SAFETY CHECKS IN SERVICING RADIO RECEIVERS

Definition: Safety checks in servicing radio receivers (Electronic equipment) are the basic rules one ought to know so as to maintain/repair electronic appliances safely.

SAFETY CHECKS RULES

 1. Keep work area clean, dry, well illuminated and ventilated.
 2. Check dead and live circuit. A servicing engineer must be able to know conductors that have current flowing through it (live wires) and those that have no current flowing through it (dead circuit).
 3. Discharge capacitor charges: capacitors are electronic components known for being able to store charges (electrons). Therefore, is should be noted that these stored charges can cause serious electrocution if touched with bare hand.
 4. Servicing engineer should protect one’s eyes from molten solder and sudden explosion of power supply filter capacitor. This can be achieved by using appropriate glasses.
 5. Handle electronic measuring instrument with care.

Assignment

Apart from the importance of obeying workshop rules discussed in the class, write down five other importances you can think of.

ELEMENTS OF ELECTRONIC CIRCUIT

RESISTORS: these are passive electronic components that limit the flow of electrons in an electronic circuit. The quantity of current different electronic component can handle varies extensively; therefore it is very necessary to regulate the amount of current flowing through a component to avoid damaging it. Resistors are electronic components capable of handling these regulations. They are measured in ohms.
TYPE OF RESISTORS
FIXED RESISTORS: these kinds of resistors have its values unchanging/unchangeable after they have been manufactured. They usually have color band that helps in calculating their approximate value.

RESISTOR SYMBOL

VARIABLE RESISTOR: these resistors have rotating part that enables the overall values to be changeable by tuning the rotating part.
VARIABLE RESISTOR

THERMISTORS (heat dependent resistor): these resistors have its values changing with change in temperature.
THERMISTOR

PHOTO RESISTORS (Light Dependent Resistor): these resistors have its values changing with changes in light intensity.
PHOTO-RESISTOR

DETERMINATION OF VALUES OF RESISTORS VALUES

There are five ways values of resistors can be determined.
 1. Using manufacturer’s data book generally called datasheet.
 2. By color code
 3. By reading the numbers inscribed on them
 4. By using ohmmeter or multi-meters
 5. By reading there exact values usually written on them for larger resistors.

 A. DETERMINATION OF RESISTOR VALUE BY DATA SHEET

This is a record book issued by manufacturers explaining in details the component that they have designed so that future users of these components may have more understanding on best ways to effectively use the components. Some basic information data sheet possesses are the component values, the maximum current, maximum voltage, maximum temperature and so many other information necessary. It should be noted that every component has a unique identification code, and with this code one can easily download from the internet the datasheet that is if getting to the manufacturer becomes impossible.
Thermistors and photo resistors' resistive values can only be determined by consulting the manufacturers' data sheet (a record describing a particular electronic component, its use, rating etc. written by the manufacturers).


 A. DETERMINATION OF RESISTOR VALUE BY COLOR BAND

COLOR BANDED

RESISTOR COLOR TABLE

RESISTORS COLOR MATCHING

Examples 1: for 4 color band resistor

Solution: Using the above table to match the colors correctly
Green (5): the first color is just an ordinary color to be written down.
Violet (7): the second color also is an ordinary number to be written down adjoining the first color.
Red (2): the third color band is the multiplier here (x102) for four color band resistors.


Therefore, we have 57 x 102 ohms = 5700 Ω = 5.7 kilo-Ω
Gold: the last color, which serves as the tolerance range of most resistors. But other colors a times replace this gold color but no so common.

Examples 1: for 5 color band resistor

Solution: Using the above table to match the colors correctly
Green (5): the first color is just an ordinary color to be written down.
Violet (7): the second color also is an ordinary number to be written down adjoining the first color.
Black (0): the third color is also an ordinary number just to be written down.
Brown (1): the third color band is the multiplier here (x101) for four color band resistors.

Therefore, we have 570 x 101 ohms = 5700 Ω = 5.7 kilo-Ω
Gold: the last color, which serves as the tolerance range of most resistors. But other colors a times replace this gold color but no so common.

ASSIGNMENT
A resitor has a value of 68Ω, what will be the color bands combinations in
(a) four color banded resistor?
(b)five color banded Resistor?

