFmax), and maximum voltage in the reverse direction (URmax).The important characteristics for a Zener diode are Zener voltage (UZ), Zener current (IZ) and maximum dissipation power (PD).When working with capacitive diodes it is important to know their maximum and minimum capacitance, as well as values of DC voltage during which these capacitances occur.With LEDs it is important to know the maximum value of current it is capable of passing. The natural characteristic voltage across a LED depends on the colour and starts at 1.7V for red to more than 2.4v for green and blue. Current starts at 1mA for a very small glow and goes to about 40mA. High brightness LEDs and "power LEDs" require up to 1 amp and more. You must know the exact current required by the LED you are using as the wrong dropper resistor will allow too much current to flow and the LED will be damaged instantly. The value of this resistors will be covered in another chapter. 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) on circuit diagrams. DUS = Diode Universal Silicon DUG = Diode Universal Germanium

5.1 Diode identification

European diodes are marked using two or three letters and a number. The first letter is used to identify the material used in manufacturing the component (A - germanium, B - silicon), or, in case of letter Z, a Zener diode. The second and third letters specify the type and usage of the diode. Some of the varities are:A - low power diode, like the AA111, AA113, AA121, etc. - they are used in the detector of a radio receiver; BA124, BA125 : varicap diodes used instead of variable capacitors in receiving devices, oscillators, etc., BAY80, BAY93, etc. - switching diodes used in devices using logic circuits. BA157, BA158, etc. - these are switching diodes with short recovery 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 a maximum current of 0.8A. If there is another Y, the diode is intended for higher current. 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.) it means it is intended for higher current.G, G, PD - different tolerance marks for Zener diodes. Some of these are ZF12 (5% tolerance), ZG18 (10% tolerance), ZPD9.1 (5% tolerance).The third letter is used to specify a property (high current, for example).American markings begin 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 that instead of N there is S, 1S241 being one of them.

5. Diodes

As with transistors, diodes are fabricated from semi-conducting material. So, the first letter in their identification is A for germanium diode or B for silicon diode. They can be encased in glass, metal or a plastic housing. They have two leads: cathode (k) and an anode (A). The most important property of all diodes is their resistance is very low in one direction and very large in the opposite direction. When a diode is measured with a multimeter and it reads a low value of ohms, this is not really the resistance of the diode. It represents the voltage drop across the junction of the diode. This means a multimeter can only be used to detect if the junction is not damaged. If the reading is low in one direction and very high in the other direction, the diode is operational. When a diode is placed in a circuit and the voltage on the anode is higher than the cathode, it acts like a low value resistor and current will flow. If it is connected in the opposite direction it acts like a large value resistor and current does not flow. In the first case the diode is said to be "forward biased" and in the second case it is "reverse biased." Figure 5.1 shows several different diodes:

Fig. 5.1: Several different types of diodes

The diodes above are all single diodes, however 4 diodes are available in a single package. This is called a BRIDGE or BRIDGE RECTIFIER. Examples of a bridge are shown in the diagram below:

You must be able to identify each of the 4 leads on a bridge so that it can be inserted into a circuit around the correct way. The surface-mount device above is identified by a cut @ 45° along one side. The leaded bridge has one leg longer than the others and the top is marked with AC marks and "+." The high-current bridge has a corner cut off and the other surface-mount device has a cut or notch at one end.
These devices are added to a circuit as shown in the next diagram:

The 4 diodes face the same direction and this means a single diode can be shown on the circuit diagram:

Symbols in 5.2 show a number of diodes. There are a number of specially-designed diodes: for high current, high-speed, low voltage-drop, light-detection, and varying capacitance as the voltage is altered. Most diodes are made from silicon as it will withstand high temperature, however germanium is used if a low voltage-drop is required. There is also a light emitting diode called a LED, but this is a completely different type of diode.

