In this article, we shall discuss the testing of a transistor (BJT) by using a digital multimeter (DMM). But before that let us first know a bit about transistor (BJT) and a digital multimeter (DMM) individually.
What is a Transistor?
A transistor is a
three-terminal, two-junction solid-state device that can work as either a
static switch or an amplifier. The transistor is also referred to as a Bipolar Junction Transistor (BJT).
A transistor is constructed by fusing three alternate layers
of P-type and N-type semiconductors together. Hence, a typical transistor
consists of three semiconductor layers which are named emitter-region,
base-region, and collector-region. A metal contact is attached to each layer to
form the three terminals of the transistor.
Each terminal of the transistor is named based on the
semiconductor region to which it is connected. Hence, the three terminals of
the transistor are named Emitter (E), Base (B), and Collector (C).
Depending on the construction, there are two types of
transistors namely, NPN Transistor
and PNP Transistor.
NPN Transistor:
The schematic diagram of an NPN transistor is shown in figure-1.
NPN Transistor is built by sandwiching a layer of P-type semiconductor between two layers of N-type semiconductor. Hence, in the NPN transistor, the emitter region and collector region are of N-type, and the base region is of P-type. It has two PN junctions namely, the emitter-base junction and the collector-base junction.
PNP Transistor:
The schematic diagram of a PNP transistor is shown in figure-2.
PNP Transistor is built by sandwiching a layer of N-type semiconductor between two layers of P-type semiconductor. Hence, in the PNP transistor, the emitter region and collector region are of P-type, and the base region is of N-type. It also has two PN junctions namely, the emitter-base junction and collector-base junction.
What is a Digital Multimeter?
A digital multimeter
(DMM) is an electronic testing device utilized to measure electrical circuit parameters such as current, voltage, resistance, etc. and it is also used
for testing electronic components such as transistors, diodes, electric wires,
etc. A typical digital multimeter is shown in figure-3.
A digital multimeter combines functions of different testing devices such as voltmeter, ammeter, ohmmeter, etc. into a single unit. Digital multimeters are easy and simple to use because they show the readings of measurement on a digital LCD or LED display.
The face of the digital multimeter shown in figure-3 has the
following four fundamental components:
- Display Screen – It shows readings of the measurement conducted.
- A knob or Rotary Switch – This is used to set measurement values, i.e. amps, volts, ohms, continuity, etc.
- Input Jacks – These are ports where test probes (Red is the positive probe and black is the negative probe) or leads are inserted.
Now, let us discuss the testing procedure of transistors by using a digital multimeter.
Testing of NPN Transistor with Digital Multimeter
The testing of an NPN transistor using a digital multimeter
is performed as per the following steps:
Step 1 – First,
turn on the digital multimeter, and set its knob (rotary switch) to diode mode.
Step 2 – For
testing Base to Emitter, connect the
positive probe (red) to the base terminal of the transistor, and the negative
probe (black) to the emitter terminal of the transistor. If the DMM shows a transistor
voltage value, generally ranging from 0.45 V to 0.9 V, the NPN transistor is good.
Step 3 – For
testing the Base to Collector, connect
the positive probe from the DMM to the base terminal of the transistor and
connect the negative probe from DMM to the collector terminal of the transistor.
If DMM shows a transistor voltage value, (between 0.45 V and 0.9 V), the NPN transistor is good.
Step 4 – For
testing Emitter to Base, connect the
positive probe from the DMM to the emitter terminal of the transistor and the
negative probe to the base terminal of the transistor. If the DMM shows OL
(Over Limit) on the display screen, the NPN
transistor is good.
Step 5 – For
testing Collector to Base, connect
the positive probe from the multimeter to the collector terminal of the
transistor, and the negative probe to the base terminal of the transistor. If
DMM displays OL, the NPN transistor is
good.
Step 6 – For
testing Collector to Emitter, connect
the positive probe from the multimeter to the collector terminal of the transistor,
and the negative probe to the emitter terminal of the transistor. If the DMM
reads OL (Over Limit), the NPN
transistor is good.
Step 7 – For
testing Emitter to Collector,
connect the positive probe from the digital multimeter to the emitter terminal
of the transistor and the negative probe to the collector terminal of the
transistor. If the digital multimeter reads OL (Over Limit), then the NPN transistor is good.
If the given NPN transistor measures contrary to these
steps, then the transistor is considered to be bad.
Testing of PNP Transistor with Digital Multimeter
The testing of a PNP transistor using a digital multimeter
is performed as per the following steps:
Step 1 – First,
turn on the digital multimeter, and set its knob (rotary switch) to diode mode.
Step 2 – For
testing Base to Emitter, connect the
positive probe (red) to the base terminal of the transistor, and the negative
probe (black) to the emitter terminal of the transistor. The PNP transistor is good if the DMM shows OL (Over Limit).
Step 3 – For
testing the Base to Collector, connect
the positive probe from the DMM to the base terminal of the transistor and
connect the negative probe from DMM to the collector terminal of the
transistor. If DMM shows OL (Over Limit), the PNP transistor is good.
Step 4 – For
testing Emitter to Base, connect the
positive probe from the DMM to the emitter terminal of the transistor and the
negative probe to the base terminal of the transistor. If the DMM shows the
transistor voltage drop between 0.45 V and 0.9 V, the PNP transistor is good.
Step 5 – For
testing Collector to Base, connect
the positive probe from the multimeter to the collector terminal of the
transistor, and the negative probe to the base terminal of the transistor. If
DMM displays the transistor voltage drop between 0.45 V and 0.9 V, the PNP transistor is good.
Step 6 – For
testing Collector to Emitter,
connect the positive probe from the multimeter to the collector terminal of the
transistor, and the negative probe to the emitter terminal of the transistor. The PNP transistor is good if the DMM reads OL (Over Limit).
Step 7 – For
testing Emitter to Collector,
connect the positive probe from the digital multimeter to the emitter terminal
of the transistor and the negative probe to the collector terminal of the
transistor. If the digital multimeter reads OL (Over Limit), then the PNP transistor is good.
If the given PNP transistor measures contrary to these
steps, then the transistor is considered to be bad.
Important Note
The testing of a transistor with a digital multimeter as
described above only verifies that the transistor junctions are not open or
shorted.
This test does not guarantee that the given transistor is
working within its designed parameters. Hence, the test performed to check the
transistor using a digital multimeter should only be used to decide whether the
transistor needs to be replaced or not.
Another important point to note is that this test is applicable
for BJT (Bipolar Junction Transistor).
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