How to test transistors with a multimeter - algorithm of actions. Checking the transistor: determining operability and basic parameters How to check whether the transistor is working

Before you begin repairing an electronic device or assembling a circuit, you should make sure that all elements that will be installed are in good condition. If new parts are used, it is necessary to ensure their functionality. The transistor is one of the main components of many electrical circuits, so it should be called first. This article will tell you in detail how to check a transistor with a multimeter.

The main component in any electrical circuit is a transistor, which, under the influence of an external signal, controls the current in the electrical circuit. Transistors are divided into two types: field-effect and bipolar.

A bipolar transistor has three terminals: base, emitter and collector. A small current is supplied to the base, which causes a change in the emitter-collector resistance zone, which leads to a change in the flowing current. The current flows in one direction, which is determined by the type of transition and corresponds to the polarity of the connection.

A transistor of this type is equipped with two p-n junctions. When electronic conductivity (n) predominates in the outer region of the device, and hole conductivity (p) predominates in the middle region, the transistor is called n-p-n (reverse conductivity). If it’s the other way around, then the device is called a p-n-p transistor (direct conduction).

Field-effect transistors have characteristic differences from bipolar ones. They are equipped with two working terminals - source and drain and one control terminal (gate). In this case, the gate is affected by voltage rather than current, which is typical for the bipolar type. Electric current flows between source and drain with a certain intensity, which depends on the signal. This signal is generated between gate and source or gate and drain. A transistor of this type can be with a control pn junction or with an insulated gate. In the first case, the working leads are connected to a semiconductor wafer, which can be p- or n-type.

The main feature of field-effect transistors is that they are controlled not by current, but by voltage. The minimal use of electricity allows it to be used in radio components with quiet and compact power supplies. Such devices may have different polarities.

How to check a transistor with a multimeter

Many modern testers are equipped with specialized connectors, which are used to test the functionality of radio components, including transistors.

To determine the operating condition of a semiconductor device, it is necessary to test each of its elements. A bipolar transistor has two p-n junctions in the form of diodes (semiconductors), which are connected back to back to the base. Hence, one semiconductor is formed by the collector and base terminals, and the other by the emitter and base.

When using a transistor to assemble a circuit board, you must clearly know the purpose of each pin. Incorrect placement of the element may cause it to burn out. Using a tester, you can find out the purpose of each pin.

Important! This procedure is only possible for a working transistor.

To do this, the device is switched to resistance measurement mode at the maximum limit. Touch the left pin with the red probe and measure the resistance at the right and middle pins. For example, the display showed values ​​of 1 and 817 Ohms.

Then the red probe should be moved to the middle, and using the black probe, measure the resistance on the right and left terminals. Here the result can be: infinity and 806 Ohms. Move the red probe to the right contact and take measurements of the remaining combination. Here, in both cases, the display will show a value of 1 ohm.

Drawing a conclusion from all measurements, the base is located on the right terminal. Now to determine other pins you need to install the black probe on the base. One pin showed a value of 817 Ohms - this is the emitter junction, the other corresponds to 806 Ohms, the collector junction.

Important! The resistance of the emitter junction will always be greater than the collector junction.

How to test a transistor with a multimeter

To ensure that the device is in good condition, it is enough to find out the forward and reverse resistance of its semiconductors. To do this, the tester is switched to resistance measurement mode and set to the limit of 2000. Next, you should ring each pair of contacts in both directions. This makes six measurements:

• the base-collector connection must conduct electric current in one direction;
• The base-emitter connection conducts electrical current in one direction;
• The emitter-collector connection does not conduct electrical current in any direction.

How to use a multimeter to test transistors whose conductivity is p-n-p (the arrow of the emitter junction is directed towards the base)? To do this, you need to touch the base with the black probe, and alternately touch the emitter and collector junctions with the red one. If they are working properly, then the tester screen will display a direct resistance of 500-1200 Ohms.

To check the reverse resistance, touch the red probe to the base, and touch the black probe alternately to the emitter and collector terminals. Now the device should show a large resistance value at both junctions, displaying “1” on the screen. This means that both junctions are working and the transistor is not damaged.

This technique allows you to solve the question: how to check a transistor with a multimeter without removing it from the board. This is possible due to the fact that the device transitions are not bypassed with low-resistance resistors. However, if during the measurements the tester shows too small values ​​of the forward and reverse resistance of the emitter and collector junctions, the transistor will have to be removed from the circuit.

Before checking the n-p-n transistor with a multimeter (the arrow of the emitter junction is directed from the base), the red probe of the tester is connected to the base to determine the direct resistance. The performance of the device is checked using the same method as a transistor with p-n-p conductivity.

A transistor malfunction is indicated by a break in one of the transitions, where a large value of forward or reverse resistance is detected. If this value is 0, the junction is open and the transistor is faulty.

This technique is suitable exclusively for bipolar transistors. Therefore, before checking, you need to make sure whether it is a composite or field device. Next, you need to check the resistance between the emitter and collector. There should be no short circuits here.