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

LEDs (Light Emitting Diodes) are constructed from a crystalline substance that emits light when a current flows through it. Depending on the crystalline material: red, yellow, green, blue or orange light is produced. The photo below shows some of the colours hat can be produced by LEDs:

It is not possible to produce white light from any of these materials, so a triad of red, blue and green is placed inside a case and they are all illuminated at the same time to produce white light. Recently, while light has been produced from a LED by a very complex and interesting process that can be found on Wikipedia: http://en.wikipedia.org/wiki/LED

LEDs have a cathode and anode lead and must be connected to DC around the correct way. The cathode lead is identified on the body by a flat-spot on the side of the LED. The cathode lead is the shorter lead.

One of the most important things to remember about a LED is the characteristic voltage that appears across it when connected to a voltage. This does not change with brightness and cannot be altered.For a red LED, this voltage is 1.7v and if you supply it with more than this voltage, it will be damaged. The easy solution is to place a resistor on one lead as shown in the diagram below:

The LED will allow the exact voltage to appear across it and the brightness will depend on the value of the resistor. Zener diodes (5.2c and 5.2d) are designed to stabilize a voltage. Diodes marked as ZPD5.6V or ZPY15V have operating voltages of 5.6V and 15V. Photo diodes (5.2e) are constructed in a way that they allow light to fall on the P-N connection. When there is no light, a photo diode acts as a regular diode. It has high resistance in one direction, and low resistance in opposite direction. When there is light, both resistances are low. Photo diodes and LEDs are the main items in an optocoupler (to be discussed in more detail in chapter 9).Tunnel diodes (5.2f and 5.2g) are commonly used in oscillators for very high frequencies. Schottky diodes (5.2h) are used in high frequency circuits and for its low voltage drop in the forward direction. Breakdown diodes (5.2i) are actually Zener diodes. They are used in various devices for protection and voltage regulation. It passes current only when voltage rises above a pre-defined value. A Varicap diode (5.2j) is used instead of a variable capacitor in high frequency circuits. When the voltage across it is changed, the capacitance between cathode and anode is changed. This diode is commonly used in radio receivers, transceivers and oscillators.The cathode of a low power diode is marked with a ring painted on the case, but it is worth noting that some manufacturers label the anode this way, so it is best to test it with a multimeter.Power diodes are marked with a symbol engraved on the housing. If a diode is housed in a metal package, the case is generally the cathode and anode is the lead coming from the housing.

4.5 Practical example

The most common role of a transistor in an analog circuit is as an active (amplifying) component. Diagram 4.8 shows a simple radio receiver - commonly called a "Crystal Set with amplifier."
Variable capacitor C and coil L form a parallel oscillating circuit which is used to pick out the signal of a radio station out of many different signals of different frequencies. A diode, 100pF capacitor and a 470k resistor form a diode detector which is used to transform the low frequency voltage into information (music, speech). Information across the 470k resistor passes through a 1uF capacitor to the base of a transistor. The transistor and its associated components create a low frequency amplifier which amplifies the signal.On figure 4.8 there are symbols for a common ground and grounding. Beginners usually assume these two are the same which is a mistake. On the circuit board the common ground is a copper track whose size is significantly wider than the other tracks. When this radio receiver is built on a circuit board, common ground is a copper strip connecting holes where the lower end of the capacitor C, coil L 100pF capacitor and 470k resistor are soldered. On the other hand, grounding is a metal rod stuck in a wet earth (connecting your circuits grounding point to the plumbing or heating system of your house is also a good way to ground your project).Resistor R2 biases the transistor. This voltage should be around 0.7V, so that voltage on the collector is approximately equal to half the battery voltage.

Fig. 4.8: Detector receiver with a simple amplifier

4.4 TUN and TUP

As we previously said, many electronic devices work perfectly even if the transistor is replaced with a similar device. Because of this, many magazines use the identification TUN and TUP in their schematics. These are general purpose transistors. TUN identifies a general purpose NPN transistor, and TUP is a general purpose PNP transistor. TUN = Transistor Universal NPN and TUP = Transistor Universal PNP. These transistors have following characteristics:

4.3 The safest way to test transistors

Another way to test transistor is to put it into a circuit and detect the operation. The following circuit is a multivibrator. The "test transistor" is T2. The supply voltage can be up to 12v. The LED will blink when a good transistor is fitted to the circuit.

