Coils

Coils

A coil is nothing more than copper wire wound in a spiral. This symbol is used to indicate a coil in a circuit diagram. Inductance value is designated in units called the Henry(H). The more wire the coil contains, the stronger its characteristics become. The inductance value can become quite large. If a coil is wound around an iron rod, or ferrite core (strengthened with iron powder), the inductance of the coil will be greatly increased. Coils used in typical electric circuits varely widely in values, ranging from a few micro-henry (µH) to many henry (H). Coils are sometimes called "inductors." Inductance is the measure of the strength of a coil. Capacitors have capacitance, resistors have resistance, and Inductors (coils) have inductance. When alternating current flows through a coil, the magnetic flux that occurs in the coil changes with the current. When a second coil is put close to the first coil (with the changing flux), alternating voltage is caused to flow in the second coil by an effect known as "mutual induction." Mutual inductance (inductance) is measured in units of the Henry. The changing magnetic flux in a coil affects itself as well as other coils. This is called self induction, the degree of this self induction is called Self Inductance. Self inductance is a measure of a coil's ability to establish an induced voltage as a result of a change in its current. Self inductance is commonly referred to as simply "inductance," and is symbolized by "L". The unit of inductance is the Henry (H). The definition of "Henry" is "When a current of 1 ampere flows through a given coil in 1 second such that 1 volt is induced to flow in a second coil, the mutual inductance between the coils is said to be 1 Henry." The definition of self inductance is the same, except that the 1 volt is induced in the first coil; there is no second coil. Characteristic of coils When wire is coiled, it takes on various characteristics that are different from straight wire. Below I will explain some of the characteristics coils that I know. Current Stabilization Characteristic When current begins to flow in the coil, the coil resists the flow. When current decreases, the coil makes current continue to flow (briefly) at the previous rate. This is called "Lenz's law". 'The direction of induced current in a coil is such that is opposes the change in the magnetic field that procduced it.' This characteristic is used for the ripple filter circuit of a power supply where it transforms alternating current(AC) to direct current(DC). When a rectifier is used to make DC from AC, the output of the rectifier without a ripple filter circuit is ripple current. Ripple current is DC that has a large AC component. A ripple filter circuit often combines coil and capacitors. The coil resists the change of current and capacitors supplement the flow of current by discharging into the circuit if the input voltage drops. Thus, clear, ripple-free DC is obtained from the ripple filter circuit. Resistor is used instead of coil in simple ripple filter circuit. Mutual induction As I wrote above, electric power can be transfered between two coils by mutual induction. The transformer utilizes this characteristic. The input coil that gives the electric power is called the primary side, while the output coil that takes out the electric power is called the secondary side. The output voltage is determined by the ratio of turns of wire between the primary coil and secondary coil. Some transformers have a tap (or several) on the secondary coil to provide multiple voltage levels. Electromagnet When current flows through a conductor, a magnetic field is created. This field is much stronger in a coil. An electromagnet is just like a regular magnet. It attracts iron, nickel, and some other metals. Relays utilize this characteristic. When the current flows to the coil of relay, the magnetic field attracts a steel plate, and the switch that is attached to the steel plate goes ON. And the doorbell chime also utilizes electromagnets. Resonance When a coil and a capacitor are combined, the resulting circuit has special characteristics. The impedance (resistance to current flow) of the circuit changes with the frequency of the voltage. Current will flow easily at a given frequency, but has difficulty flowing at another frequency. The tunning circut that select a particular radio station utilizes this characteristic. Explaning resonance in more detail is very difficult. If you want to know more detail, please read further in a book about electronics. High Frequency Coils The photograph shows an example of a small coil component. The component on the left is wound with thin copper wire to a small barbell-shaped ferrite core, and has a value of 100µH. It is used for high frequency resonance, or for detering of high frequency. As for size, the diameter is about 4 mm, the height about 7 mm. The value of the small coil like this is indicated with a color code, just like a resistor. The strengh of this type of coil varies from 1µH to several hundred µH. 1µH, 2.2µH, 3.3µH, 3.9µH, 4.7µH, 5.6µH, 6.8µH, 8.2µH, 10µH, 15µH, 18µH, 22µH, 27µH, 33µH, 39µH, 46µH, 56µH, 68µH, 82µH, 100µH other. The second coil from the left has thin copper wire wound around a stick-shaped ferrite core. It is used the same as the component above. The value is 470µH. The diameter of the core is 4 mm, height is 10 mm, and the diameter of the coil is 8 mm. The two devices on the right in the photograph are high frequency transformers. They are used for intermediate frequency (455KHz) tuning of transistor radios, or for oscillator circuits. To shield the coils from magnetic flux, and to prevent the coils from interfering with other circuits, the high frequency coils are housed in a metal case called shield case. This case must be connected to ground. As for tuning or oscillation, this type of transformer can change its value of inductance. Adjustment of the Inductance Value The ferrite core of the coil is made like a screw. The core can be made to move in or out of the coil by turning it with a screwdriver. A special plastic screwdriver is better to use for adjustment of the coils. By moving the ferrite core in or out of the coil, the value of the coil's inductance can be changed. The value of inductance can also be changed by changing the number of turns of wire that comprise the coil, but in fact it is not possible in any practical way. You want try it ? The tuner of an FM radio handles very high frequencies (about 70MHz to 100MHz). The coils used in the tuning circuit are hollow; i.e. they have no ferrite core. A coil with a ferrite core has too much inductance too be used in such a circuit. To adjust the inductance value of a hollow coil, the spacing between the loops of the coil is changed. When you disassemble an FM radio, you may find these coils to appear a bit untidy. Do not try to "fix" the coil by making it a perfect set of loops. The coil has been bent intentionally, in order to be adjusted precisely. The Toroidal Coil The toroidal coil consists of copper wire wrapped around a cylindrical core. It is possible to make it so that the magnetic flux which occurs within the coil doesn't leak out, the coil efficiency is good, and that the magnetic flux has little influence on other components.

3. Coils and transformers

3. Coils and transformers

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 X_L, 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

3.2 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.

3.2.1 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.

3.3 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.

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