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: