COMPONENT TESTING USING CRO

A component tester is used to test if components are working correctly in circuit. This is very nice if you do electronic repairs. You can test components directly on a soldered PCB.

You can see if components work correctly by comparing them with the same component on a working PCB, or the more difficult way- by analyzing the characteristics

We tell how you can make your own component tester, if your oscilloscope does not have one.

The necessary ingredients:-

• Oscilloscope with 2 channels, X-Y function, Channel Invert function • 1k resistor • 12 to 15 volt transformer.

Connect the diagram above, set your oscilloscope to use X-Y function and invert channel Y. You can now use 'Component tester probe'-connections to test your components.

Test Procedure :-
Caution! Do not test any component in live circuitry, remove all grounds, power and signals connected to the component under test. Set up Component Tester as stated. Connect test leads across component to be tested. Observe oscilloscope display. – Only discharged capacitors should be tested!

Test Pattern Displays:-

The following ”Test patterns” show typical patterns displayed by the various components under test.
Open circuit is indicated by a straight horizontal line. Short circuit is shown by a straight vertical line.

Testing Resistors:-

The test pattern expected from a resistor is therefore a sloping straight line. The angle of slope is determined by the value of the resistor under test.

With high values of resistance, the slope will tend towards the horizontal axis, and with low values, the slope will move towards the vertical axis. Values of resistance from 20 Ohm to 4.7 kOhm can be approximately evaluated.
The determination of actual values will come with experience, or by direct omparison with a component of known value.

Testing Capacitors and Inductors:-

Capacitors and inductors cause a phase difference between current and voltage, and therefore between the X and Y deflection, giving an ellipse shaped display. The position and opening width of the ellipse will vary according to the impedance value (at 50Hz) of the component under test.

A horizontal ellipse indicates a high impedance or a relatively small capacitance or a relatively high inductance.

A vertical ellipse indicates a low impedance or a relatively large capacitance or a relatively small inductance.

A sloping ellipse means that the component has a considerable ohmic resistance in addition to its reactance.

The values of capacitance of normal or electrolytic capacitors from 0.1μF to 1000μF can be displayed and approximate values obtained.

Inductive components (coils, transformers) can also be tested. The determination of the value of inductance needs some experience, because inductors have usually a higher ohmic series resistance.

Testing Semiconductors:-

Most semiconductor devices, such as diodes, Z-diodes, transistors and FETs can be tested. The test pattern displays vary according to the component type as shown in the figures below.
The main characteristic displayed during semiconductor testing is the voltage dependent knee caused by the junction changing from the conducting state to the non conducting state.
It should be noted that both the forward and reverse characteristic are displayed simultaneously. This is a two terminal test, therefore testing of transistor amplification is not possible, but testing of a single junction is easily and quickly possible.
More important is testing components for open or short circuit, which from experience is most frequently needed.

Testing Diodes:-

Diodes normally show at least their knee in the forward characteristic. This is not valid for some high voltage diode types, because they contain a series connection of several diodes

Possibly only a small portion of the knee is visible.
Zener diodes always show their forward knee and, depending on the test voltage, their zener breakdown forms a second knee in the opposite direction.
If the breakdown voltage is higher than the positive or negative voltage peak of the test voltage, it can not be displayed.
The polarity of an unknown diode can be identified by comparison with a known diode.

Testing Transistors:-

Three different tests can be made to transistors: base-emitter,base-collector and emitter-collector.
The resulting test pattern sare shown below. The basic equivalent circuit of a transistor is a Z-diode between base and emitter and a normal diode with reverse polarity between base and collector in series connection.
There are three different test patterns:-

For a transistor the figures b-e and b-c are important. The figure e-c can vary; but a vertical line only shows short circuit condition.
These transistor test patterns are valid in most cases, but there are exceptions to the rule (e.g. Darlington, FETs).

In-Circuit Tests

Caution!

During in circuit tests make sure the circuit is dead. No power from mains/line or battery and no signal inputs are permitted. Remove all ground connections including Safety Earth (pull out power plug from outlet). Remove all measuring cables including probes between oscilloscope and circuit under test. Otherwise both COMPONENT TESTER leads are not isolated against the circuit under test.