TOLERANCE CALCULATION

All we have discussed so far has not included the tolerance bands, which are usually the 4th color for 4 colors banded resistors and the 5th color for 5 color banded resistor. Using color code to identify the value does not mean that the resistor will be exactly that value if cross checked using Multi-meter. The reason why the values are not exactly the same is because of manufacturing errors. These slight changes in actual values and color code value does not mean the resistor is faulty or bad, though there is a maximum limit of difference that can be tolerated and that is why it is called tolerance color.

PROBLEM: the only challenge one will encounter is identifying the tolerance color out of the four or five banded resistors. Look carefully at the below picture of a real life resistor, you will notice that one of the colors is not uniformly gapped as the other three. Therefore, the band with the wider gap is actually the tolerance color.
Examples 1: Detecting the tolerance Color

GOLD: this color is the most popular tolerance color that most resistors in the market have. And it has the value of 5%. Check the above table to look up other tolerance band colors and their values.


Example 3: Therefore, if a resistor is 1000Ω and the tolerance is GOLD.
Then the resistor value is 1000 ± 5% Ω.

It means one will have to find 5% of 1000
Tolerance=5/100 x 1000 = 50 Ω Therefore, the resistor value can be in the range of 1000 ± 50Ω. (950 Ω to 1050 Ω)
That is 1000 - 50 = 950Ω
1000 + 50 = 1050Ω
Finally, the resistor value when checked with Multi-meter should not be less than 950 Ω or greater than 1050 Ω.

Example 4:

Therefore, if a resistor is 5600Ω and the tolerance is RED.
Then the resistor value is 5600 ± 2% Ω.
It means one will have to find 2% of 5600
Tolerance=2/100 x 5600
= 112 Ω
Therefore, the resistor value can be in the range of 5600 ± 112 Ω. (5488 Ω to 5712 Ω)
That is 5600 - 112 = 5488 Ω
5600 + 112 = 5712Ω
Finally, the resistor value when checked with Multi-meter should not be less than 5488Ω or greater than 5712 Ω.


DETERMINING RESISTOR VALUES USING CODED NUMBER INSCRIBED


Resistors exact values can also be determined using coded number inscribed on them. These codes have universal accepted steps for interpreting their values correctly.
Example 1: A resistor with a coded number 102 what will be the exact value?
Example 2: A resistor with inscription of 105, what will be the exact value?


DETERMINING RESISTOR VALUES READING EXACT VALUE INSCRIBED

When larger resistors are built, using code or code number becomes unnecessary since enough space to write their precise value is available. Some variable resistors have its exact values inscribed on it; example is the large variable resistor we used in the design of our frequency reading lamp. Therefore, it requires no effort to determining its values.


DETERMINING RESISTOR VALUES USING MULTIMETER

PRACTICAL: to understand fully how to use multi-meter ensure you attend your teacher's meter use demonstration class.


Class work:

Use color code to determine the value of the above sketched resistor.
Use your multi-meter to determine the exact value.
Compare the two values obtained using different methods.

INDUCTORS

These are passive electronic components that convert electrical energy to magnetic energy; it can also store electrical energy in the form of magnetic field. It is measured in Henrys. Inductors are the simplest electronic component to make, is made simply by coiling wires around a given material like iron or ferrite core in some cases without cores. Most practically used inductors are usually in milli-henry and micro-henrys. Just like resistors their values can be known using datasheet, coded number inscription, exact value inscription, capacitance meter (though expensive), and color codes. If you desire to work more with FM radios and AM radio detail knowledge of inductors and inductors making can ever be overemphasized.




CAPACITORS

These are passive electronic components that can store and discharge current. It is important to note that it discharge slowly but charges very fast. They are measured in farads. Practically reading the value of a given capacitor is exactly like that of resistor. That is capacitor values can be known using datasheet, coded number inscription, exact value inscription, capacitance meter (though expensive), and color codes. In most cases, the most popular methods in use are coded number inscription and exact value inscription; therefore we will focus more on these two methods. If you desire to employ the color code method then refer to the resistor color code section since both are read almost exactly the same.

