Hameg HM504-2, Руководство пользователя

Страница: 35 / 42

33
Subject to change without notice
reality, other deviations such as unavoidable offset voltages must
be taken into account, which may cause a display deviating from
0 Volt without signal applied at the input.
The display shows the arithmetic (linear) mean value. The DC
content is displayed if DC or AC superimposed DC voltages are
applied. In case of square wave voltages, the mean value depends
on the pulse duty factor.
Component Tester
General
The instrument specific information regarding the control
and terminals are part of item [37] in section ”Controls and
Readout”.
The instrument has a built in electronic Component Tester, which
is used for instant display of a test pattern to indicate whether or
not components are faulty. It can be used for quick checks of
semiconductors (e.g. diodes and transistors), resistors, capacitors,
and inductors. Certain tests can also be made to integrated
circuits. All these components can be tested individually, or in
circuit provided that it is unpowered.
The test principle is fascinatingly simple. A built in generator
provides a sine voltage, which is applied across the component
under test and a built in fixed resistor. The sine voltage across the
test object is used for the horizontal deflection, and the voltage
drop across the resistor (i.e. current through test object) is used
for Y deflection of the oscilloscope. The test pattern shows the
current/voltage characteristic of the test object.
The measurement range of the component tester is limited and
depends on the maximum test voltage and current (please note
data sheet). The impedance of the component under test is
limited to a range from approx. 20 Ohm to 4.7 kOhm. Below and
above these values, the test pattern shows only short circuit or
open circuit. For the interpretation of the displayed test pattern,
these limits should always be born in mind. However, most
electronic components can normally be tested without any
restriction.
Using the Component Tester
After the component tester is switched on, the Y amplifier and
the time base generator are inoperative. A shortened horizontal
trace will be observed. It is not necessary to disconnect scope
input cables unless in circuit measurements are to be carried out.
For the component connection, two simple test leads with 4 mm
Ø banana plugs, and test prods, alligator clips or sprung hooks,
are required.
The test leads are connected as described in section
”Controls and Readout”.
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.
I
Open circuit is indicated by a straight horizontal line.
I
Short circuit is shown by a straight vertical line.
Testing Resistors
If the test object has a linear ohmic resistance, both deflecting
voltages are in the same phase. 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 comparison 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.
I
A horizontal ellipse indicates a high impedance or a relatively
small capacitance or a relatively high inductance.
I
A vertical ellipse indicates a low impedance or a relatively
large capacitance or a relatively small inductance.
I
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. More precise measurement can be obtained in a
smaller range by comparing the capacitor under test with a
capacitor of known value. 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. However, the impedance
value (at 50 Hz) of an inductor in the range from 20 Ohm to
4.7 kOhm can easily be obtained or compared.
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. Since the test voltage
applied is only very low, all sections of most semiconductors can
be tested without damage. However, checking the breakdown or
reverse voltage of high voltage semiconductors is not possible.
More important is testing components for open or short circuit,
which from experience is most frequently needed.
Component Tester