Checking field-effect transistors without soldering. How to test different types of transistors with a multimeter? How to test a transistor without desoldering it from the circuit

Exist two types of bipolar transistors: PNP-transistor and NPN-transistor.

In the picture below structural scheme PNP transistor:

The schematic designation of a PNP transistor in a circuit looks like this:

where E is the emitter, B is the base, K is the collector.

There is also another type of bipolar transistor: NPN transistor. Here the material P is already enclosed between two materials N.

Here is its schematic representation on the diagrams

Since the diode consists of one PN junction, and the transistor consists of two, it means you can imagine a transistor as two diodes! Eureka!

Now you and I can check the transistor by checking these two diodes, of which, roughly speaking, the transistor consists. You can read how to check a diode.

Checking the working transistor

Well, let's determine in practice the performance of our transistor. And here is our patient:

We carefully read what is written on the transistor: C4106. Now open the search engine and look for a document describing this transistor. In English it is called “datasheet”. Just like that, we type “C4106 datasheet” into the search engine. Keep in mind that imported transistors are written in English letters.

We are most interested in the pinout of the transistor's terminals, as well as its type: NPN or PNP. That is, we need to find out what conclusion is what. For a given transistor, we need to find out where its base is, where its emitter is, and where its collector is.

And here is the pinout diagram from the datasheet:

Now we understand that the first pin is the base, the second pin is the collector, and the third is the emitter

Let's return to our drawing

We learned from the datasheet that our transistor is NPN conductive.

We set the multimeter to test and start checking the “diodes” of the transistor. To begin with, we put “plus” to the base, and “minus” to the collector

Everything is OK, the direct PN junction should have a small voltage drop. For silicon transistors this value is 0.5-0.7 Volts, and for germanium transistors 0.3-0.4 Volts. In the photo there are 543 millivolts or 0.54 volts.

We check the base-emitter transition by placing a “plus” on the base and a “minus” on the emitter.

We again see the voltage drop of the direct PN junction. All OK.

We swap the probes. We put “minus” on the base, and “plus” on the collector. Now we measure the reverse voltage drop at the PN junction.

Everything is OK, since we see one.

We now check the reverse voltage drop of the base-emitter junction.

Here our multimeter also shows one. This means that the transistor can be diagnosed as healthy.

Checking the faulty transistor

Let's check one more transistor. It is similar to the transistor that we discussed above. Its pinout (that is, the position and meaning of the pins) is the same as that of our first hero. We also set the multimeter for continuity and cling to our ward.

Zeros... This is not good. This indicates that the PN junction is broken. You can safely throw such a transistor in the trash.

Checking the transistor using a transistor meter

It is very convenient to check transistors, having

Conclusion

In conclusion of the article, I would like to add that it is always better to find a datasheet for the transistor being tested. There are so-called composite transistors. This means that two or more transistors can be mounted in one transistor housing. Please also keep in mind that some radio elements have the same housing as transistors. These could be thyristors, voltage converters, or even some foreign 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:

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

The simplest representative semiconductor elements is a diode containing one pn junction.

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

Bipolar

Also divided into two subgroups:

1. with n-p-n junction;
2. with p-n-p junction.

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

1. n-channel;
2. 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.

Modern design field effect transistors supplemented with 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:

1. Set up the multimeter: the red probe is connected to the connector with the “V/” icon 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").
2. 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.
3. 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 using the same circuit as that of 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:

1. 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 ».
2. 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.
3. Connect a probe corresponding to its type of conductivity to the base: “hole” base (n-p-n type transistor) - red probe, “electronic” (transistor p-n-p type) - black.
4. 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.
5. 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:

1. Static electricity is removed from the element.
2. 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).
3. Check the resistance between drain and source: normally the tester displays 400 - 700 Ohms.
4. 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.
5. 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.
6. 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.
7. The red probe is connected to the gate, which will lead to a partial opening of the transition.
8. 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).
9. The black probe is connected to the gate, which will close the transition.
10. 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:

1. n-p-n;
2. p-n-p.

