How to test a transistor with a multimeter: testing various types of devices. Checking the transistor: determining performance and basic parameters How transistors are checked

When repairing and designing electronics, you often have to check the transistor for serviceability.

Let's consider a technique for testing bipolar transistors with a conventional digital multimeter, which almost every beginning radio amateur has.

Despite the fact that the technique for testing a bipolar transistor is quite simple, beginning radio amateurs can sometimes encounter some difficulties.

The features of testing bipolar transistors will be discussed a little later, but for now we will consider the simplest technology for testing with a conventional digital multimeter.

First you need to understand that a bipolar transistor can be conventionally represented as two diodes, since it consists of two p-n junctions. And a diode, as you know, is nothing more than an ordinary p-n junction.

Here is a schematic diagram of a bipolar transistor that will help you understand the principle of testing. In the figure, the p-n junctions of the transistor are depicted as semiconductor diodes.

Bipolar transistor device p-n-p The structure using diodes is depicted as follows.

As you know, bipolar transistors come in two types of conductivity: n-p-n And p-n-p. This fact must be taken into account when checking. Therefore, we will show the conditional equivalent of an n-p-n structure transistor made up of diodes. We will need this drawing for subsequent testing.

Transistor with structure n-p-n in the form of two diodes.

The essence of the method comes down to checking the integrity of these same p-n junctions, which are conventionally depicted in the figure as diodes. And, as you know, A diode allows current to flow in only one direction. If you connect plus ( + ) to the anode terminal of the diode, and minus (-) to the cathode, then the p-n junction will open and the diode will begin to pass current. If you do it the other way around, connect the plus ( + ) to the cathode of the diode, and minus (-) to the anode, then the p-n junction will be closed and the diode will not pass current.

If suddenly, when checking, it turns out that the p-n junction passes current in both directions, then it means it is “broken”. If the p-n junction does not pass current in any direction, then the junction is in an open circuit. Naturally, if there is a breakdown or break in at least one of the p-n junctions, the transistor will not work.

Please note that conditional diagram of diodes is necessary only for a more visual representation of the transistor testing technique. In reality, the transistor has a more sophisticated design.

The functionality of almost any multimeter supports diode testing. On the multimeter panel, the diode test mode is depicted in the form of a symbolic image that looks like this.

I think it’s already clear that we will be checking the transistor using this function.

A little clarification. The digital multimeter has several sockets for connecting test leads. Three, or even more. When checking a transistor, you need a negative probe ( black) connect to the socket COM(from the English words common– “general”), and the positive probe ( red) into the slot marked with the letter Omega Ω , letters V and possibly other letters. It all depends on the functionality of the device.

Why do I talk in such detail about how to connect test leads to a multimeter? Yes, because the probes can simply be mixed up and you can connect the black probe, which is conventionally considered “negative,” to the socket to which you need to connect the red, “positive” probe. This will ultimately cause confusion and, as a result, errors. Be careful!

Now that the dry theory has been outlined, let's move on to practice.

What multimeter will we use?

First, we will check a domestically produced silicon bipolar transistor KT503. It has structure n-p-n. Here is its pinout.

For those who do not know what this obscure word means pinout, I’ll explain. Pinout is the location of the functional pins on the body of the radio element. For a transistor, the functional terminals will be the collector ( TO or English- WITH), emitter ( E or English- E), base ( B or English- IN).

First we connect red (+ ) probe to the base of the KT503 transistor, and black(-) probe to the collector terminal. This is how we check the operation of the p-n junction in direct connection (that is, when the junction conducts current). The breakdown voltage value appears on the display. In this case it is equal to 687 millivolts (687 mV).

As you can see, the p-n junction between the base and emitter also conducts current. The display again shows the breakdown voltage value equal to 691 mV. Thus, we checked the B-K and B-E transitions when connected directly.

To make sure that the p-n junctions of the KT503 transistor are in good condition, let’s check them in the so-called reverse switching. In this mode, the pn junction does not conduct current, and the display should show nothing other than “ 1 " If the display shows " 1 ", this means that the junction resistance is high and it does not allow current to pass through.

To check the p-n junctions B-K and B-E in reverse connection, we change the polarity of connecting the probes to the terminals of the KT503 transistor. We connect the negative (“black”) probe to the base, and the positive (“red”) probe first connects to the collector terminal...

...And then, without disconnecting the negative probe from the base output, to the emitter.

As we can see from the photographs, in both cases the display showed one “ 1 ", which, as already mentioned, indicates that the p-n junction does not pass current. So we checked the transitions B-K and B-E in reverse switching.