Fig. 4.7: Oscillator to test transistors

To test PNP transistors, same would go, only the transistor which would need to be replaced is the T1, and the battery, LED, C1 and C2 should be reversed.

4.2 Basic characteristics of transistors

Selecting the correct transistor for a circuit is based on the following characteristics: maximum voltage rating between the collector and the emitter UCEmax, maximum collector current ICmax and the maximum power rating PCmax. If you need to change a faulty transistor, or you feel comfortable enough to build a new circuit, pay attention to these three values. Your circuit must not exceed the maximum rating values of the transistor. If this is disregarded there are possibilities of permanent circuit damage. Beside the values we mentioned, it is sometimes important to know the current amplification, and maximum frequency of operation.When there is a DC voltage UCE between the collector (C) and emitter (E) with a collector current, the transistor acts as a small electrical heater whose power is given with this equation:

Because of that, the transistor is heating itself and everything in its proximity. When UCE or ICE rise (or both of them), the transistor may overheat and become damaged. Maximum power rating for a transistor, is PCmax (found in a datasheet). What this means is that the product of UCE and IC should should not be higher than PCmax:
So, if the voltage across the transistor is increased, the current must be dropped.For example, maximum ratings for a BC107 transistor are:ICmax=100mA,UCEmax = 45V andPCmax = 300mWIf we need a Ic=60mA , the maximum voltage is:

For UCE = 30V, the maximum current is:

Among its other characteristics, this transistor has current amplification coefficient in range between hFE= 100 to 450, and it can be used for frequencies under 300MHz. According to the recommended values given by the manufacturer, optimum results (stability, low distortion and noise, high gain, etc.) are with UCE=5V and IC=2mA.There are occasions when the heat generated by a transistor cannot be overcome by adjusting voltages and current. In this case the transistors have a metal plate with hole, which is used to attach it to a heat-sink to allow the heat to be passed to a larger surface. Current amplification is of importance when used in some circuits, where there is a need for equal amplification of two transistors. For example, 2N3055H transistors have hFE within range between 20 and 70, which means that there is a possibility that one of them has 20 and other 70. This means that in cases when two identical coefficients are needed, they should be measured. Some multimeters have the option for measuring this, but most don't. Because of this we have provided a simple circuit (4.6) for testing transistors. All you need is an option on your multimeter for measuring DC current up to 5mA. Both diodes (1N4001, or similar general purpose silicon diodes) and 1k resistors are used to protect the instrument if the transistor is "damaged". As we said, current gain is equal to hFE = IC / IB. In the circuit, when the switch S is pressed, current flows through the base and is approximately equal to IB=10uA, so if the collector current is displayed in milliamps. The gain is equal to:

For example, if the multimeter shows 2.4mA, hFE = 2.4*100 = 2400.

Fig. 4.6: Measuring the hFE

While measuring NPN transistors, the supply should be connected as shown in the diagram. For PNP transistors the battery is reversed. In that case, probes should be reversed as well if you're using analog instrument (one with a needle). If you are using a digital meter (highly recommended) it doesn't matter which probe goes where, but if you do it the same way as you did with NPN there would be a minus in front of the read value, which means that current flows in the opposite direction.

4.1 The working principle of a transistor

Transistors are used in analog circuits to amplify a signal. They are also used in power supplies as a regulator and you will also find them used as a switch in digital circuits. The best way to explore the basics of transistors is by experimenting. A simple circuit is shown below. It uses a power transistor to illuminate a globe. You will also need a battery, a small light bulb (taken from a flashlight) with properties near 4.5V/0.3A, a linear potentiometer (5k) and a 470 ohm resistor. These components should be connected as shown in figure 4.4a.

Fig. 4.4: Working principle of a transistor: potentiometer moves toward its upper position - voltage on the base increases - current through the base increases - current through the collector increases - the brightness of the globe increases.

Resistor (R) isn't really necessary, but if you don't use it, you mustn't turn the potentiometer (pot) to its high position, because that would destroy the transistor - this is because the DC voltage UBE (voltage between the base and the emitter), should not be higher than 0.6V, for silicon transistors. Turn the potentiometer to its lowest position. This brings the voltage on the base (or more correctly between the base and ground) to zero volts (UBE = 0). The bulb doesn't light, which means there is no current passing through the transistor.As we already mentioned, the potentiometers lowest position means that UBE is equal to zero. When we turn the knob from its lowest position UBE gradually increases. When UBE reaches 0.6v, current starts to enter the transistor and the globe starts to glow. As the pot is turned further, the voltage on the base remains at 0.6v but the current increases and this increases the current through the collector-emitter circuit. If the pot is turned fully, the base voltage will increase slightly to about 0.75v but the current will increase significantly and the globe will glow brightly. If we connected an ammeter between the collector and the bulb (to measure IC), another ammeter between the pot and the base (for measuring IB), and a voltmeter between the ground and the base and repeat the whole experiment, we will find some interesting data. When the pot is in its low position UBE is equal to 0V, as well as currents IC and IB. When the pot is turned, these values start to rise until the bulb starts to glow when they are: UBE = 0.6V, IB = 0.8mA and IB = 36 mA (if your values differ from these values, it is because the 2N3055 the writer used doesn't have the same specifications as the one you use, which is common when working with transistors).The end result we get from this experiment is that when the current on the base is changed, current on the collector is changed as well. Let's look at another experiment which will broaden our knowledge of the transistor. It requires a BC107 transistor (or any similar low power transistor), supply source (same as in previous experiment), 1M resistor, headphones and an electrolytic capacitor whose value may range between 10u to 100µF with any operating voltage. A simple low frequency amplifier can be built from these components as shown in diagram 4.5.

Fig. 4.5: A simple transistor amplifier

It should be noted that the schematic 4.5a is similar to the one on 4.4a. The main difference is that the collector is connected to headphones. The "turn-on" resistor - the resistor on the base, is 1M. When there is no resistor, there is no current flow IB, and no Ic current. When the resistor is connected to the circuit, base voltage is equal to 0.6V, and the base current IB = 4µA. The transistor has a gain of 250 and this means the collector current will be 1 mA. Since both of these currents enter the transistor, it is obvious that the emitter current is equal to IE = IC + IB. And since the base current is in most cases insignificant compared to the collector current, it is considered that:

The relationship between the current flowing through the collector and the current flowing through the base is called the transistor's current amplification coefficient, and is marked as hFE. In our example, this coefficient is equal to:

Put the headphones on and place a fingertip on point 1. You will hear a noise. You body picks up the 50Hz AC "mains" voltage. The noise heard from the headphones is that voltage, only amplified by the transistor. Let's explain this circuit a bit more. Ac voltage with frequency 50Hz is connected to transistor's base via the capacitor C. Voltage on the base is now equal to the sum of a DC voltage (0.6 approx.) via resistor R, and AC voltage "from" the finger. This means that this base voltage is higher than 0.6V, fifty times per second, and fifty times slightly lower than that. Because of this, current on the collector is higher than 1mA fifty times per second, and fifty times lower. This variable current is used to shift the membrane of the speakerphones forward fifty times per second and fifty times backwards, meaning that we can hear the 50Hz tone on the output.Listening to a 50Hz noise is not very interesting, so you could connect to points 1 and 2 some low frequency signal source (CD player or a microphone).There are literally thousands of different circuits using a transistor as an active, amplifying device. And all these transistors operate in a manner shown in our experiments, which means that by building this example, you're actually building a basic building block of electronics.