If to assemble an electrical circuit it is necessary to use a transistor that has a gain close to the current value, you can use a tester to determine the required element. To do this, the tester is switched to hFE mode. The transistor is connected to the connector corresponding to the specific type of device located on the device. The multimeter screen should display the value of parameter h21.

How to check a thyristor with a multimeter? It is equipped with three p-n junctions, which differs from a bipolar transistor. Here the structures alternate with each other in the manner of a zebra. Its main difference from a transistor is that the mode remains unchanged after the control pulse hits. The thyristor will remain open until the current in it drops to a certain value, which is called the holding current. Using a thyristor allows you to assemble more economical electrical circuits.

The multimeter is set to the resistance measurement scale in the range of 2000 Ohms. To open the thyristor, the black probe is connected to the cathode, and the red probe to the anode. It should be remembered that the thyristor can be opened by a positive and negative pulse. Therefore, in both cases, the resistance of the device will be less than 1. The thyristor remains open if the control signal current exceeds the holding threshold. If the current is less, the switch will close.

How to test an IGBT transistor with a multimeter

An insulated gate bipolar transistor (IGBT) is a three-electrode power semiconductor device in which two transistors are connected in one structure according to the cascade principle: field-effect and bipolar. The first forms the control channel, and the second – the power channel.

To test the transistor, the multimeter must be set to semiconductor testing mode. After this, use probes to measure the resistance between the emitter and the gate in the forward and reverse directions to identify a short circuit.

Now connect the red wire of the device to the emitter, and briefly touch the black wire to the gate. The gate will be charged with a negative voltage, allowing the transistor to remain off.

Important! If the transistor is equipped with a built-in back-to-back diode, the anode of which is connected to the emitter of the transistor, and the cathode to the collector, then it must be ringed accordingly.

Now you need to verify the functionality of the transistor. First, you should charge the gate-emitter input capacitance with positive voltage. For this purpose, simultaneously and briefly touch the red probe to the gate, and the black probe to the emitter. Now you need to check the collector-emitter junction by connecting the black probe to the emitter and the red probe to the collector. The multimeter screen should display a slight voltage drop of 0.5-1.5 V. This value should remain stable for several seconds. This indicates that there is no leakage in the input capacitance of the transistor.

Helpful advice! If the multimeter voltage is not enough to open the IGBT transistor, then a DC voltage source of 9-15 V can be used to charge its input capacitance.

How to check a field-effect transistor with a multimeter

Field-effect transistors are highly sensitive to static electricity, so grounding is required first.

Before you start checking the field-effect transistor, you should determine its pinout. On imported devices, marks are usually applied that identify the terminals of the device. The letter S represents the source of the device, the letter D represents the drain, and the letter G represents the gate. If there is no pinout, then you need to use the documentation for the device.

Radio amateurs know that they often have to spend a lot of time searching for faults that arise in electronic circuits for various reasons. If the circuit is assembled independently, then the final stage of the work will be to check its functionality. And you need to start with selecting known-good electronic components. Semiconductor devices are widely used in amateur radio designs. Checking a transistor, how to ring a transistor with a multimeter - these are important questions.

Types of transistors

As electronics develops, more and more varieties of this type of semiconductor devices appear. The emergence of each new group is due to increasing requirements for the operation of electronic devices and their technical characteristics.

Bipolar devices

Bipolar semiconductor transistors are the most commonly found elements of electronic circuits. Even if we consider the construction of various large microcircuits, we can see a huge number of representatives of semiconductors of this type.

The definition of “bipolar” comes from the types of electric current carriers 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 terminal, 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. The emitter and collector are the outer regions. 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 between semiconductors of different 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 a maximum area, since at high currents it is necessary to remove excess thermal energy using an external radiator to prevent overheating of the device.

In the diagrams, the emitter pin is indicated by an arrow., which determines the direction of the main current through the device. The main current is in the collector - emitter section (or emitter - collector, depending on the direction of the arrow). But it occurs only if control current flows 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 field devices require the presence of a control voltage (electric field) for current to flow through the semiconductor.

The electrodes have names: gate, source, drain. And the voltage that opens the “n” type or “p” type channel is applied to the gate area and determines the intensity of the current with 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. If you know the principles of its operation, this task is not difficult.

Bipolar type devices

Their circuit can be simplified in the form of two semiconductor diodes connected towards each other. For “p - n - p” conductivity devices, the cathodes will be connected, and for the “n - p - n” structure, the anodes of the diodes will have a common point. In any case, the connection point will be the base electrode terminal, and the other two terminals will be the emitter and collector, respectively.

For the “p - n - p” structure in the diagram, the emitter arrow is directed towards the base terminal. Accordingly, for conductivity “n - p - n” the emitter arrow will change its direction to the opposite. To determine the state of a semiconductor transistor, information about its type and, accordingly, 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 “p - n - p” conductivity devices, the open state will correspond to the connection of the “negative” (black) probe of the tester to the base terminal. The “positive” (red) tip is alternately connected to the collector and emitter. This will be a direct connection of “p - n” transitions.