If you carefully followed the presentation, you will have noticed that we tested the transistor according to the previously outlined method. As you can see, the KT503 transistor turned out to be working.

Breakdown of the P-N junction of the transistor.

If any of the junctions (B-K or B-E) are broken, then when checking them on the multimeter display, it will be found that in both directions, both in direct connection and in reverse, they show a non-breakdown voltage of the p-n junction, and resistance. This resistance is either equal to zero “0” (the buzzer will squeak) or will be very small.

Broken P-N junction of the transistor.

If there is a break, the p-n junction does not pass current either in the forward or reverse direction - the display in both cases will show “ 1 " With such a defect, the pn junction turns into an insulator.

Testing bipolar transistors of the p-n-p structure is carried out in a similar way. But at the same time polarity needs to be changed connecting measuring probes to the terminals of the transistor. Let us recall the drawing of a conventional image of a p-n-p transistor in the form of two diodes. If you forgot, then look again and you will see that the cathodes of the diodes are connected together.

As a sample for our experiments, we will take a domestic silicon transistor KT3107 p-n-p structures. Here is its pinout.

In the pictures, checking the transistor will look like this. We check the B-K transition when connected directly.

As you can see, the transition is correct. The multimeter showed the breakdown voltage of the junction - 722 mV.

We do the same for the B-E transition.

As you can see, it is also working properly. The display shows 724 mV.

Now let's check the serviceability of the transitions in the opposite direction - for the presence of a “breakdown” of the transition.

Transition B-K when switched back on...

Transition B-E during reverse switching.

In both cases, the device display shows one “ 1 " The transistor is OK.

Let's summarize and describe a short algorithm for checking a transistor with a digital multimeter:

Determination of the transistor pinout and its structure;

Checking transitions B-K and B-E in direct connection using the diode test function;

Checking the B-K and B-E transitions in reverse connection (for the presence of a “breakdown”) using the diode test function;

When checking, it is necessary to remember that in addition to conventional bipolar transistors, there are various modifications of these semiconductor components. These include compound transistors (Darlington transistors), “digital” transistors, line transistors (the so-called “line transistors”), etc.

They all have their own characteristics, such as built-in protective diodes and resistors. The presence of these elements in the structure of the transistor sometimes complicates their testing using this technique. Therefore, before checking a transistor unknown to you, it is advisable to read the documentation for it (datasheet). I talked about how to find a datasheet for a specific electronic component or microcircuit.

In the process of repairing electronics, it is often necessary to check the functionality of the most common radio components - transistors.

There is a device specially designed for this - R/L/C/Transistor-meter, but it is not always available.

Therefore, it is useful to know how to test transistors, which will be discussed below.

A transistor consists of materials with special electrical properties - semiconductors. The latter are of two types:

with n-conductivity (electronic);
with p-conductivity (hole).

The simplest representative of semiconductor elements is a diode containing one p-n junction.

Transistors are more complex. There are two types of them: bipolar and field.

Bipolar

Also divided into two subgroups:

with n-p-n junction;
with p-n-p junction.

The components of a bipolar transistor are called emitter, collector and base. If we imagine this element in the form of two connected diodes, then the base will be their junction point.

To check a bipolar device, you need to recognize its type (n-p-n or p-n-p) and determine the purpose of the terminals (base, emitter and collector).

Field

Also divided into two types:

n-channel;
p-channel.

In a field-effect transistor, the resistance of the current-carrying section is regulated by the electric field.

The components of the element are called source, drain and gate. The current moves from source to drain, regulation is carried out by the gate.

The design of modern field-effect transistors is supplemented by a diode installed between the source and drain.

Determining the base (gate) output

The easiest way to determine the purpose of the transistor's terminals (pinout) is to download the documentation for it. The search is carried out using markings on the body. This alphanumeric code is typed into the search bar and then “datasheet” is added.

If the documentation cannot be found, the base and other terminals of the bipolar transistor are recognized based on its features:

pnp transistor: opens by applying negative to the base;
npn transistor: opens by applying a positive voltage to the base.

