ELECTRICAL MEASURING INSTRUMENTSDefinition: These are instruments used in the measurement of electrical quantities and characteristics.
An ammeter is a measuring instrument used to measure electric CURRENT in a circuit. Electric current is measured in amperes(A).
An ohmmeter is an electrical measuring instrument that measures electrical resistance, the opposition to an electric current. Electrical resistance is measured in OHMs.
A voltmeter is an electrical instrument used for measuring electrical potential difference between two points in an electric circuit. Electrical potential difference is measured in Volts.
This is an electrical instrument used in the measurement of electrical power. Electrical power is measured in watts.
A multi-meter or a multi-tester is an electronic measuring instrument that can measure more than one kind of electrical quantity/characteristics. A typical multi-meter would include basic features such as been able to measure voltage, current and resistance. A more complex multi-meter can measure current, resistance, voltage, power, transistors, frequency, inductance, capacitance, inductance e.t.c. combined.
PLACING AMMETER AND VOLTMETER IN A CIRCUIT
To measure accurately the current flowing into a load or out of a load, we need to open circuit the points of interest to us and use the ammeter to complete the circuit.
To measure voltage, we mean to measure simply the potential difference between two points in a circuit. Therefore, we don’t need to open circuit; we simply place the meter probes at these two points. It is pertinent to note that the power supply (i.e. battery, ac or converted ac (dc)) must be connected to the circuit.
To measure resistance, connection of the power supply is utterly irrelevant in most cases. Our target here is the resistances culminating from the wires, semiconductors, resistors and other components used in the making of the circuit.
USING OHMMETER TO TEST SEMICONDUCTOR
Due to the forward/reverse behaviours of semiconductor devices, it is very easy to test them using ohmmeter. When the test leads (probes) of the ohmmeter are connected to the diode, a resistance will be measured that is different from the resistance indicated if the leads is reversed. The smaller value is called the forward resistance, and the larger value is the back resistance. If the ratio of the back-forward resistance is greater than 10 to 1, the diode is capable of performing as a rectifier.
AMPLIFIER DESIGNAn amplifier is an electronic circuit capable of receiving a small signal from a pickup transducer or other input source and provides a larger version of the signal to an output device or to another amplifier stage.
How does a transistor do this? Well, technically a transistor has three terminals. A current or voltage applied through two terminals creates and controls a larger current or voltage produced through the third terminal.
CLASSES OF POWER AMPLIFICATIONClasses of amplifications are the various ways the base of every amplifier is turned on during amplification processes. In some cases, the transistor is turned on from the beginning that the amplifier device is on until it is switched off again; while in other time the transistor is turned on once the person using the amplifier device is talking. These different ways a transistor is turned on gave birth to what is called classes of amplifier. Generally, we have four recognized classes of amplifiers. But in this class we will be discussing five classes and in the future you will be meeting other types.
CLASS A AMPLIFIER
This is a type of amplifier where output transistor is on full time and the output current flows for entire cycle of the input wave form. This means the conduction angle is 3600.
1. Class A is the simplest
2. High fidelity because input signal will be exactly reproduced at the output.
3. Improved high frequency response.
1. Very low efficiency
2. They are expensive
3. They are bulky
The efficiency of this amplifier can be improved by transformer coupling.
CLASS B AMPLIFIER
This is a kind of power amplifier where the transistor conducts only for one half cycle of the input signal. This means the conduction angle is 1800. Without input signal, the transistor has zero DC bias.
In the circuit above one of the transistor conducts only for 1800 only when sound signal (current) is entering the base of the transistor. When the positive signal is going in NPN transistor is ON while PNP is OFF, When the negative signal is going in PNP transistor is ON while NPN is OFF and no signal is going in both transistors will be switched OFF.
1. High efficiency when compared with Class A.
1. It still has low fidelity due to cross of distortion.
This class of amplifier's efficiency can also be improved by transformer coupling.
CLASS AB AMPLIFIER
This is an amplifier whose transistor base voltage is slightly above zero DC bias there by keeping the transistors slightly turned ON. Unlike class B that is zero biased thereby leaving it utterly turned OFF. For class AB operation, the output signal swings occurs between 1800 and 3600. It is neither class A nor Class AB. It applied partially the concept of Class A DC bias and incorporated the push pull concept of Class B. since both transistors are slightly conducting at zero voice signal input, no information in the signal will be lost at the output, this cross over distortion of class B is so eliminated at a cost of slightly reduced efficiency.
1. No cross over distortion
2. No need for the bulky coupling transformer
3. No humming in the output
1. Efficiency is slightly reduced.
2. There will be some DC component in the output due to direct coupling.
CLASS C AMPLIFIER
This amplifier is biased for operation for less than 1800 of the input signal cycle. For this reason will only operate with tuned or resonant circuit which provides a full cycle operation for the tuned or resonant frequency. Therefore, this kind of amplifier is employed in radio communication.
1. High efficiency
2. Very suitable for RF amplification
3. Lowest size of all classes of amplifiers.
1. Lowest linearity
2. Not good for audio amplifier
1. Radio Frequency oscillator
2. Radio Frequency amplifier
3. FM transmitters
4. Booster amplifier
5. High frequency repeaters
6. Tuned amplifier
CLASS D AMPLIFIER
This amplifier is designed to operate with digital pulses. Using digital techniques makes is possible to have a signal that varies over the entire cycle.
1. It is the most efficient of all the classes of amplifier.
2. Low heat dissipation
3. Reduced size and weight
1. Requires more complex engineering techniques.
2. Requires a very clean and stable power supply.
DESIGN OF AN AUDIO POWER AMPLIFIERDEFINATION: Audio Amplifier is an electronic equipment capable of magnifying the original weak human voice into a very loud sound for the purpose of covering a larger area. Imagine a large congregation/audience listening to a person using his/her natural voice. The listener will be in stress trying to hear the speaker; the speaker also will be in a stress trying to be heard. These stresses can be put away with a little voice amplification device. Therefore, it can be put simply that an audio amplifier is employed in making communication especially to a large audience more convenient.
COMPONENTS USED IN THE DESIGN
1. Step Down Transformer: an electromagnetic device used in stepping down 220 volts AC to any other lower voltage which depends on the amplifier’s power requirements.
2. Bridge Rectifier: this is a semiconductor electronic component that converts AC to DC (usually ripple dc). It can come in an interconnection of four individual rectifying diodes or in an integrated circuit version of it.
BRIDGE RECTIFIER:3. Filtering Capacitor: in other to obtain a better DC than the one gotten after passing the AC through the bridge rectifier, a filtering capacitor is used.
CAPACITOR:4. Resistor: an electronic component that limit the flow of electric current. It is also used in dividing voltage which is relevant for sound amplification.
FIXED RESISTOR:5. Integrated circuit (TDA 2030): this is a 5 pins integrated circuit housing the actual operational amplifiers needed for the amplification of audio signal. DESIGN OF AUDIO AMPLIFIER
COMPLETE CIRCUIT OF 10W AMPLIFIER. The above circuit diagram shows how the audio amplifiers circuit will be built. There are millions of other possibilities different from the one shown above that does amplification even better than this one.