How to effectively test a transistor

Transistors, developed in the middle of the 20th century, became the basis for the rapid development of electronics. By replacing electron tubes, they started an era of miniaturisation of electronic devices that continues to this day. Like any other component, the transistor may also malfunction or be damaged. In this article, we will show you how to check the correct functioning of these electronic components.

What is a transistor — basic knowledge

The simplest answer is: a transistor is a 3-pin (sometimes 4-pin), semiconductor device with the ability to amplify electrical signals and to act as a current-controlling device.

The commercialisation of transistors in the middle of the last century completely changed the development of all technologies, including electronics. The replacement of large and energy-consuming electron tubes with these miniature components (nanometre-scale transistors), as is the case with computer microprocessors, has accelerated the development of technology and brought us to our current level of technological progress.

Transistors are classified according to several criteria, with the following being the most important:

1. Bipolar and unipolar transistors.
2. Gallium and silicon transistors for very high frequency applications (silicon carbide, gallium nitride, gallium arsenide).
3. Low or high power transistors, low or high frequency transistors.

The last two concern the materials used to make the transistors and their basic parameters, and are not as important as the systematics described in the first point. The typology in point 1 is crucial because it actually describes two basic types of transistor, which differ in their operating principle: field-effect (unipolar) transistors and bipolar transistors. Of course, within each of these types there are additional sub-types of transistors (MOSFET, JFET, IGBT, etc), but the key principle of operation is the same.

In field-effect transistors, the current flows through a semiconductor of a single conductivity type; therefore the output current is a function of the control voltage. The principle of operation is simple: the semiconductor is equipped with two electrodes — source S and drain D — and the current flows along a semiconductor path (the so-called channel). An additional third electrode (G – gate) runs along the channel, which, under the influence of the applied voltage, alters the conductivity of the channel and thus affects the flowing current. In this simple way, the field effect transistor makes it possible to control the current within a given circuit.

Bipolar transistors have a more complex structure. They are made of three semiconductor layers of different types of conductivity: n or p (n – negative, p – positive). Depending on how these layers are arranged, we get two main types of bipolar transistor — PNP or NPN; however, regardless of which one we deal with, the E (emitter), B (base) and C (collector) layers are always distinguished here. The transistor allows a small current flowing between the base and the emitter to control a much larger current flowing between two other terminals. If a DC voltage flows between the terminals of a transistor in such a way that the p-type is connected with the positive terminal and the n-type is connected with the negative terminal, we get a current flow and a kind of open gate. In the case of reverse polarity, the gate closes due to high resistance and current flow is prevented.

Thanks to their amplifying properties, transistors are used, among other things, in the construction of all kinds of amplifiers. They are the basic structural element of many electronic circuits, such as current sources, generators, stabilisers or electronic torques, which started to be used in the construction of logic gates. From here it is not far to the most widely known application of transistors: in the construction of semiconductor RAM and ROM memories, ie, in microprocessors. Their implementation would not be possible without integration technology (integrated circuits), the use of which has already become widespread.

How to test a transistor — methods for testing transistor operation

In order to check the correct operation of a transistor, one of the two most popular methods can be used: checking it with a classic multimeter or using specially designed testers for testing various electronic components, including transistors. Using these methods, the transistor should be soldered out of the circuit and removed from the circuit board, although, as we will outline later, it is also possible to test these components without this step.

How to test the transistor with the meter

Such a bipolar transistor test can be performed either by switching the multimeter to ohmmeter mode (resistance test) or by switching to a diode test; in the first case, the limit should be set to 2 kohm. The next step is to determine if you are dealing with a npn or pnp transistor; technical documentation can help with this. Assuming we are dealing with a pnp type and we decide to test in ohmmeter mode, we proceed as follows:

• Connect the negative probe of the multimeter to the base output (usually it is a black probe), and the positive (red) first to the collector and then to the emitter. Obtaining a value in the range of ~500–1500 ohm confirms correct operation of the transistor.
• Connect the red probe to the base, and the black probe first to the collector and then to the emitter. For a properly functioning transistor, the multimeter should indicate that the measured value is outside the specified range.
• Both the positive and negative probes touch the pins of the transistor, which are the equivalents of the collector and emitter. The measured result should be 1, regardless of whether we applied a positive or negative probe.
• We test the resistance in both directions. Obtaining a result of 1 in both directions (resistance tends to infinity) indicates a faulty transistor. A reading of zero or near zero is interpreted identically.
   