4. Transistors

Transistors are active components and are found everywhere in electronic circuits. They are used as amplifiers and switching devices. As amplifiers, they are used in high and low frequency stages, oscillators, modulators, detectors and in any circuit needing to perform a function. In digital circuits they are used as switches. There is a large number of manufacturers around the world who produce semiconductors (transistors are members of this family of components), so there are literally thousands of different types. There are low, medium and high power transistors, for working with high and low frequencies, for working with very high current and/or high voltages. Several different transistors are shown on 4.1.The most common type of transistor is called bipolar and these are divided into NPN and PNP types.Their construction-material is most commonly silicon (their marking has the letter B) or germanium (their marking has the letter A). Original transistor were made from germanium, but they were very temperature-sensitive. Silicon transistors are much more temperature-tolerant and much cheaper to manufacture.

Fig. 4.1: Different transistors

Fig. 4.2: Transistor symbols: a - bipolar, b - FET, c - MOSFET, d - dual gate MOSFET, e - inductive channel MOSFET, f - single connection transistor

The second letter in transistor’s marking describes its primary use:C - low and medium power LF transistor,D - high power LF transistor,F - low power HF transistor,G - other transistors,L - high power HF transistors,P - photo transistor,S - switch transistor,U - high voltage transistor.Here are few examples:AC540 - germanium core, LF, low power,AF125 - germanium core, HF, low power,BC107 - silicon, LF, low power (0.3W),BD675 - silicon, LF, high power (40W),BF199 - silicon, HF (to 550 MHz), BU208 - silicon (for voltages up to 700V),BSY54 - silicon, switching transistor.There is a possibility of a third letter (R and Q - microwave transistors, or X - switch transistor), but these letters vary from manufacturer to manufacturer.The number following the letter is of no importance to users.American transistor manufacturers have different marks, with a 2N prefix followed by a number (2N3055, for example). This mark is similar to diode marks, which have a 1N prefix (e.g. 1N4004).Japanese bipolar transistor are prefixed with a: 2SA, 2SB, 2SC or 2SD, and FET-s with 3S:2SA - PNP, HF transistors,2SB - PNP, LF transistors,2SC - NPN, HF transistors,2SD - NPN, HF transistors.Several different transistors are shown in photo 4.1, and symbols for schematics are on 4.2. Low power transistors are housed in a small plastic or metallic cases of various shapes. Bipolar transistors have three leads: for base (B), emitter (E), and for collector (C). Sometimes, HF transistors have another lead which is connected to the metal housing. This lead is connected to the ground of the circuit, to protect the transistor from possible external electrical interference. Four leads emerge from some other types, such as two-gate FETs. High power transistors are different from low-to-medium power, both in size and in shape.It is important to have the manufacturer’s catalog or a datasheet to know which lead is connected to what part of the transistor. These documents hold the information about the component's correct use (maximum current rating, power, amplification, etc.) as well as a diagram of the pinout. Placement of leads and different housing types for some commonly used transistors are in diagram 4.3.

Fig. 4.3: Pinouts of some common packages

It might be useful to remember the pinout for TO-1, TO-5, TO-18 and TO-72 packages and compare them with the drawing 4.2 (a). These transistors are the ones you will come across frequently in everyday work.The TO-3 package, which is used to house high-power transistors, has only two pins, one for base, and one for emitter. The collector is connected to the package, and this is connected to the rest of the circuit via one of the screws which fasten the transistor to the heat-sink.Transistors used with very high frequencies (like BFR14) have pins shaped differently. One of the breakthroughs in the field of electronic components was the invention of SMD (surface mount devices) circuits. This technology allowed manufacturers to achieve tiny components with the same properties as their larger counterparts, and therefore reduce the size and cost of the design. One of the SMD housings is the SOT23 package. There is, however, a trade-off to this, SMD components are difficult to solder to the PC board and they usually need special soldering equipment.As we said, there are literally thousands of different transistors, many of them have similar characteristics, which makes it possible to replace a faulty transistor with a different one. The characteristics and similarities can be found in comparison charts. If you do not have one these charts, you can try some of the transistors you already have. If the circuit continues to operate correctly, everything is ok. You can only replace an NPN transistor with an NPN transistor. The same goes if the transistor is PNP or a FET. It is also necessary to make sure the pinout is correct, before you solder it in place and power up the project.As a helpful guide, there is a chart in this chapter which shows a list of replacements for some frequently used transistors.

