Tuesday, November 4, 2008

Checking Electronic Components

CHECKING ELECTRONIC COMPONENTS

In the process of repairing and servicing electronic instrument and appliances it is very important to have knowledge as to how the defectiveness of electronic component can be determined.

Resister





This can be done using Digital Multi Meter (DMM) or Analog Multimeter (AMM). To achieve this purpose the resistance scale has to be selected. If the resistor is working properly, the meter reading should agree with the value read using resistor color code.



Capacitors




Electrolytic Capacitors


For this purpose the Digital Multi Meter has to be put the ohms scale and the two leads of the capacitor is now connected to the probes, with correct polarity. If the capacitor is in good condition it will show high impedance indicating no leakage, which is in most cases ranging up to Mage Ohms. If this is not the case capacitor may be defective.

In the case if the Analog Multimeter is used, if the capacitor is good, it will cause the pointer to give of full-scale deflection suddenly for a small duration.


Non polarity Capacitors

Checking non-polarity capacitors is same as checking polarity capacitors. The only difference is there’s no a polarity. So if the capacity is good, it will display a high impedance value.
If an Analogue Multi Meter is used, it will give slight reading indicating correctness of the component.

Diodes

Use the Digital Multimeter in diode mode and the diode is connected with the probes in correct polarity. If the diode were Silicon one it will show band gap value of 0.6V, whereas 0.2V will be shown if Germanium. If the readings are ok, the diode is ok.


But we can check a diode in resistance scale also. The resistance will display as overloaded or will display a very high resistance in the reverse bias direction and it will show a low resistance in forward bias direction. If the readings are correct, the diode is good.

In the case of an Analog Multimeter, above measurement is performed in resistance scale. So a low value will be the result in forward bias direction and a high resistance value for reverse bias direction.


Transistors

Bipolar transistors are constructed of three-layers of semiconductors. There are two as PNP or NPN as they constructed. Actually they cab be taken as two diodes connected back-to-back as shown in following figures. Therefore we can use the method we used to check the diodes.







Checking a Known Transistor

This figure shows the two occasions of the forward biasing of the forward biasing of the two diodes. Here they are tested in the resistance mode. Then if the transistor is in good condition the values must be very low and closer to zero and the following parameters must be correct too. If we use the diode mode, the correct biasing voltage must be there.






This figure shows the reverse biasing of the two diodes. Here the meter reading must be overload or the resistance must be very high.


If the both condition in the forward biasing and reverse biasing directions are ok, then the transistor is in good condition.



Identify an Unknown Resistor
The following example describes how to identify an unknown transistor.




The six combination of the meter checking as follows.

· Meter touching wire 1 (+) and 2 (-): "OL"
· Meter touching wire 1 (-) and 2 (+): "OL"
· Meter touching wire 1 (+) and 3 (-): 0.655 volts
· Meter touching wire 1 (-) and 3 (+): "OL"
· Meter touching wire 2 (+) and 3 (-): 0.621 volts
· Meter touching wire 2 (-) and 3 (+): "OL"
The only combinations of test points giving conducting meter readings are wires 1 and 3 (red test lead on 1 and black test lead on 3), and wires 2 and 3 (red test lead on 2 and black test lead on 3). These two readings must indicate forward biasing of the emitter-to-base junction (0.655 volts) and the collector-to-base junction (0.621 volts).

Now if we look for the one wire common to both sets of conductive readings. It must be the base connection of the transistor, because the base is the only layer of the three-layer device common to both sets of PN junctions (emitter-base and collector-base). Here in this example, that wire is number 3, being common to both the 1-3 and the 2-3 test point combinations. In both those sets of meter readings, the black (-) meter test lead was touching wire 3, which tells us that the base of this transistor is made of N-type semiconductor material (black = negative).
Then what are the emitter and collector? The emitter must be 1 and collector must be 2, because the emitter-base biasing voltage must be higher than the collector-base biasing voltage.
Thus, the transistor is a PNP type with base on wire 3, emitter on wire 1 and collector on wire 2:

Testing Instruments

TESTING INSTRUMENTS
Testing instruments is a very important task in electronic field. Therefore, there are so many electrical measuring equipments in the modern world. Following are some of the basic and very important measuring instruments, we usually use.
- Multi-meter (analog/digital)
- Ossiloscope
- Waveform Generator


Multimeter
The multimeter is the most basic electrical measuring instrument. They are small enough to carry in a hand and battery operated. The basic functions of multimeters are measuring Voltages, Currents and Resistance. But most multimeters have some other functions like diode test, transistor check and capacitance check, especially in digital multimeters.
There are 2 kind of multimeters as how they display the values. They are the Analog Multimeter and the Digital Multimeter.


Analog Multimeter


Analog Multimeter

An analog multimeter uses a needle and a printed set of scales to display it is values. When comparing to Digital Multimeter, this is someone difficult to read the values, because we can’t read the values directly. Another disadvantage is that we have to use the polarity in the correct way; otherwise the meter may cause damage. This is a difficult task, because if we don’t know the polarity of the circuit. But if we use a DMM it displays a minus value, if the polarity is changed.
Advantages are also there when comparing to DMM. One is when using this as an ammeter, as the internal resistance is law, it gives a very accurate value.


Digital Multimeter
Digital Multimeter




The Digital Multimeters are the easier to get the values, because it gives the values directly in digital format. There are also auto ranging function so that we can read the value in les effort.
As a voltmeter, DMMs are much accurate than AMM as its internal resistance is higher. Most of them have a battery indicator. Therefore we can get an idea about the internal battery. That avoids wrong meter readings due to malfunction of the battery.
There are also multimeters are available with the Digital and Anolog together. They are more useful as we can get the both advantages in a one unit.


Oscilloscope



The oscilloscope is a graph displaying devices, which displays the voltage against the time of an external signal. Therefore the X axis represents the time and Y axis represents the voltage.
The earlier times, they were used cathode-ray tube to display the graphs. But now a days we use more accurate oscilloscopes. Most of they are used Digital Phosphor displays replacing the Cathode-ray. There are so many functions and much easier to user.
Following are some of the basic applications of an oscilloscope.
· Determine the frequency of an oscillating signal.
· Determine amplitude of a signal.
· To find out malfunctioning component is distorting the signal.
· To find out the DC and AC voltages of a AC and DC coupled signal.
· To study about the noises added to the signal.
· To compare two or more signals.

Waveform Generator



Basically a waveform Generator is an oscillator generates waveforms continuously. A typical waveform generator generates 3 kinds of wave forms. They are as,
- Sinusoidal

- Square

-Triangle


The following is a block diagram of a basic waveform generator. The IC max038 is the basic unit there. It is a high frequency waveform generating IC manufactured by Maxim. The output frequency can be controlled over the frequency range of 0.1Hz to 20MHz.



The waveform generators are commonly used in testing amplifiers and transmitters. They are using to give input signals to those circuits and after checking the output we can get an idea about the instrument. To check the output, oscilloscopes are commonly used. These waveform generators also can be used to give clock pulses to a circuit. There are lot of other applications with the use of this instrument.