Field effect transistor testing circuit

This is a type of bipolar transistor that needs to be connected to this series 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 transistor, 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:

1. Using a switch, “plus” is supplied to the base. If it is working properly, the light will come on.
2. 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.

Seal

The fastest and effective way checking the serviceability of transistors is checking (the continuity) of its transitions with a multimeter, although in some cases this does not give a 100% guarantee, but more on that below.

So, how to test a transistor with a multimeter.

A transistor can be represented as two diodes connected in opposite directions (p-n-p - forward) and in the opposite direction (n-p-n - reverse). On circuit diagrams The structure of transistors is indicated by an emitter junction arrow. If the arrow is directed towards the base, then it is a p-n-p structure, and if it is directed away from the base, then it is a transistor n-p-n structures. See pictures

To check the P-N-P transistor with a multimeter, with the negative probe (black) we touch the base terminal, and with the positive probe (red) we touch the collector and emitter terminals in turn. If the transistor is intact, then the voltage drop in the test mode (testing) in millivolts will be in the range of 500 - 1200 Ohms, and the difference between these values ​​should be small. After this, we swap the probes; the multimeter should not show any drop. Next, we check the collector - emitter in both directions (swap the probes), there should also be no values ​​here.

Checking N-P-N transistors with a multimeter is identical, with the only difference being that the multimeter should show the voltage drop across the transitions when the positive probe touches the base of the transistor, and the black probe alternately touches the collector and emitter.

Look short video checking the transistor with a multimeter.

At the beginning I mentioned that in some cases, such a check may give a false conclusion. It happens when repairing a TV, when checking a soldered transistor with a multimeter, all transitions show normal values, but it does not work in the circuit. This can only be revealed by replacement.

The compound transistor is checked by inserting it into the holes on the panel of a multimeter or other device. To do this, you need to know what conductivity it is and then insert it, not forgetting to switch the tester to the appropriate position.

You can check the power transistor, as well as the line transistor, using the same method by examining transitions B-K, B-E, K-E, but since these transistors in most cases have built-in diodes (K-E) and resistances (B-E), all this must be taken into account. If an element is unfamiliar, it is better to look at its datasheet.

How to check on the board

You can check the transistor on the board in a similar way, but in some cases low-resistance resistors, chokes or transformers installed nearby in the harness can introduce false values. Therefore, it is better to have special devices designed for such checks, such as ESR-mikro v4.0.

ESR-mikro v4.0 can check a bipolar transistor without desoldering

Field check

It is difficult to assess the serviceability of a field-effect transistor, and while it is quite safe with powerful ones, it is more difficult with low-power ones. The fact is that these elements are gate-controlled by voltage and are easily broken through by static voltage.

The performance of field-effect transistors is checked with care, preferably on an antistatic table with an antistatic wrist strap (although for the most part this applies to low-power elements).

The transitions themselves will show infinite resistance, but as can be seen from those proposed above, the high-current field-effect transistor has a diode, it can be checked. An indicator that there is no short circuit is already a good sign.

We switch the device to the diode “testing” mode and enter the field transformer into saturation mode. If it is N-type, then we touch the drain with the minus, and the shutter with the plus. A working transistor should open. Next, we transfer the positive one, without disconnecting the negative one, to the source, the multimeter will show some resistance. Next you need to lock the radio component. Without taking the “plus” away from the source, the negative ones need to touch the gate and return it to the drain. The transistor will be turned off.

For P-type elements, the probes are swapped.

Instructions

It is not possible to check the transistor when it is soldered into an electronic circuit, so unsolder it before checking. Inspect the body. If there is a melted crystal on the case, then there is no point in checking the transistor. If the body is intact, then you can start checking.

The vast majority of power field-effect transistors are MOS-FET and n-channel insulated gate structures. Less common with the p-channel, mainly in the final stages of audio. Different field effect transistor structures require different ways their checks.

After desoldering the transistor, let it cool.

Place the transistor on a dry piece of paper. Insert the red ohmmeter leads into the positive connector and the black leads into the negative connector. Set the measurement limit to 1kOhm. The channel resistance of an open transistor depends on the voltage applied to the gate relative to the source, so while working with the transistor, you can set a measurement limit that is more convenient for you. The connection of the electrodes inside the housing is shown in the photo.