REAL LIFE PICTURES OF AVAILABLE CAPACITORS IN THE MARKET




TYPES OF CAPACITOR
Fixed capacitor: this kind of capacitor weather polarized or un-polarized has it value unchanging after it had been manufactured.
Variable capacitor: this kind of capacitor can be tuned to change its value.


DETERMINING THE VALUES OF CAPACITOR USING CODED NUMBER INSCRIPTION










Note: some capacitors come with color band just like resistors and are read exactly like resistors.


DETERMINING THE VALUES OF CAPACITOR EXACT VALUE INSCRIPTION

Finally, most capacitors with large surface area have their values boldly written on the body. Example: most electrolytic capacitors just like the below figure.



DIODES

These are semiconductor electronic components that allow current to flow in one direction.
TYPES OF DIODES
 1. RECTIFYING DIODES: these diodes function strictly on allowing current to flow in one direction and is mostly used in converting AC to DC.


 2. ZENER DIODES: these kinds of diodes apart from allowing current to flow in one direction can also be connected in such a way that it can stabilize (regulate) voltage. So these components are used as regulators.


 3. LIGHT EMITTING DIODES: these kinds of diodes serve as indicators in circuits because of their light emitting ability. They are also used in the design of TV and computers display unit, static and moving message displays.


 4. SILICON CONTROL RECTIFIER (SCR): this is another kind of rectifying diode that is used as switch; it does not conduct until the third pin (gate) is turned on.



TRANSISTOR

This is a semiconductor electronic component that is used as a switch, an amplifier or a buffer.

TRANSISTOR CONSTRUCTION TYPES

Every transistor is made up of three (3) layers, 2 similar extrinsic materials with another kind of extrinsic material at the center.
That is; NPN or PNP
NPN: two N-type material + One P-type Material.
PNP: two P-type material + One N-type material.

With the two probe of your analogue or digital multi-meter you can identify and differentiate NPN transistor from PNP transistor since these pins positions are not constant.

Each of the pin has a unique name emitter, base or collector. With your multi-meter probe, you can identify the base but it is hard to identify the collector and emitter by the same means.

DESCRIPTION OF TRANSISTOR

Transistor is a three terminal semiconductor electronic component used as switch or amplifier in circuit. Being able to identify these terminals is a must if a technologist is to use it correctly.

As stated earlier, a transistor can either be NPN or PNP.

IDENTIFYING A TRANSISTOR AND ITS THREE TERMINALS USING DIGITAL MULTIMETER
 1. Switch your meter to diode symbol. If the black meter's probe is placed in any of the three pins of a transistor and touching the other two pins with the red probe gives readings, it means that the transistor is PNP. The pin that the black probe was placed is the base and emitter and collector not yet known.

 2. Your meter still switched to diode reading. If the red probe of the meter is placed in any of the pins of a transistor and touching the other two pins with the black probe gives reading, it means it is NPN transistor. The pin that the red probe was placed is the base but emitter and collector yet to be known.

 3. Switch the meter to hFe symbol and plug in your transistor into any of the holes that has been provided on your digital meter for identifying all the pins of the transistors simultaneously. If the if the meter has not giving a steady value at its read out keep shaking and changing the holes the transistor terminals were initially plugged to until a stead value shows up. At this point look at the holes it has inscription of e, b and c at each of the holes and the three pins will be distinctively known.

PLACING A TRANSISTOR IN A CIRCUIT
There are three ways (configurations) a transistor can be placed in a circuit. For instance, using NPN in the below analysis.


COMMON BASE CONFIGURATION: in this placement, the base is usually grounded, the emitter serves as the input while the collector serves as the output.


COMMON EMITTER CONFIGURATION In this configuration, the output is the collector; the input is the base while emitter is grounded. This kind of configuration makes the output direct opposite of the input (inverts input signal). But this concept is the most widely used of all transistor configurations because of the high current gain.


COMMON COLLECTOR CONFIGURATION In this configuration, base is the input and emitter is the output while collector is common.


TYPES OF TRANSISTOR
Bipolar Junction transistor (BJT)
Field Effect Transistor (FET)
ASSIGNMENT
Discuss in brief your own observable differences in the transistor configurations.