User manual

Diodes can also be measured with the multimeter's impedance function. When very
low impedances are determined, measurement takes place in flow direction, for
very high impedances, measurement takes place in reverse direction.
First build a simple LED circuit. It comprises an LED with 1 kΩ resistor. Add a
diode installed in the line strand. Put a second LED strand on the experiment
board in parallel. Now, however, install the diode in the reverse direction. You
can recognise diode direction at a ring on one side of the cylinder. Once you
have connected the battery, only one LED will be lit. the other one remains dark,
because its diode is operated in reverse direction. Remember which LED is lit
and which one is not. Then disconnect the battery from the circuit.
Now put the multimeter to impedance measuring and put the strings to both ends
of a diode as if to measure impedance. To this with both diodes in turn. When
the display shows "1---", you have measured the diode in reverse direction. If a
measured value is displayed, you have measured in passage direction.
If you have measured the reverse direction in the lit LED, this shows that the
measuring lines are attached to the diode swapped. Always observe correct
polarity when testing diodes. The red measuring line must be connected to the
ring side. Generally, however, you need to observe the current flow direction in
a circuit.
This measuring method can be used to test diodes for function. Only when they
only permit current to flow in one direction and block it in the other direction
are they OK. Any other measuring result indicates a defective component.
Figure 68: The diode test circuit looks rather complicated. However, it should
help to not only recognise what happens when the diode is installed correctly
and incorrectly, but also how to touch the measuring strings to the diode
correctly.
Figure 69: This circuit uses two simple LED circuits. One diode was installed in
each strand - one in passage and one in reverse direction.
Figure 70: This diode is operated in reverse direction. The impedance determined
is infinite.
Figure 71: If the red string is touched to the diode side marked with the ring,,
it is measured in passage direction.
Figure 72: A second option is the diode test function of the Voltcraft VC-11. If
a measured value is displayed, the diode is measured in passage direction.
18 Inspecting Transistors
Only few multimeters can be connected for precise measurement of transistors.
Nevertheless, it is possible to inspect their general function with a simple
multimeter. However,. you need to limit yourself to the statement of "works" or
"doesn't work".
Imagine a transistor comprising of two diodes, which also corresponds to its
equivalent circuit diagram. First set the multimeter for diode test. In the
Voltcraft VC-11, it is marked with the red circuit diagram of a diode. To test
an NPN transistor, apply the rated measuring string to the base connection, the
black one alternatingly to the collector and emitter. In both cases, the