Transistors and their test with a multimeter; how to check a transistor with a tester without unsoldering

Radio amateurs know that often a lot of time has to be spent on troubleshooting electronic circuits for various reasons. If the circuit is assembled independently, then the final stage of the work will be to check its operability. And you need to start with the selection of known serviceable electronic components. Semiconductor devices are widely used in amateur radio designs. Checking a transistor, how to ring a transistor with a multimeter are important questions.

Types of transistors

There are more and more varieties of this type of semiconductor devices with the development of electronics. The emergence of each new group is due to the increased requirements for the operation of electronic devices and their technical characteristics.

Bipolar devices

Bipolar semiconductor transistors are the most common elements in electronic circuits. Even if you consider the construction of various large microcircuits, you can see a huge number of representatives of this type of semiconductor.

The definition of "bipolar" comes from the types of carriers of electric current that are present in them. This current is determined by the movement of negative and positive charges in the body of the semiconductor.

Each area of ​​the three-layer structure has its own metal lead, with the help of which the device is connected to other elements of the electronic circuit. These pins have their own names: emitter, base, collector. Emitter and collector are outer areas. The inner area is the base.

Bipolar transistors form two groups depending on the type of semiconductor. They are designated "p - n - p" and "n - p - n". The areas of contact of semiconductors of various types are called "p - n" junctions.

The base area is the thinnest. Its thickness determines the frequency properties of the device, that is, the maximum frequency of the radio signal at which the transistor can operate as an amplifying element. The collector area has the maximum area, since at high currents it is necessary to remove excess heat energy using an external radiator to prevent overheating of the device.

In the diagrams, the emitter terminal is indicated by an arrow, which determines the direction of the main current through the device. The main current is the collector - emitter (or emitter - collector, depending on the direction of the arrow). But it occurs only in the case of a control current flowing in the base circuit. The ratio of these currents determines the amplifying properties of the transistor. Thus, a bipolar transistor is a current device.

Field effect transistors

Transistors of this type differ significantly from bipolar devices. If the latter are devices controlled by a weak base current of a certain polarity, then the field devices for the flow of current through a semiconductor require a control voltage (electrical fields).

The electrodes are named: gate, source, drain. And the voltage that opens the channel "n" type or "p" type is applied to the gate area and determines the intensity of the current at the correct polarity. These devices are also called unipolar.

Checking with a multimeter

Transistors are active elements of an electronic circuit. Their serviceability determines its correct operation. How to check a transistor with a tester - this question is important. With knowledge of the principles of its work, this task is not difficult.

Bipolar devices

Their circuit can be simplified as two semiconductor diodes connected towards each other. For "p - n - p" devices, the cathodes will be connected, and for the "n - p - n" structure, the diode anodes will have a common point. In any case, the junction point will be the lead of the base electrode, and the other two leads, respectively, the emitter and collector.

For the structure "p - n - p" in the diagram, the arrow of the emitter is directed to the output of the base. Accordingly, for conductivity "n - p - n", the arrow of the emitter will change its direction to the opposite. To determine the state of a semiconductor transistor, information about its type and, accordingly, about the marking of its electrodes is of great importance. This information can be found from numerous reference books or from communication on thematic forums.

For bipolar devices "p - n - p" of conductivity, the open state will correspond to the connection of the "minus" (black) probe of the tester to the base output. The “positive” (red) tip is alternately connected to the collector and emitter. This will be a direct inclusion of "p - n" transitions.

In this case, the resistance of each will be in the range (600-1200) Ohm. The exact value depends on the manufacturer of the electronic components. The collector junction resistance will be slightly less than the emitter junction resistance.

Since the bipolar transistor is presented in the form of an opposite connection of two semiconductor diodes with one-sided conductivity, then at changing the polarity of the probes of the resistance tester "p - n" junctions in normally operating transistors will ideally tend to infinity.

The same pattern should be observed when measuring the resistance between the emitter and collector leads. Moreover, this large value does not depend on the polarity change of the measuring probes. All this applies to serviceable transistors.

The process of checking the health (or malfunction) of a bipolar semiconductor element using a multimeter is as follows:

• determination of the type of device and the diagram of its outputs;
• checking the resistance of its "p - n" junctions in the forward direction;
• changing the polarity of the probes and determining the resistance of the transitions with such a connection;
• checking the collector-emitter resistance in both directions.

Determination of the serviceability of devices "n - p - n" structure differs only in that for the direct inclusion of transitions to the output of the base it is necessary to connect the red "positive" wire of the multimeter, and alternately connect the black one to the emitter and collector terminals (negative). The picture with the resistance values ​​for this conductivity should be repeated.

Signs of malfunction of bipolar transistors include the following:

• "Continuity" of "p - n" junctions shows too low resistance values;
• "P - n" junction is not "ringing" in both directions.

In the first case, we can talk about an electrical breakdown of the junction, or even about a short circuit.

The second case shows an internal break in the structure of the device.

In both cases, this instance cannot be used to work in the circuit.

Field effect transistors

To check the performance of this element, we use the same multimeter as for the bipolar device. It must be remembered that field workers can be n-channel and p-channel.

To check an element of the first type, you need to do the following:

• determine the resistance of the "drain - source" section of the closed transistor;
• open the transition;
• determine the resistance of an open field worker;
• close the transition;
• re-measure the resistance of the closed field-effect transistor.

To determine the resistance of a closed device with an n-channel, the red wire touches the "source" output, and the black wire - the "drain".

The field device is opened by applying a positive potential to its gate (red wire).

To check the open state of the transistor, the resistance of the "drain - source" section is re-measured (black wire - drain, red - source). The resistance of the ajar n-channel decreases slightly compared to the first measurement.

The device is closed by applying a negative potential to its "gate" (black wire of the multimeter). After that, the resistance of the "drain - source" section will return to its original value.

When checking a p-channel device, repeat all the previous steps, reversing the polarity of the tester's measuring probes.

Before checking field devices, take steps to protect against static electricity, which can introduce significant difficulties in the verification process, or even completely remove the tested product from building. These proven measures include simply touching the central heating battery with your hand. Specialists use an anti-static bracelet.

When testing high-power transistors of this type, it is often possible to determine the presence of resistance when the semiconductor channel is completely blocked. This means that between the "source" and "drain" there is a protective diode built into the body of the device. You can verify this by changing the polarity of the tester leads.

Checking devices in the circuit

How to check a transistor with a multimeter without soldering, how to check a field-effect transistor - these questions arise among radio amateurs quite often. Removing a semiconductor device from a circuit requires great care and experience. You must have in your arsenal a low-voltage soldering iron with a thin tip, a bracelet that protects against static discharges. The conductors of the printed circuit board can be overheated during operation, or even accidentally short-circuited among themselves.

Although with experience in such work - the task is quite solvable. Of course, you need to be able to read electrical diagrams and imagine the work of each of its components.

Evaluation of the performance of bipolar transistors of low and medium power differs little from checking these elements "on the table", when all the terminals of the device are in an accessible position for checking.

It is more difficult to check directly in the circuit of high-power devices used in the circuits of the output stages of amplifiers, switching power supplies. In these circuits, there are elements that protect the transistors from the latter reaching the maximum allowable modes. When checking the states of "p - n" transitions in these cases, you can get absolutely incorrect results. As a way out - desoldering the output of the base.

Inspection of field devices can give a result that is far from the real state of affairs. The reason is the presence in the circuits of a large number of elements for correcting the operation of transistors, including low resistance inductors.

There are still a large number of different types of transistors, for assessing the state of which you have to use various special probes. But this is a topic for a separate material.