3.3 Practical examples with coils and transformers

On the figure 2.6b coils, along with the capacitor, form two filters for conducting the currents to the speakers. The coil and capacitor C on figure 2.6c form a parallel oscillatory circuit for "amplifying" a particular radio signal, while rejecting all other frequencies.

Fig. 2.6: a. Amplifier with headphones, b. Band-switch, c. Detector radio-receiver

The most obvious application for a transformer is in a power supply. A typical transformer is shown in figure 3.8 and is used for converting 220V to 24V.

Fig. 3.8: Stabilized converter with circuit LM317

Output DC voltage can be adjusted via a linear potentiometer P, in 3~30V range.

Fig. 3.9: a. Stabilized converter with regulator 7806, b. auto-transformer, c. transformer for devices working at 110V, d. isolating transformer

Figure 3.9a shows a simple power supply, using a transformer with a centre-tap on the secondary winding. This makes possible the use two diodes instead of the bridge in figure 3.8.
Special types of transformers, mainly used in laboratories, are auto-transformers. The diagram for an auto-transformer is shown in figure 3.9b. It features only one winding, wound on an iron core. Voltage is taken from the transformer via a slider. When the slider is in its lowest position, voltage equals zero. Moving the slider upwards increases the voltage U, to 220V. Further moving the slider increases the voltage U above 220V.
The transformer in figure 3.9c converts 220v to 110v and is used for supplying devices designed to work on 110V.
As a final example, figure 3.9d represents an isolating transformer. This transformer features the same number of turns on primary and secondary windings. Secondary voltage is the same as the primary, 220V, but is completely isolated from the "mains," minimizing the risks of electrical shock. As a result, a person can stand on a wet floor and touch any part of the secondary without risk, which is not the case with the normal power outlet.

3.2.1 Working principles and basic characteristics

As already stated, transformers consist of two windings, primary and the secondary (figure 3.7). When the voltage Up is connected to the primary winding (in our case the "mains" is 220V), AC current Ip flows through it. This current creates a magnetic field which passes to the secondary winding via the core of the transformer, inducing voltage Us (24V in our example). The "load" is connected to the secondary winding, shown in the diagram as Rp (30Ω in our example). A typical load could be an electric bulb working at 24V with a consumption of 19.2W.

Fig. 3.7: Transformer: a. Working principles, b. Symbol

Transfer of electrical energy from the primary to the secondary is done via a magnetic field (called "flux") and a magnetic circuit called the "core of the transformer." To prevent losses, it is necessary to make sure the whole magnetic field created by the primary passes to the secondary. This is achieved by using an iron core, which has much lower magnetic resistance than air.
Primary voltage is the "mains" voltage. This value can be 220V or 110V, depending on the country. Secondary voltage is usually much lower, such as 6V, 9V, 15V, 24V, etc, but can also be higher than 220V, depending on the transformer's purpose. Relation of the primary and secondary voltage is given with the following formula:

where Ns and Np represent the number of turns on the primary and secondary winding, respectively. For instance, if Ns equals 80 and Np equals 743, secondary voltage will be:

Relationship between the primary and secondary current is determined by the following formula:

For instance, if Rp equals 30Ω, then the secondary current equals Ip = Up/Rp = 24V/30Ω = 0.8A. If Ns equals 80 and Np equals 743, primary current will be:

Transformer wattage can be calculated by the following formulae:

In our example, the power equals:

Everything up to this point relates to the ideal transformer. Clearly, there is no such thing as perfect, as losses are inevitable. They are present due to the fact that the windings exhibit a certain resistance value, which makes the transformer warm up during operation, and the fact that the magnetic field created by the primary does not entirely pass to the secondary. This is why the output wattage is less than the input wattage. Their ratio is called EFFICIENCY:

For transformers delivering hundreds of watts, efficiency is about µ=0.85, meaning that 85% of the electrical energy taken from the mains gets to the consumer, while the 15% is lost due to previously mentioned factors in the form of heat. For example, if power required by the consumer equals Up*Ip = 30W, then the power which the transformer draws from the maains equals:

To avoid any confusion here, bear in mind that manufacturers have already taken every measure in minimizing the losses of transformers and other electronic components and that, practically, this is the highest possible efficiency. When acquiring a transformer, you should only worry about the required voltage and the maximal current of the secondary. Dividing the wattage and the secondary voltage gets you the maximal current value for the consumer. Dividing the wattage and the primary voltage gets you the current that the transformer draws from network, which is important to know when buying the fuse. Anyhow, you should be able to calculate any value you might need using the appropriate formulae from above.

3.2 Transformers

For electronic devices to function it is necessary to have a DC power supply. Batteries and rechargeable cells can fulfill the role, but a much more efficient way is to use a POWER SUPPLY. The basic component of a power supplyr is a transformer to transform the 220V "mains" to a lower value, say 12V. A common type of transformer has one primary winding which connects to the 220V and one (or several) secondary windings for the lower voltages. Most commonly, cores are made of E and I laminations, but some are made of ferromagnetic material. There are also iron core transformers used for higher frequencies. Various types of transformers are shown on the picture below.

Fig. 3.5: Various types of transformers

Symbols for a transformer are shown on the figure 3.6 Two vertical lines indicate that primary and secondary windings share the same core.

Fig. 3.6: Transformer symbols

With the transformer, manufacturers usually supply a diagram containing information about the primary and secondary windings, the voltages and maximal currents. In the case where the diagram is missing, there is a simple method for determining which winding is the primary and which is the secondary: a primary winding consists of thinner wire and more turns than the secondary. It has a higher resistance - and can be easily be tested by ohmmeter. Figure 3.6d shows the symbol for a transformer with two independent secondary windings, one of them has three tappings, giving a total of 4 different output voltages. The 5v secondary is made of thinner wire with a maximal current of 0.3A, while the other winding is made of thicker wire with a maximal current of 1.5A. Maximum voltage on the larger secondary is 48V, as shown on the figure. Note that voltages other than those marked on the diagram can be produced - for example, a voltage between tappings marked 27V and 36V equals 9V, voltage between tappings marked 27V and 42V equals 15V, etc.

3.1 Coils

Coils are not a very common component in electronic circuits, however when they are used, they need to be understood. They are encountered in oscillators, radio-receivers, transmitter and similar devices containing oscillatory circuits. In amateur devices, coils can be made by winding one or more layers of insulated copper wire onto a former such as PVC, cardboard, etc. Factory-made coils come in different shapes and sizes, but the common feature for all is an insulated body with turns of copper wire.
The basic characteristic of every coil is its inductance. Inductance is measured in Henry (H), but more common are millihenry (mH) and microhenry (µH) as one Henry is quite a high inductance value. As a reminder:

1H = 1000mH = 106 µH

Coil inductance is marked by XL, and can be calculated using the following formula:

where f represents the frequency of the voltage in Hz and the L represents the coil inductance in H.
For example, if f equals 684 kHz, while L=0.6 mH, coil impedance will be:The same coil would have three times higher impedance at three times higher frequency. As can be seen from the formula above, coil impedance is in direct proportion to frequency, so that coils, as well as capacitors, are used in circuits for filtering at specified frequencies. Note that coil impedance equals zero for DC (f=0).
Several coils are shown on the figures 3.1, 3.2, 3.3, and 3.4.
The simplest coil is a single-layer air core coil. It is made on a cylindrical insulator (PVC, cardboard, etc.), as shown in figure 3.1. In the figure 3.1a, turns have space left between them, while the common practice is to wind the wire with no space between turns. To prevent the coil unwinding, the ends should be put through small holes as shown in the figure.