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

Since a bipolar transistor is presented in the form of a back-to-back connection of two semiconductor diodes with one-way conductivity, when changing the polarity of the resistance tester probes, the “p - n” junctions of normally operating transistors will ideally tend to infinity.

The same picture should be observed when measuring the resistance between the emitter and collector terminals. Moreover, this large value does not depend on changing the polarity of the measuring probes. All this applies to working transistors.

The process of checking the serviceability (or malfunction) of a bipolar semiconductor element using a multimeter comes down to the following:

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

Determining the health of devices of the “n - p - n” structure differs only in that in order to directly switch on the transitions, it is necessary to connect the red “positive” wire of the multimeter to the base terminal, and alternately connect the black (negative) wire to the emitter and collector terminals. The picture with the resistance values ​​for this conductivity should be repeated.

Signs of a faulty bipolar transistor include the following:

• "continuity" of "p - n" transitions shows too low resistance values;
• The “p - n” transition does not “ring” in both directions.

In the first case, we can talk about an electrical breakdown of the junction, or even 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 functionality 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 must perform the following steps:

To determine the resistance of a closed device with an n-channel, touch the “source” terminal with the red wire, and the “drain” terminal with the black wire.

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 measured again (black wire - drain, red - source). The resistance of the slightly open n-channel decreases slightly compared to the first measurement.

Closing the device is achieved by applying a negative potential to its “gate” (black wire of the multimeter). After this, the resistance of the drain-source section will return to its original value.

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

Before testing field devices, it is necessary to take measures to protect against the effects of static charges, which can introduce significant difficulties into the testing process, or even completely damage the product being tested. Such proven measures include simply touching the central heating radiator with your hand. Experts use a bracelet that has antistatic properties.

When testing high-power transistors of this type, it is often possible to determine the presence of resistance when the semiconductor channel is completely closed. This means that a protective diode built into the device body is connected between the “source” and “drain”. Changing the polarity of the tester leads helps to verify this.

Checking devices in the circuit

How to test a transistor with a multimeter without desoldering it, how to test a field-effect transistor - these questions arise among radio amateurs quite often. Removing a semiconductor device from a circuit requires great care and experience. It is necessary to have in your arsenal a low-voltage soldering iron with a thin tip and a bracelet that protects against static discharges. The conductors of the printed circuit board can overheat during operation, or even accidentally short-circuit with each other.

Although, if you have experience in such work, the task is completely solvable. Of course, you need to be able to read electrical diagrams and imagine the operation of each of its components.

Assessing the performance of low- and medium-power bipolar transistors differs little from checking these elements “on the table”, when all the terminals of the device are in a position accessible for testing.

It is more difficult to check directly in the circuit of high-power devices used in the circuits of output stages of amplifiers and switching power supplies. These circuits contain elements that protect transistors from reaching the maximum permissible modes. When checking the states of “p - n” transitions in these cases, you can get absolutely incorrect results. The way out is to solder the base output.

Checking field devices can give results that are 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, to assess the condition of which it is necessary to use various special probes. But this is a topic for a separate article.

Semiconductor elements are used in almost all electronic circuits. Those who call them the most important and most common radio components are absolutely right. But any components do not last forever; overvoltage and current, temperature violations and other factors can damage them. We will tell you (without overloading with theory) how to check the performance of various types of transistors (npn, pnp, polar and composite) using a tester or multimeter.

Where to begin?

Before checking any element with a multimeter for serviceability, be it a transistor, thyristor, capacitor or resistor, it is necessary to determine its type and characteristics. This can be done by marking. Once you know it, it won’t be difficult to find a technical description (datasheet) on thematic sites. With its help, we will find out the type, pinout, main characteristics and other useful information, including replacement analogues.

For example, the scanning on the TV stopped working. Suspicion is raised by the line transistor marked D2499 (by the way, a fairly common case). Having found a specification on the Internet (a fragment of it is shown in Figure 2), we receive all the information necessary for testing.

Figure 2. Specification fragment for 2SD2499

There is a high probability that the datasheet found will be in English, no problem, the technical text is easy to understand even without knowledge of the language.

Having determined the type and pinout, we solder the part and begin testing. Below are the instructions with which we will test the most common semiconductor elements.

Checking a bipolar transistor with a multimeter

This is the most common component, for example the KT315, KT361 series, etc.

There will be no problems with testing this type; it is enough to imagine the pn junction as a diode. Then the pnp and npn structures will look like two counter- or reverse-connected diodes with a midpoint (see Fig. 3).

Figure 3. “Diode analogues” of pnp and npn junctions

We connect the probes to the multimeter, the black one to “COM” (this will be a minus), and the red one to the “VΩmA” socket (plus). We turn on the testing device, switch it to the dialing or resistance measurement mode (it is enough to set the limit to 2 kOhm), and begin testing. Let's start with pnp conductivity:

1. We attach the black probe to terminal “B”, and the red one (from the “VΩmA” socket) to leg “E”. We look at the multimeter readings; it should display the value of the junction resistance. The normal range is 0.6 kOhm to 1.3 kOhm.
2. In the same way we take measurements between terminals “B” and “K”. The readings should be in the same range.

