Other components

There is an enormous amount of different electronic components in use nowadays all around the world. This unfortunately means that many of them will remain unexplained in this book, because such task would quickly become impossible to achieve. But we will cover some of the most important “other” components which you will come to meet in future editions of “Practical ELECTRONICS”. Their symbols are on 10.1.

Fuses (10.1a) have single role in circuit - to apprehend heavy failures and damages of the circuits. What this means is that some high power devices, like different amplifiers, radio receivers, TVs, and such, rise the amount of current flow from the mains during some instabilities, which could result in serious damages of your appliance. To prevent this from happening, fuses are installed inside of the device. Fuse is usually made out of a glass tube holding a thin wire which burns out on current flow anomalies and shuts down the whole device, and by sacrificing itself, saves other, more expensive components. Most commonly fuses could be found in the transformer part of the device. One of the two metal caps has fuse's specifications printed on it: it's specified work load, and burnout point.
Ordinary electric light bulb with a heating thread is on 10.1b. It's resistance depends on the temperature it is heated to. Usual light bulb's resistance when heated is ten to twenty times higher than when it is cold.
VDR resistors' resistance (10.1d) depends on the voltage brought to them: higher the voltage, higher the resistance. They found their usage in different voltage protection devices. If they, for example, receive voltage higher than 220V, their resistance rises and this “soaks” the excess voltage.
Symbol for a DC voltage battery is on 10.1e.
Quartz crystal oscillator is on 10.1f. Quartz oscillator is a thin quartz plate pressed between two metal plates and packed in a metal case connected to the plates. Quartz crystal oscillator is commonly used as a stabilizer for some electronic oscillator units, or as a clock source in microprocessor designs. Nowadays it's price is very low, but in past it has been acquired by dismantling some old electronic clock.
Instrument for measuring current (A) and voltage (V) is on 10.1g. This symbol dates from times when analog equipment with needle was in usage. Symbol remains the same nowadays, although digital instruments have replaced analog ones long time ago.
AC voltage symbol is on 10.1h. If there was more than one sine-wave, it is a high-frequency (HF) AC voltage source. Square-wave and sawtooth instead of a sine-wave represent square-wave or sawtooth voltage.
The simplest form of a switch device is displayed on 10.1i. It's lever has two positions, one when it is non-conductive, thus the controlled device is switched off and conductive, when the device is on. Because of the wide usage of switches, there are many different ones among them. For example, two pole switch (10.1j) has two operating positions, in one position it connects points 1 and 2, and in the other it connects points 1 and 3.
There are switches with even more operating positions, on 10.1k there is an example switch with a rotation switch with four positions.

Momentary switches, or push buttons (10.1l) have a built-in spring, which makes the switch conduct only while it is being pressed (your standard doorbell has this kind of switch). There is an opposite effect component, whose points are constantly connected, while button pressing opens the circuit (these switches are commonly referred to as reset buttons). Another possibility is a three-contact push button (10.1m). When button is not pressed it connects points 1 and 2, and when it is pressed it connects points 1 and 3.
Four diodes in a single case (10.1n), connected in a Gretz (or often named bridge connection) are used as two way directors in transformers. It's plastic case has four connectors on it's sides: two marked with sine waves, used to connect the AC voltage, and two marked with a + and a -, where comes an electrolithic capacitor, which produces a DC voltage.
Relay symbols are represented on 10.1o. When electromagnet receives sufficient voltage on points 4 and 5, connection between points 2 and 3 is opened, and at the same time closed between points 3 and 1. Relay is actually an electromagnetic switch. This means that you can design a circuit which automatically toggles on and off based on some conditions which were predefined when the circuit was designed.
Symbols for a receiver (10.1p) and transmitter (10.1q) antenna.
Grounding symbols (10.1r). Grounding and common ground aren't the same thing, but if both exist in a circuit, they are always connected to each other. With electronic devices housed in a metal case, grounding is connected to the metal housing. You will be able to spot a ground on the printed circuit board by looking for the broadest line on the board. Negative pole of the battery (or transformer) is connected to this line.
Shielded cables (10.1t) are usually used when a signal is weak or for some reason far from it's amplifier. Typical example is a microphone cable, which sends audio signal to the amplifier. If normal cable was used instead of a shielded one, it would act as a receiving antenna, and therefore receive large amount of unwanted signals, like different voltages from the mains grid, various interferences like cars, elevators, home appliances, mobile phones, etc. End result of this would have been an irritating noise produced by the speakers. Because of this, shielded cables have a conductive outer layer which looks like a net and is always connected to the ground. Because of this, the outer layer now forms a Farad's cage, which blocks outer electromagnetic interferences from reaching core wire, enabling it now to carry only the clear signal from the source to the amplifier.
Schematic symbol for a beginning or ending of some conducting line. Symbols for pairs of crossing, but not connected, wires are on 10.1v and 10.1w. Connected crosses are on 10.1x and 10.1y.
Schematic symbols representing logic gates and different digital integrated circuits are on 10.1z. It should be kept in mind that basic logic gates (AND, OR, XOR, Inverter, etc.) aren't manufactured as single standalone components. They are always integrated in groups in one IC (7.4a), but for the sake of clarity, they are represented as separate blocks on the schematics. These components demand a DC voltage, which might or might not be represented on the schematic, but is always considered as given. These voltages might be different depending on the internal structure and technology used between different family types. Detailed info on this can be found in component's datasheet provided by the manufacturer. As said, four NAND gates on 7.4a are separate blocks on the schematic, but different numbers on their inputs and outputs state that they come from the same case, and those numbers represent pin numbers of that case. Supply for all four gates is connected between pins 14 (positive) and 7 (negative)