Assuming that our transistor is the npn type, and deciding to test the diode (because this type of transistor resembles a system with two parallel diodes), we must first switch the multimeter to the appropriate mode, then connect the red diode to the base and the black one to the emitter. After this procedure, the meter should show a specific DC voltage value on its display, which should be compared with the data in the technical documentation of the tested transistor. This is to verify that the measurement obtained is between the minimum and maximum specified by the component manufacturer. If so, the transistor operates correctly.

Apart from the above tests for correct operation of a transistor, one may also measure the current gain denoted by h21, but for this purpose the meter must be equipped with a special socket for testing such elements. If this is the case, switch the device to the hFE mode, then plug the transistor terminals into the appropriate sockets marked with symbols B, E and C (base, emitter, collector) and read the measured DC gain value from the LCD screen.

MOSFETs and JFET transistors

MOSFET transistors might be tricky. In their case, we also set the meter to the “diode test” position, and then perform the following sequence:

• positive probe to the drain, negative to the gate (for about 2 seconds),
• negative probe to the source (2 seconds),
• negative probe to drain, positive to gate,
• positive probe to source.
   

After performing this combination, the last time the red probe is connected to the source — not before — some measured value should appear on the multimeter screen. If any readings appeared earlier, the tested transistor is defective. This is due to a simple fact: the gate should be isolated from the other circuits and nothing should be displayed.

With JFET transistors it is different. If you want to test them, keep in mind that they have a low resistance between drain and source, and the channel of these transistors is “pinched off” under the influence of the applied voltage. Therefore, if there is a breakdown, then we can conclude that such a transistor needs to be replaced with a new one, because the tested one is certainly defective.

It is worth adding that unipolar (field effect) transistors are sensitive to static electricity. Careless or inadequate measurement may therefore damage a previously functional component. This is even more true of IGBT transistors.

Testing transistors using electronic component testers

Multifunction electronic component testers are small devices resembling classical multimeters, used for the testing of transistors, resistors, capacitors, diodes and many other elements used in conventional electronics. They can measure voltage, resistance and several other parameters, and present the measured parameters on their displays. They are usually battery-powered (usually 9 V or 12 V), have a high level of automatic operation, have special sockets on the front and are thus very easy to use. Some have classic probes instead of sockets, but even with these everything is automatic. Simply keep any probe on any pin and the tester will automatically identify all pins, recognise the type of semiconductor junction, determine the transistor type and test the conduction voltage, cut-off voltage (for MOSFETs), leakage current, threshold voltage, and resistance, or measure the current gain.

How to test a transistor without soldering

Testing the efficiency of a transistor without removing it from the circuit is very troublesome and subject to high risk of error, because the measurement results may be affected by other elements in the circuit. For such a test to be valid, it is therefore important to know the scheme of the system and the specifics of its individual components and their interactions. However, there are devices available on the market that have a function allowing us to check the proper functioning of transistors without the need to solder them. These may include, for example, Rohde & Schwarz oscilloscopes with component test function. Importantly, the technical documentation of these meters will include diagrams showing the correct functioning of selected components.

Naturally, purchasing an oscilloscope with the component test function is associated with considerable expense, but in the case of professional services, for example, it is an excellent investment, as the component test function enables a quick comparison of the characteristics of devices whose efficiency we have no reservations about with the characteristics of those requiring repair. They are ideal diagnostic tools that will significantly reduce the time spent on repairs.

How to test a transistor with a multimeter from TME

In the TME catalogue, set the “transistor test” filter in the “portable digital multimeters” product category. We then obtain a list of those models that allow you to perform a transistor test, both using classical probes and using special pin sockets that work with pnp and npn transistors. You may be interested in products of such brands as Peaktech, B&K Precision, Axiomet, or Uni-T. All selected models are compact units with backlit liquid crystal displays, several outlets, and an easy-to-read main dial for mode selection. Most of them are also protected by heavy-duty, impact-absorbing plastic covers for increased durability.

For the original tutorial, visit: https://www.tme.eu/en/news/library-articles/page/44797/a-quick-tutorial-on-how-to-test-a-transistor/