If during the first and/or second measurement the multimeter displays minimum resistance, then there is a breakdown in the transition(s) and the part requires replacement.

1. We reverse the polarity (red and black probe) and repeat the measurements. If the electronic component is working properly, the resistance will be displayed, tending to the minimum value. If the reading is “1” (the measured value exceeds the capabilities of the device), an internal break in the circuit can be stated, therefore, the radio element will need to be replaced.

Testing a reverse conduction device follows the same principle, with a slight modification:

1. We connect the red probe to leg “B” and check the resistance with the black probe (touching terminals “K” and “E” alternately), it should be minimal.
2. We change the polarity and repeat the measurements, the multimeter will show a resistance in the range of 0.6-1.3 kOhm.

Deviations from these values ​​indicate a component failure.

Checking the functionality of the field-effect transistor

This type of semiconductor elements is also called mosfet and mosfet components. Figure 4 shows the graphic designation of n- and p-channel field switches in circuit diagrams.

Figure 4. Field-effect transistors (N- and P-channel)

To test these devices, we connect the probes to the multimeter in the same way as when testing bipolar semiconductors, and set the test type to “continuity”. Next, we proceed according to the following algorithm (for an n-channel element):

1. We touch the black wire to the “c” pin, and the red wire to the “i” pin. The resistance on the built-in diode will be displayed, remember the reading.
2. Now you need to “open” the transition (this will only be possible partially), for this we connect the probe with the red wire to terminal “z”.
3. We repeat the measurement carried out in step 1, the reading will change downwards, which indicates a partial “opening” of the field worker.
4. Now you need to “close” the component, for this purpose we connect the negative probe (black wire) to the “z” leg.
5. We repeat steps 1, the original value will be displayed, therefore, “closing” has occurred, which indicates the serviceability of the component.

To test p-channel elements, the sequence of actions remains the same, with the exception of the polarity of the probes, it must be reversed.

Note that insulated gate bipolar elements (IGBT) are tested in the same way as described above. Figure 5 shows the SC12850 component in this class.

Fig 5. IGBT transistor SC12850

For testing, it is necessary to perform the same steps as for a field-effect semiconductor element, taking into account that the drain and source of the latter will correspond to the collector and emitter.

In some cases, the potential on the multimeter probes may not be enough (for example, to “open” a powerful power transistor); in such a situation, additional power will be needed (12 volts will be enough). It must be connected through a resistance of 1500-2000 Ohms.

Checking a Composite Transistor

Such a semiconductor element is also called a “Darlington transistor”; in fact, it is two elements assembled in one package. For example, Figure 6 shows a fragment of the specification for KT827A, which displays the equivalent circuit of its device.

Figure 6. Equivalent circuit of the KT827A transistor

It will not be possible to check such an element with a multimeter; you will need to make a simple probe, its diagram is shown in Figure 7.

Rice. 7. Circuit for testing a composite transistor

Designation:

• T is the element being tested, in our case KT827A.
• L – light bulb.
• R is a resistor, its value is calculated using the formula h21E*U/I, that is, we multiply the input voltage by the minimum gain value (for KT827A - 750), divide the resulting result by the load current. Let's say we use a light bulb from the side lights of a car with a power of 5 W, the load current will be 0.42 A (5/12). Therefore, we will need a 21 kOhm resistor (750 * 12 / 0.42).

Testing is carried out as follows:

1. We connect the plus from the source to the base, as a result the light bulb should light up.
2. We apply minus - the light goes out.

This result indicates the functionality of the radio component; other results will require replacement.

How to test a unijunction transistor

Let's take KT117 as an example; a fragment from its specification is shown in Figure 8.

Fig 8. KT117, graphical representation and equivalent circuit

The element is checked as follows:

We switch the multimeter to continuity mode and check the resistance between legs “B1” and “B2”; if it is insignificant, we can state a breakdown.

How to test a transistor with a multimeter without desoldering their circuits?

This question is quite relevant, especially in cases where it is necessary to test the integrity of SMD elements. Unfortunately, only bipolar transistors can be checked with a multimeter without removing them from the board. But even in this case, one cannot be sure of the result, since there are often cases when the p-n junction of an element is shunted with a low-resistance resistance.

Often when repairing various electronic equipment, suspicion arises of a malfunction of bipolar or field-effect (Mosfet) transistors. In addition to specialized instruments and probes for testing transistors, there are methods available to everyone; the simplest tester or multimeter will do the minimum.

As we know, transistors mainly come in two varieties: bipolar and field-effect, their operating principle is similar, but the testing methods are significantly different, so we will consider different testing methods for each transistor separately.

Checking bipolar transistors

The methods for testing bipolar transistors are quite simple and for convenience you need to remember that a bipolar transistor is conventionally two diodes with a point in the middle, essentially made of two p-n junctions.

Bipolar transistors have two types of conductivity: p-n-p and n-p-n, which must be remembered and taken into account when checking.

And the diode, as we know, passes current only in one direction, which we will check.
If it turns out that the current flows in both sides of the junction, then this clearly indicates that the transistor is “broken,” but these are all conventions, in reality, when measuring resistance, there should not be “zero” resistance in any of the positions of the transitions being tested - that’s why this There is the easiest way to detect a transistor failure.
Well, now let’s look at more reliable methods of verification in more detail.

And so we set the tester or multimeter to continuity mode (checking diodes), then you need to make sure that the probes are inserted into the correct connectors (red and black), and there is no “discharged” icon on the display. The display should show one, and when the probes are closed, zeros (or values ​​close to zero) should appear, and a sound signal should also sound. And so we are convinced that the correct mode of the multimeter has been selected, we can begin testing.

And so we check all the transitions of the transistor one by one:

• Base - Emitter - a serviceable junction behaves like a diode, that is, it conducts current only in one direction.

• Base - Collector - a serviceable junction behaves like a diode, that is, it conducts current only in one direction.

• Emitter - Collector - in good condition, the resistance of the transition should be “infinite”, that is, the transition should not pass current or ring in any of the polarity positions.

Depending on the polarity of the transistor (p-n-p or n-p-n), only the direction of the “continuity” of the base-emitter and base-collector junctions depends; with different polarity of transistors, the direction will be the opposite.

How is a “broken” transition determined?
If the multimeter detects that any of the transitions (B-K or B-E) in both polarity switches has “zero” resistance and the sound indication beeps, then such a transition is broken and the transistor is faulty.

How to determine a broken p-n junction?
If one of the transitions is broken, it will not pass current and will ring in either direction of polarity, no matter how you change the polarity of the probes.

I think everyone understands how to check the transitions of a transistor, the essence of the test is the same as for diodes, we put the black (negative) probe, for example, on the collector, and the red probe (positive) on the base and look at the readings on the display. Then we swap the tester probes and look at the readings again. In a working transistor, in one case there should be some value, usually more than 100, in another case the display should show “1”, which indicates “infinite” resistance.

Checking the transistor with a dial tester

The testing principle is still the same, we check transitions (like diodes)
The only difference is that such “ohmmeters” do not have a diode continuity mode and their “infinite” resistance is in the initial state of the needle, and the maximum deviation of the needle means “zero” resistance. You just need to get used to this and remember this feature when checking.
It is best to take measurements in the “1Ohm” mode (you can try up to *1000Ohm limit).

To check in the circuit (without desoldering) Using a pointer tester, you can even more accurately determine the resistance of the junction if it is shunted in the circuit with a low-resistance resistor, for example, a resistance reading of 20 ohms will already indicate that the resistance of the junction is not “zero”, which means there is a high probability that the junction is working. With a multimeter in diode testing mode, the picture is that it will simply show “short circuit” and squeak (of course, it also depends on the accuracy of the device).

If you don’t know where the base is and where the emitter and collector are. Transistor pinout?

For medium and high power transistors, the collector output is always on the body, which is redesigned for mounting on the radiator, so this does not cause problems. And already knowing the location of the collector, it will be much easier to find the base and emitter.
Well, if there is a low-power transistor in a plastic case where all the terminals are the same, we will use this method:
All we need is to measure all combinations of transitions one by one by touching the probes alternately to different terminals of the transistor.

We need to find two transitions that will show infinity "1". For example: we found infinity between right-left and right-middle, that is, in essence, we found and measured the reverse resistance of two p-n junctions (like diodes), from which the placement of the base becomes obvious - the base is on the right.
Next we look for where the collector is and where the emitter is, for this we measure the direct resistance of the transitions from the base and here everything becomes clear since the resistance of the base-Collector junction is always less compared to the base-Emitter junction.

Fast, accurate transistor testing

If you have a multimeter at hand with a function for testing the gain of transistors, great, the test will take a few seconds, here you just need to determine the correct pinout (unless, of course, it is known).
For such multimeters, the test sockets consist of two sections p-n-p and n-p-n, and in addition, each section has three combinations of how a transistor can be inserted there, that is, together there are no more than 6 combinations, and only one correct one, which should show the gain of the transistor, for which conditions it's fine.

Simple sample

In this circuit, the transistor will work as a key; the circuit is very simple and convenient if you need to check transistors often and a lot.

If the transistor is working, when the button is pressed the LED lights up, when released it goes out.
The circuit is presented for n-p-n transistors, but it is universal, all you need to do is put another LED in reverse polarity in parallel with the LED, and when checking the p-n-p transistor, simply change the polarity of the power source.

If something goes wrong using this method, think about whether the transistor is in front of you and by chance it may not be bipolar, but field-effect or composite.
When checking, composite transistors are often confused when trying to check them in the standard way, but first of all you need to look at a reference book or “datasheet” with the entire description of the transistor.