10.1 Relays

Relay is an electro mechanic device which is commonly used to bridge the gap in the circuit, where an electronic circuit is connected to some device connected to the mains grid, whose 220V AC may prove deadly for other components intended for much lower DC voltages.
Simply put, relay is an electro-mechanical switch which is opened and closed using a magnet. Schematic of a relay is on 10.2. Copper wire coil on ferrite core is used as a magnet. Above the magnet itself is a plastic lever with iron head and a spring. In the middle of that lever there is a metal fin with small platinum cylinders. Same cylinders are on pins 1 and 2. When current is conducted through pins 4 and 5 to the coil, lever is drawn to the magnet, which disconnects fin from pin 2, thus opening the circuit between 3 and two. Fin is connected to the cylinder on pin 1 and closes the circuit between pins 3 and 1. When there is no voltage on 4 and 5, magnet is off and the lever springs back to it's original position closing the circuit between pins 3 and 2 and opening one between 3 and 1.

Simplest relay is displayed on 10.2a. There are far more complicated relays in existence, where an electro magnet activates more switches. On 10.2b is a relay with three pairs of switches.
Relay is usually connected as a collector load of a transistor, as shown on 10.3. When the input voltage is high enough (between points A and B), so that the base voltage (voltage between base and ground, or between base and emitter, because its connected to ground as well) is higher than 0.7V, current IB flows through base, and IC flows through the collector. This current flows through the electro magnet's coil, this draws the lever with the fin, and this breaks the connection between pins 2 and 3 and connects pins 1 and 3.
Lower part of the 10.2 represents a shape and pin placement of a relay.
Since relay is an electro mechanic component which is consisted of moving parts, it has a limited operational life span, and cannot be used for rapid switching. It would not be very effective using it in a, for example, light show which has frequent switching frequency (several hundreds or thousands times per hour). Each opening and closing of the contact is followed by sparks which would dramatically shorten the life of such device.
When choosing a relay, two value numbers are most important to us:

      1. electro magnet's coil, and

      2. relay contacts to which is connected the controlled device.

      1. Coil values are “input values” or voltage and resistance values at which relay draws the lever and switches. Usual coil voltage values are 3V, 5V, 6V, 12V and 24V. They can be found printed on the relay's housing. These are all DC voltages, but there are AC voltage designed relays with 230V/250V. Depending on the resistance of the coil is the current flow value. Usually resistance is, with the voltage, marked on the housing of the component, and if this is not the case, it can be easily measured. Current flowing through the relay is calculated using Ohm's law, by dividing relay's voltage with it's resistance. For example, relay we used was 12V, using the multimeter we measured it's resistance was 300Ohm, which means that current flow through the coil is:

      2. Voltage on relay's contacts, also marked on the housing, is a maximum value allowed, this value should not be neglected, since disregarding it would cause intense sparks inside of the relay itself and possibly serious damage to all devices connected to it. If you want to switch some home appliance operating off the mains grid, relay contact's maximum rating should be 250V (this is, as with all other components, valid if mains is 220V in your country, if you live in United States, you should use a relay intended for 120V, this goes pretty much for Japan as well)

Maximum current rating relay's can sustain is marked on the housing with all other info, usually above 1A, and commonly higher than 10A.
Maximum power rating of a controlled device is calculated by multiplying the maximum current rating with relay's pin voltage. If, for example, maximum allowed current flow rating is 8A, and the voltage is 220V, highest allowable power rating is:

P=I.U=8.220=1760 W.