They work like this:

Set up the multimeter: the red probe is connected to the connector with the “V/ Ω " (positive potential), black - to the COM connector (negative potential), and the switch is set to the "continuity" mode or, if this is not the case, to the resistance measurement sector (icon " Ω ") to the top position (usually "2000 ohms").
Define the base. The red probe is connected to the first terminal of the transistor, the black one - alternately to the others. Then red is connected to the second terminal, black again in turn to the 1st and 3rd. A sign that the red one is connected to the base is the same behavior of the device when the black probe comes into contact with other terminals. The device beeped both times or showed a certain final resistance on the display - the transistor is of the n-p-n type; The device was silent both times or displayed “1” on the display (no conductivity) - the transistor belongs to the pnp type.
Collector and emitter recognition. To do this, connect a probe corresponding to the type of conductivity to the base: for an n-p-n transistor - red, for a p-n-p transistor: black.

Design of a field-effect transistor with a control p-n junction and an n-type channel a) with a gate on the substrate side; b) with diffusion seal

The second probe is alternately connected to the other terminals. When contacting the collector, the display shows a lower resistance value than contacting the emitter.

The terminals of the field-effect transistor are usually marked:

G: shutter;
S: source;
D: drain.

If there is no marking, the gate is detected in the same way as a bipolar transistor.

Field effect transistors are sensitive to static electricity. Because of this, their terminals are short-circuited with foil during storage, and before starting manipulations, wear an antistatic bracelet or at least touch a grounded metal object (instrument cabinet) to remove the static charge.

Checking the transistor with a multimeter

If the purpose of the terminals is known, the bipolar transistor is checked as follows:

Prepare the multimeter as described above: the switch is moved to the “2K” position in the “ Ω "(resistance measurement) or in continuity mode, the black probe is plugged into the "COM" connector, the red one into the "V/ Ω ».
Connect the probes to the emitter and collector, then swap them. Normally, in both cases the device does not produce a signal and displays “1”. Some terminal resistance indicates a breakdown.
Connect a probe corresponding to its type of conductivity to the base: “hole” base (n-p-n type transistor) – red probe, “electronic” (p-n-p type transistor) – black.
The second probe is connected in turn to the emitter and collector. Test results: the multimeter emits a signal, the display shows a resistance from 500 to 1200 Ohms - the transistor is working; there is no signal and the display shows one - an open circuit.
Another probe is connected to the base, and the second one is short-circuited in turn with the emitter and collector. Results: no signal, “1” on the display – the transistor is working; the device beeps, the display shows a certain final resistance value - the transistor is broken.

The field device is checked as follows:

Static electricity is removed from the element.
Set up the multimeter according to the usual scheme: black probe - into the “COM” port; red - to port “V/ Ω "; switch - to position “2K” of sector “ Ω "(resistance measurement).
Check the resistance between drain and source: normally the tester displays 400 - 700 Ohms.
The source and drain are short-circuited in order to reset the junction capacitances, after which the polarity is changed and the measurements are repeated. If the transistor is working properly, the readings change up or down by about 10% (40 - 70 Ohms). An infinitely high resistance between the source and drain (the display shows “1”) indicates a malfunction of the device.
Check for one-way continuity between source and gate, then between drain and gate. With one polarity of measurements, the multimeter will show a resistance of 400 - 700 Ohms, with the other - unity. Which probe is connected to the gate depends on the type of transistor (n-channel or p-channel). If the conductivity on the drain-gate or source-gate lines is two-way, that is, the device displays a certain final resistance value at any polarity, the transistor is broken.
When checking an n-channel field switch, the black probe is connected to the drain, the red probe to the source. The channel resistance value is recorded.
The red probe is connected to the gate, which will lead to a partial opening of the transition.
Return the red probe to the source and measure the channel resistance. If the transistor is working properly, the resistance will decrease (due to partial opening).
The black probe is connected to the gate, which will close the transition.
Return the black probe to the drain and measure the resistance. If the transistor is working properly, it takes on the original value that was recorded.

Transistor test circuit

Check points 6 - 10 for a p-channel field-effect transistor are performed with the opposite polarity - swapping the red and black probes.

The voltage generated by the multimeter is not enough to open the power transistors. In this case, a 12 V power supply is used, connected through a resistor with a resistance of 1500 - 2000 Ohms.

Testing without soldering

A bipolar transistor can be checked without soldering if the circuit is not shunted with low-resistance resistors. Otherwise, instead of a resistance of 500 - 1200 Ohms, the multimeter will show only a few tens or even units. Then it is required.

Field-effect transistors are almost always bypassed, so they have to be desoldered before testing.

Gain Determination

When a device fails, another one with a similar gain is selected to replace it. To determine this parameter, you need a multimeter with a transistor testing function. On the switch panel of such a device there is a sector marked “hFE”. It has two rows of ports of three each, which are designated as follows:

n-p-n;
p-n-p.

Field effect transistor testing circuit

This is a type of bipolar transistor that needs to be connected to a given row of ports. The purpose of each port is judged by its letter designation:

B: base;
C: manifold;
E: emitter.

By connecting the transistor leads to the corresponding ports of the appropriate row, the user sees the gain value on the display.

Checking a Composite Transistor

A compound transistor includes two conventional bipolar transistors, and sometimes more. The standard method of checking with a multimeter is not applicable to it. It is necessary to assemble an electrical circuit powered from a constant 12 V power source. “Plus” is connected through a light bulb to the collector, “minus” - to the emitter. The base is connected through a resistor to a switch, which allows you to apply either “plus” or “minus” to it.

The resistor resistance is calculated using the formula:

R = U x h21E /I,

U - input voltage, V;
H21E - minimum gain of this transistor;
I - load current, A.

Consider the following example:

tested composite transistor: KT827A (h21E = 750);
lamp power: 5 W.

The load current will be: I = 5 / 12 = 0.42 A.

Then the resistor resistance: R = 12 * 750 / 0.42 = 21600 Ohm, take R = 21 kOhm.

The verification is carried out in two stages:

Using a switch, “plus” is supplied to the base. If it is working properly, the light will come on.
The switch short-circuits the base to minus.

If it is working properly, the light will go out.

Even the simplest multimeter, which is not equipped with a function for determining the parameters of semiconductor devices, will help check the performance of the transistor. If you need to select an equivalent one instead of a burnt-out transistor, you will have to look for a tester model with the mentioned function.

Testing transistors is an important point in electronics and radio engineering. Try to figure out on your own how to test a transistor with a multimeter without desoldering. This is a fairly simple procedure that can be performed in various ways. The most practical option is to check the transistor with a multimeter. It is this method that will be discussed in this article.

General information

Today there are two types of transistors - bipolar and field-effect. In the first, the output current is created with the participation of both charges in the form of holes and electrons, while in the other version only one of the carriers participates.

Checking the bipolar transistor

Specified procedure for bipolar transistors It starts with proper setup of the device. The device is switched to semiconductor testing mode; one should appear on the display. The terminals are connected in a similar way to the resistance measurement mode. A black wire is connected to the COM port, and a red wire is connected to the output to measure voltage, resistance and frequency. If the multimeter does not have a corresponding mode, then the process should be carried out in resistance measurement mode when set to maximum.

It is also important that the multimeter battery is fully charged and the probes are in good working order. When connecting the tips, serviceability is indicated by the squeak of the device and zeros on the screen. The procedure in this case follows the following steps:

As a result, there is no need to solder the element to ensure its serviceability. If you want to use for checking light bulbs and other elements, it is not recommended to do this, since there is a risk of permanently damaging the bipolar transistor.

Field device test

Procedure for such elements similar to bipolar. However, there are some peculiarities here:

Due to these points, it is possible to carry out a high-quality check of field devices without involving desoldering. If you have a composite device, then the test is similar to the procedure for bipolar devices.

Advantage of the method

Testing a transistor using a multimeter is advantageous in that there is no need to solder the element, and it is quite accurate. The methodology for testing bipolar and field devices is similar, but it is necessary to take into account a number of points and nuances that help improve the methodology. Proper setting of the multimeter and the ability to work with various elements will allow you to perform the most accurate and high-quality check of the serviceability of devices of any type.

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.

Useful 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.

A transistor is a semiconductor device whose main purpose is to be used in circuits to amplify or generate signals, as well as for electronic switches.

Unlike a diode, a transistor has two pn junctions connected in series. Between the transitions there are zones with different conductivities (type “n” or type “p”), to which the terminals for connection are connected. The output from the middle zone is called the “base”, and from the extreme ones - the “collector” and “emitter”.

The difference between the “n” and “p” zones is that the first has free electrons, and the second has so-called “holes”. Physically, a "hole" means there is a lack of an electron in the crystal. Electrons, under the influence of the field created by a voltage source, move from minus to plus, and “holes” - vice versa. When regions with different conductivities are connected to each other, electrons and “holes” diffuse and a region called a p-n junction is formed at the boundary of the connection. Due to diffusion, the “n” region turns out to be positively charged, and the “p” region is negatively charged, and between regions with different conductivities, an own electric field arises, concentrated in the region of the p-n junction.

When the positive terminal of the source is connected to the “p” region, and the negative terminal to the “n” region, its electric field compensates for the own field of the p-n junction, and an electric current passes through it. When connected in reverse, the field from the power source is added to its own, increasing it. The junction is locked and no current passes through it.

The transistor contains two junctions: collector and emitter. If you connect the power source only between the collector and emitter, then no current will flow through it. One of the passages turns out to be locked. To open it, potential is applied to the base. As a result, a current arises in the collector-emitter section, which is hundreds of times greater than the base current. If the base current changes over time, then the emitter current exactly repeats it, but with a larger amplitude. This is what determines the reinforcing properties.

Depending on the combination of alternating conduction zones, p-n-p or n-p-n transistors are distinguished. P-n-p transistors open when the base potential is positive, and n-p-n transistors open when the base potential is negative.

Let's look at several ways to test a transistor with a multimeter.

Checking the transistor with an ohmmeter

Since the transistor contains two p-n junctions, their serviceability can be checked using the method used for testing semiconductor diodes. To do this, it can be thought of as the equivalent of a back-to-back connection of two semiconductor diodes.

The serviceability criteria for them are:

Low (hundreds of Ohms) resistance when connecting a DC source in the forward direction;
Infinitely high resistance when connecting a DC source in the reverse direction.

A multimeter or tester measures resistance using its own auxiliary power source - a battery. Its voltage is small, but it is enough to open the pn junction. By changing the polarity of connecting the probes from the multimeter to a working semiconductor diode, in one position we get a resistance of a hundred Ohms, and in the other - infinitely large.

A semiconductor diode is rejected if

in both directions the device will show a break or zero;
in the opposite direction, the device will show any significant resistance value, but not infinity;
The device readings will be unstable.

When checking a transistor, six resistance measurements with a multimeter will be required:

base-emitter direct;
base-collector direct;
base-emitter reverse;
base-collector reverse;
emitter-collector direct;
emitter-collector reverse.

The criterion for serviceability when measuring the resistance of the collector-emitter section is an open circuit (infinity) in both directions.

Transistor Gain

There are three schemes for connecting a transistor to amplifier stages:

with a common emitter;
with a common collector;
with a common base.

They all have their own characteristics, and the most common is the common emitter circuit. Any transistor is characterized by a parameter that determines its amplification properties - gain. It shows how many times the current at the output of the circuit will be greater than at the input. For each of the switching schemes there is its own coefficient, different for the same element.

The reference books give the coefficient h21e - the gain factor for a circuit with a common emitter.

How to Test a Transistor by Measuring Gain

One of the methods for checking the health of a transistor is to measure its gain h21e and compare it with the passport data. The reference books give the range in which the measured value can be for a given type of semiconductor device. If the measured value is within the range, then it is working.

The gain is also measured to select components with the same parameters. This is necessary for building some amplifier and oscillator circuits.

To measure the h21e coefficient, the multimeter has a special measurement limit designated hFE. The letter F stands for “forward” (straight polarity), and the “E” stands for common emitter circuit.

To connect the transistor to the multimeter, a universal connector is installed on its front panel, the contacts of which are marked with the letters “EVSE”. According to this marking, the terminals of the transistor “emitter-base-collector” or “base-collector-emitter” are connected, depending on their location on a particular part. To determine the correct location of the pins, you will have to use a reference book, where you can also find out the gain factor.

Then we connect the transistor to the connector, selecting the measurement limit of the multimeter hFE. If its readings correspond to the reference values, the electronic component being tested is operational. If not, or the device shows something unintelligible, the transistor has failed.

Field effect transistor

A field-effect transistor differs from a bipolar transistor in its operating principle. Inside the crystal plate of one conductivity (“p” or “n”), a section with a different conductivity, called a gate, is introduced in the middle. At the edges of the crystal, pins are connected, called source and drain. When the gate potential changes, the size of the current-carrying channel between the drain and the source and the current through it change.

The input resistance of the field-effect transistor is very high, and as a result it has a high voltage gain.

How to test a field effect transistor

Let's consider testing using the example of a field-effect transistor with an n-channel. The procedure will be as follows:

We switch the multimeter to diode testing mode.
We connect the positive terminal from the multimeter to the source, and the negative terminal to the drain. The device will show 0.5-0.7 V.
Change the polarity of the connection to the opposite. The device will show a break.
We open the transistor by connecting the negative wire to the source and touching the gate with the positive wire. Due to the existence of the input capacitance, the element remains open for some time; this property is used for testing.
We move the positive wire to the drain. The multimeter will show 0-800 mV.
Change the polarity of the connection. The device readings should not change.
We close the field-effect transistor: the positive wire to the source, the negative wire to the gate.
We repeat points 2 and 3, nothing should change.