How to test a compound transistor

To test such a transistor it is necessary to “start” it, that is, it must seem to be working; to create such a condition there is a simple but interesting way.
Using a dial tester set to resistance testing mode (limit *1000?), we connect the probes, positive to the collector, negative to the emitter - for n-p-n (for p-n-p, vice versa) - the tester's needle will not move, remaining at the beginning of the "infinity" scale (for a digital multimeter " 1")
Now if you wet the stick and close it by touching the terminals of the base and collector, the arrow will move because the transistor opens a little.
In the same way, you can test any transistor without even desoldering the circuit.
But it should be remembered that some composite transistors include protective diodes in the emitter-collector junction, which gives them an advantage when working with an inductive load, for example, an electromagnetic relay.

Checking field effect transistors

There is one distinctive point when testing such transistors - they are very sensitive to static electricity, which can damage the transistor if you do not follow safety methods when testing, as well as desoldering and moving. And it is low-power and small-sized field-effect transistors that are more susceptible to static.

What are the security methods?
The transistors should be placed on a table on a metal sheet that is connected to ground. In order to remove the maximum static charge from a person, an antistatic bracelet is used, which is worn on the wrist.
In addition, storage and transportation of particularly sensitive field devices should be with short-circuited leads; as a rule, the leads are simply wrapped with thin copper wire.

Field effect transistor as opposed to bipolar voltage controlled, and not by current like a bipolar one, so by applying voltage to its gate we either open it (for N-channel) or close it (for P-channel).

You can check the field-effect transistor using either a pointer tester or a digital multimeter.
All field-effect transistor terminals should show infinite resistance, regardless of polarity and voltage on the probes.

But if you put the positive probe of the tester to the gate (G) of an N-type transistor, and the negative one to the source (S), the gate capacitance will charge and the transistor will open. And already by measuring the resistance between the drain (D) and the source (S), the device will show a certain resistance value, which depends on a number of factors, for example, gate capacitance and junction resistance.

For the P-channel type of transistor, the polarity of the probes is reversed. Also, for the purity of the experiment, before each test it is necessary to short-circuit the leads of the transistor with tweezers to remove the charge from the gate, after which the drain-source resistance should again become “infinite” (“1”) - if this is not the case, then the transistor is most likely faulty.

A feature of modern high-power field-effect transistors (MOSFETs) is that the drain-source channel is called a diode; the built-in diode in the field-effect transistor channel is a feature of powerful field-effect transistors (a production process phenomenon).
In order not to consider such a “continuity” of the channel as a malfunction, you just need to remember about the diode.

In a working state, the drain-source junction of the MOSFET should ring in one direction like a diode and show infinity in the other (in the closed state - after shorting the terminals). If the junction rings in both directions with “zero” resistance, then such a transistor is “broken” and faulty

Visual method (express check)

• It is necessary to short-circuit the terminals of the transistor

• Using a tester in continuity mode (diode), we place the positive probe to the source, and the negative probe to the drain (a working one will show 0.5 - 0.7 volts)

• Now we swap the probes (the correct one will show “1” or, in other words, infinite resistance)

• We place the negative probe to the source, and the positive one to the gate (open the transistor)

• We leave the negative probe at the source, and immediately put the positive one at the drain, a working transistor will be open and show 0 - 800 millivolts

• Now we can swap the positive and negative probes; in reverse polarity, the drain-source junction should have the same resistance.

• We put the positive probe to the source, and the negative probe to the gate - the transistor will close

• We can check the drain-source junction again, it should again show “infinite” resistance since the transistor is already closed (but remember about the diode in reverse polarity)

The large gate capacitance of some field-effect transistors (especially powerful ones) allows us to keep the transistor open for some long time, which allows us to open it and check the drain-source resistance after removing the positive probe from the gate. But for transistors with low gate capacitance, it is necessary to move the probes very quickly to record the correct operation of the transistor.

Note: for testing P-channel field effect transistor, the process looks the same, but the multimeter probes must be of the opposite polarity. For convenience, you can switch them in places (red to minus, and black to plus) and use the same instructions described above.

When checking a transistor using this method, the drain-source channel can be opened and closed even with your finger, for example, to open it, just touch the gate with your finger while holding the plus with your other hand, and to close it, you still need to touch the gate, but already holding the other finger or second hand minus. An interesting experience that gives an understanding that the transistor is controlled not by current (like bipolar ones) but by voltage.

A simple probe circuit for testing field effect transistors

You can put together a simple and effective circuit for checking field devices that will make it clear enough about the state of the transistor; in addition, you can transfer transistors quickly enough if they need to be checked often and a lot. In some circuits, you can check the transistor even without completely desoldering it from the board.

Before considering ways to check the health of transistors, you need to know how to check the health of a p-n junction or how to properly test diodes. This is where we will start...

Semiconductor Diode Testing

When testing diodes using pointer ampere-volt-ohmmeters, the lower measurement limits should be used. When checking a working diode, the resistance in the forward direction will be several hundred Ohms, and in the reverse direction - an infinitely large resistance. If a diode malfunctions, a pointer (analog) ammeter will show a resistance in both directions close to 0 (if the diode breaks down) or an infinitely high resistance if the circuit is broken. The resistance of transitions in the forward and reverse directions is different for germanium and silicon diodes.

Diode check using digital multimeters is carried out in their testing mode. In this case, if the diode is working, the display shows the voltage at the p-n junction when measuring in the forward direction or a gap when measuring in the reverse direction. The magnitude of the forward voltage at the junction for silicon diodes is 0.5...0.8 V, for germanium diodes - 0.2...0.4 V. When checking a diode using digital multimeters in resistance measurement mode, when checking a working diode, it is usually there is a gap in both the forward and reverse directions due to the fact that the voltage at the multimeter terminals is not enough for the junction to open.

For the most common bipolar transistors, testing them is similar to testing diodes, since the structure of the pnp or npn transistor itself can be represented as two diodes (see the figure above), with the cathode or anode terminals connected together, representing the terminal of the transistor base. When testing a transistor, the forward voltage at the junction of a working transistor will be 0.45...0.9 V. Simply put, when checking the base-emitter, base-collector junctions with an ohmmeter, a working transistor in the forward direction has a small resistance and a large transition resistance in the reverse direction . Additionally, you should check the resistance (voltage drop) between the collector and emitter, which for a working transistor should be very large, except for the cases described below. However, there are some peculiarities when checking transistors. We will dwell on them in more detail.

One of the features is that some types of powerful transistors have a built-in damper diode, which is connected between the collector and emitter, as well as a resistor with a nominal value of about 50 Ohms between the base and emitter. This is typical primarily for horizontal scan output stage transistors. These additional elements disrupt the normal testing pattern. When checking such transistors, you should compare the parameters being tested with the same parameters of a known-good transistor of the same type. When checking transistors with a resistor in the base-emitter circuit with a digital multimeter, the voltage at the base-emitter junction will be close to or equal to 0 V.

Other “unusual” transistors are composite transistors connected in a Darlington circuit. Outwardly, they look like ordinary ones, but in one case there are two transistors connected according to the circuit shown in Fig. 2. They are distinguished from ordinary ones by their high gain - more than 1000.

Testing of such transistors is no different, except that the forward voltage of the base-emitter junction is 1.2...1.4 V. It should be noted that some types of digital multimeters in test mode have a terminal voltage of less than 1.2 V , which is not enough to open the p-n junction, and in this case the device shows a gap.

Testing of unijunction and programmable unijunction transistors

A unijunction transistor (UJT) is distinguished by the presence of a section with negative resistance on its current-voltage characteristic. The presence of such a section indicates that such a semiconductor device can be used to generate oscillations (OPETs, tunnel diodes, etc.).

The unijunction transistor is used in oscillator and switching circuits. First, let's look at how a unijunction transistor differs from a programmable unijunction transistor. It is not difficult:

• What they have in common is a three-layer structure (like any transistor) with 2 p-n junctions;
• A unijunction transistor has terminals called base 1 (B1), base 2 (B2), and emitter. It enters the conducting state when the emitter voltage exceeds the critical switching voltage and remains in this state until the emitter current drops to a certain value called the turn-off current. All this is very similar to the operation of a thyristor;
• A programmable unijunction transistor has terminals called anode (A), cathode (K) and control electrode (GE). According to the principle of operation, it is closer to a thyristor. It switches when the voltage at the control electrode exceeds the voltage at the anode (by approximately 0.6 V - the forward voltage of the p-n junction). Thus, by changing the voltage at the anode using a divider, you can change the switching voltage of such a device, i.e. "program" it.

To check the serviceability of a unijunction and programmable unijunction transistor, you should measure the resistance between terminals B1 and B2 or A and K with an ohmmeter to check for breakdown. But the most accurate results can be obtained by assembling a circuit for testing unijunction and programmable unijunction transistors (see the diagram below - for OPT - figure on the left, for programmable OPT - figure on the right).

Checking digital transistors

Rice. 4 Simplified circuit of a digital transistor on the left, Test circuit on the right. The arrow means "+" of the measuring device

Other unusual transistors are digital (transistors with internal bias circuits). Figure 4 above shows a diagram of such a digital transistor. The values ​​of resistors R1 and R2 are the same and can be either 10 kOhm, or 22 kOhm, or 47 kOhm, or have mixed values.

A digital transistor is no different in appearance from a regular one, but the results of its “diagnosis” can confuse even an experienced technician. For many they were “incomprehensible” and remain so. In some articles you can find the statement - “testing digital transistors is difficult... The best option is to replace it with a known-good transistor.” Undoubtedly, this is the most reliable method of verification. Let's try to figure out if this is really so. Let's figure out how to properly test a digital transistor and what conclusions to draw from the measurement results.

To begin with, let us turn to the internal structure of the transistor, shown in Fig. 4, where the base-emitter and base-collector junctions are depicted for clarity in the form of two back-to-back diodes. Resistors R1 and R2 can be of the same value, or they can be different and be either 10 kOhm, or 22 kOhm, or 47 kOhm, or have mixed values. Let the resistance of resistor R1 be 10 kOhm, and R2 - 22 kOhm. Let us take the resistance of an open silicon junction to be 100 Ohms. In particular, this value is shown by the Ts4315 dial avometer when measuring resistance at the x1 limit.

In the forward direction, the base-collector circuit of the transistor in question consists of a series-connected resistor R1 and the resistance of the base-collector junction itself (VD1 in Fig. 1). The junction resistance, since it is significantly less than the resistance of resistor R1, can be neglected, and this measurement will give a value approximately equal to the value of the resistance of resistor R1, which in our example is 10 kOhm. In the opposite direction, the junction remains closed and no current flows through this resistor. The avometer needle should show “infinity”.

The base-emitter circuit is a mixed connection of resistors R1, R2 and the resistance of the base-emitter junction itself (VD2 in Fig. 4 on the left). Resistor R2 is connected in parallel to this junction and practically does not change its resistance. Therefore, in the forward direction, when the junction is open, the ampere-volt-ohmmeter will again show a resistance value approximately equal to the resistance value of the base resistor R1. When the polarity of the tester is changed, the base-emitter junction remains closed, and current flows through the series-connected resistors R1 and R2. In this case, the tester will show the sum of these resistances. In our example it will be approximately 32 kOhm.

As you can see, in the forward direction, a digital transistor is tested in the same way as a conventional bipolar transistor, with the only difference being that the arrow of the device shows the resistance value of the base resistor. And from the difference in the measured resistances in the forward and reverse directions, you can determine the resistance value of resistor R2.

Now let's look at testing the emitter-collector circuit. This circuit consists of two back-to-back diodes, and with any polarity of the tester, its arrow should show “infinity”. However, this statement is only true for a conventional silicon transistor.

In the case under consideration, due to the fact that the base-emitter junction (VD2) turns out to be shunted by resistor R2, it becomes possible to open the base-collector junction with the corresponding polarity of the measuring device. The transistor resistance measured in this case has some scatter, but for a preliminary assessment you can focus on a value that is approximately 10 times less than the resistance of resistor R1. When changing the polarity of the tester, the resistance of the base-collector junction should be infinitely large.

In Fig. 4 on the right summarizes the above, which is convenient to use in everyday practice. For a direct conduction transistor, the arrow will indicate “-” on the measuring device.

As a measuring device, it is necessary to use pointer (analog) AVOmeters with a head deflection current of about 50 μA (20 kOhm/V).

It should be noted that the above is somewhat idealized, and in practice, there may be situations that require logical interpretation of the measurement results. Especially in cases where the digital transistor turns out to be defective.

How to test a MOSFET

There are several different ways to test MOSFETs. For example this:

• Check the resistance between gate - source (3-I) and gate - drain (3-C). It must be infinitely large.
• Connect the gate to the source. In this case, the source-drain (IS) junction must be wired as a diode (an exception for MOS transistors that have built-in breakdown protection - a zener diode with a certain opening voltage).

The most common and characteristic malfunction of MOSFETs is a short circuit between the gate-source and gate-drain.

Another way is to use two ohmmeters. The first is switched on to measure between source and drain, the second - between source and gate. The second ohmmeter should have a high input resistance - about 20 MOhm and a voltage at the terminals of at least 5 V. When the second ohmmeter is connected in direct polarity, the transistor will open (the first ohmmeter will show a resistance close to zero), and when the polarity changes to the opposite, the transistor will close. The disadvantage of this method is the voltage requirements at the terminals of the second ohmmeter. Naturally, digital multimeters are not suitable for these purposes. This limits the use of this verification method.

Another method is similar to the second. First, the gate and source terminals are briefly connected to each other in order to remove the charge present on the gate. Next, an ohmmeter is connected to the source-drain terminals. Take a 9 V battery and briefly connect it with the plus to the gate and the minus to the source. The transistor will open and remain open for some time after the battery is disconnected due to the conservation of charge. Most MOSFETs turn on at a gate-to-source voltage of about 2 V.

When testing MOSFETs, special care must be taken to avoid damaging the transistor with static electricity.

How to determine the structure and pin locations of transistors whose type is unknown

When determining the structure of a transistor, the type of which is unknown, you should, by searching through six options, determine the base terminal, and then measure the forward voltage at the transitions. The forward voltage at the base-emitter junction is always several millivolts higher than the forward voltage at the base-collector junction (when using a dial multimeter, the resistance of the base-emitter junction in the forward direction is slightly higher than the resistance of the base-collector junction). This is due to transistor manufacturing technology, and the rule applies to ordinary bipolar transistors, with the exception of some types of power transistors that have a built-in snubber diode. The polarity of the multimeter probe connected when measuring transitions in the forward direction to the base of the transistor will indicate the type of transistor: if it is “+” - a transistor of the n-p-n structure, if “-” - the p-n-p structure.