Transistor from picture 10.2 should have at least twice high the value of voltage between collector and emitter than battery's voltage (24V in our example), and that maximum collector current rating is higher than current rating through the coil (this is 40mA in our example).

10.2 Examples

Modern telecommunication systems were born on May 24th 1844. On this day Samuel Morse established a transfer of telegraphic signals between Washington and Baltimore. This transmission has been conducted over a wire line. Half a century later, wireless, radio transmission was introduced. Soon after that, transmission of sound, and after that picture. Since the very beginnings, amateurs formed a strong base around radio transmission. At first they established communication using telegraphy, but soon equipment for audio communication was within reach of amateurs. Nowadays most radio enthusiasts communicate over satellite. Telegraphy is the most primitive form of radio communication, but still has people interested in it.
Telegraphy amateurs use series of abridged phrases, forming their own lingo, which speeds up the communication. OK, for example stands for everything is alright, TNX – thanks, QRS – drop the typing speed. One of them is QRP, or drop the transmission power, is also used as a reference for low power CW transmitters.
Schematic of an interesting 0.5W QRP transmitter is on 10.4. Central component of this transmitter is a logic gate 74HC240. According to radio amateur's holy bible “The ARRL Handbook” 1998. edition, American amateur Lew Smith, or N7KSB, successfully communicated to radio amateurs in over 30 countries on every continent.
Chip 74HC240 has eight inverting amplifiers, commonly used as buffer circuits in digital electronics. One of the buffers, on 10.4, whose input is on pin 11 and output on 9, connected to other components forms an oscillator. This oscillator's frequency needs to be very stable, it must not change on temperature or supply voltage variations, etc. This is achieved using a quartz crystal oscillator labeled as Q on the schematic.
Signal from the oscillator is amplified using the amplifiers made of four buffers connected in parallel. Inputs of these buffers are on pins 2, 4, 6 and 8. Unused buffers on 3, 5, and 7 are connected to ground and outputs remain unconnected (acronym NC you can find on some schematic means Not Connected). Amplified HF signal from the amplifier is on pins 12, 14, 16 and 18 and is connected to coils L1 and L2 and capacitors C7 and C8, to the connector U. This filter represses higher harmonies of the HF signal created by the oscillator. Signal from the oscillator produces square voltage, rich in harmonics and these could interfere with other radio devices, so that part of the circuit is highly recommended. Coaxial cable with 50W impendance is plugged into the connector U. This cable leads HF signal to the emitting antenna.

* Transmitter is supplied from 8V DC voltage. Higher voltage should not be used, since this could lead to overheating and damaging of the integrated circuit.

* Since this circuit generates an excess heat, heatsink should be added to the design. It could be bought as an “off the shelf” product or made out of an around 1mm thick aluminum sheet, which is glued to the circuit using an epoxy resin or a special paste used to glue heatsinks to your CPU of the PC.

* Coil data is given in the spreadsheet in the upper right corner of the picture. They are wound using a 1.6mm copper wire with laque insulation. As shown, you wind the coil using a 9.5mm drill bit. Make sure you wind it tight, curl to curl. Then, you remove the coil from the drill bit and stretch it, so that it's total length is as shown on the spreadsheet.

* When pin 1 is connected to ground, oscillator operates, this is achieved by pressing the momentary switch. Resistor R1 and capacitor C3 form a low pass filter. This circuit removes “clicks” which appear when switch's contacts open or close.

* Quartz crystal Q has the same resonant frequency as transmitter (30MHz, 20MHz or 15MHz). To establish transmission, transmitter must “fall into” the antenna range (28MHz, 21MHz or 14MHz) and this is achieved by tuning the trimmer capacitor Ct.

On 10.5 is an example of a classic shielded cable application. Received LF signal is routed to the amplifier circuit using an ordinary microphone cable. Shielding, made of a conducting wire laced into a net is connected to ground, and the wire core is connected to the amplifier. In the same manner, using a double core shielded cable, a potentiometer, used to regulate the volume is connected to the circuit. Pictures 9.7 and 9.8 hold information applicable to this circuit as well.


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