Fig. 3.1: Single-layer coil

Figure 3.1b shows how the coil is made. If the coil needs 120 turns with a tapping on the thirtieth turn, there are two coils L1 with 30 turns and L2 with 90 turns. When the end of the first and the beginning of the second coil are soldered, we get a "tapping."
A multilayered coil is shown in figure 3.2a. The inside of the plastic former has a screw-thread, so that the ferromagnetic core in the shape of a small screw can be inserted. Screwing the core moves it along the axis and into the center of the coil to increase the inductance. In this manner, fine changes to the inductance can be made.

Fig. 3.2: a. Multi-layered coil with core, b. Coupled coils

Figure 3.2b shows a high-frequency transformer. As can be seen, these are two coils are coupled by magnetic induction on a shared body. When the coils are required to have exact inductance values, each coil has a ferromagnetic core that can be adjusted along the coil axis.
At very high frequencies (above 50MHz) coil inductance is small, so coils need only a few turns. These coils are made of thick copper wire (approx. 0.5mm) with no coil body, as shown on the figure 3.3a. Their inductance can be adjusted by physically stretching or squeezing the turns together.

Fig. 3.3: a. High frequency coil, b. Inter-frequency transformer

Figure 3.3b shows a metal casing containing two coils, with the schematic on the right. The parallel connection of the first coil and capacitor C forms an oscillatory circuit. The second coil is used for transferring the signal to the next stage. This is used in radio-receivers and similar devices. The metal casing serves as a screen to prevent external signals affecting the coils. For the casing to be effective, it must be earthed.
Fig 3.4 shows a "pot core" inductor. The core is made in two halves and are glued together. The core is made of ferromagnetic material, commonly called "ferrite." These inductors are used at frequencies up to 100kHz. Adjustment of the inductance can be made by the brass or steel screw in the centre of the coil.

Fig. 3.4: A "pot core" inductor

3. Coils and transformers

Coils are not a very common component in electronic circuits, however when they are used, they need to be understood. They are encountered in oscillators, radio-receivers, transmitter and similar devices containing oscillatory circuits. In amateur devices, coils can be made by winding one or more layers of insulated copper wire onto a former such as PVC, cardboard, etc. Factory-made coils come in different shapes and sizes, but the common feature for all is an insulated body with turns of copper wire.

For electronic devices to function it is necessary to have a DC power supply. Batteries and rechargeable cells can fulfill the role, but a much more efficient way is to use a POWER SUPPLY. The basic component of a power supplyr is a transformer to transform the 220V "mains" to a lower value, say 12V. A common type of transformer has one primary winding which connects to the 220V and one (or several) secondary windings for the lower voltages. Most commonly, cores are made of E and I laminations, but some are made of ferromagnetic material. There are also iron core transformers used for higher frequencies. Various types of transformers are shown on the picture below.

2.4 Practical examples

Several practical examples using capacitors are shown in photo 1. A 5µF electrolytic capacitor is used for DC blocking. It allows the signal to pass from one sage to the next while prevent the DC on one stage from being passed to the next stage. This occurs because the capacitor acts like a resistor of very low resistance for the signals and as a resistor of high resistance for DC.

photo 1 : a. Amplifier with headphones, b. Electrical band-switch

The photo 1.b represents a diagram of a band-switch with two speakers, with Z1 used for reproducing low and mid-frequency signals, and Z2 for high frequency signals. 1 and 2 are connected to the audio amplifier output. Coils L1 and L2 and the capacitor C ensure that low and mid-frequency currents flow to the speaker Z1, while high frequency currents flow to Z2. How this works exactly ? In the case of a high frequency current, it can flow through either Z1 and L1 or Z2 and C. Since the frequency is high, impedance (resistance) of the coils are high, while the capacitor's reactance is low. It is clear that in this case, current will flow through Z2. In similar fashion, in case of low-frequency signals, current will flow through Z1, due to high capacitor reactance and low coil impedance.

photo 1 : c. Detector radio-receiver

The photo 1.c represents a circuit diagram for a simple detector radio-receiver (commonly called a "crystal set"), where the variable capacitor C, forming the oscillatory circuit with the coil L, is used for frequency tuning. Turning the capacitor's rotor changes the resonating frequency of the circuit, and when matching a certain radio frequency, the station can be heard.