Touch the black probe to the “source” electrode of the transistor, and touch the red electrode to the “drain” electrode. If it shows a short circuit, remove the probes and connect all three electrodes with a screwdriver. The goal is to discharge the capacitive junction of the gate, which may have been charged. After this, repeat the channel resistance measurement. If the device still shows a short circuit, then the transistor is faulty and must be replaced.

If the device shows resistance close to infinity, then check the gate junction. It is checked in the same way as a channel transition. Touch the “source” electrode of the transistor with any probe, and touch the “gate” electrode with the other. The resistance must be infinitely large. The insulated gate is not electrically connected to the transistor channel and any resistance detected in this circuit indicates a faulty transistor.

The method for checking a fully operational transistor looks like this: Touch the black ohmmeter probe to the “source” electrode of the transistor, touch the “gate” electrode with the red probe. The resistance should be infinitely large, then, without closing the “gate” to other electrodes, touch the red probe to the “drain” electrode. The device will show a small resistance in this area. The magnitude of this resistance depends on the voltage between the ohmmeter probes. Now touch the red probe to the “source” electrode and repeat the above procedure. The channel resistance will be very high, close to infinity. The method for testing a MOS-FET transistor with a p-channel differs in that during measurements it is necessary to exchange the red and black ohmmeter probes.

Content:

In electronics and radio engineering, not only the correct assembly of the circuit, but also the subsequent verification of its performance is of great importance. The entire device or its individual elements. In this regard, the question quite often arises of how to check a transistor with a multimeter without disturbing the circuit. There are various methods that apply individually to each type of element. Before starting such inspection and testing, it is recommended to study the general structure and.

Basic types of transistors

There are two main types of transistors - bipolar and field-effect. In the first case, the output current is created with the participation of carriers of both signs (holes and electrons), and in the second case - only one. Testing the transistor with a multimeter will help determine the malfunction of each of them.

Bipolar transistors are essentially semiconductor devices. They are equipped with three pins and two pn junctions. The operating principle of these devices involves the use of positive and negative charges - holes and electrons. Flowing currents are controlled using a specially dedicated control current. These devices are widely used in electronic and radio engineering circuits.

Bipolar transistors consist of three-layer semiconductors of two types - “p-p-p” and “p-p-p”. In addition, the design has two pn junctions. The semiconductor layers are connected to external terminals through non-rectifying semiconductor contacts. The middle layer is considered the base, which is connected to the corresponding pin. Two layers located at the edges are also connected to the outputs - the emitter and collector. On electrical diagrams to designate the emitter, an arrow is used to show the direction of current flowing through the transistor.

In different types of transistors, holes and electrons - carriers of electricity - can have native functions. The most common type is p-p-p due to the best parameters and technical characteristics. The leading role in such devices is played by electrons, which perform the main tasks of ensuring all electrical processes. They are approximately 2-3 times more mobile than holes, which is why they have increased activity. Qualitative improvements in devices also occur due to the collector junction area, which is significantly larger than the emitter junction area.

Each bipolar transistor has two pn junctions. When testing a transistor with a multimeter, this allows you to check the performance of the devices by monitoring the resistance values ​​of the transitions when direct and reverse voltages are connected to them. For normal operation of the p-p-p-device, a positive voltage is applied to the collector, under the influence of which the base junction opens. After the base current occurs, the collector current appears. When a negative voltage occurs in the base, the transistor closes and the current flow stops.

The base junction in pnp devices opens when exposed to negative collector voltage. Positive voltage causes the transistor to turn off. All the necessary collector characteristics at the output can be obtained by smoothly changing the current and voltage values. This allows you to effectively test the bipolar transistor with a tester.

Exist electronic devices, all processes in which are controlled by the action of an electric field directed perpendicular to the current. These devices are called field-effect or unipolar transistors. The main elements are three contacts - source, drain and gate. The design of the field-effect transistor is complemented by a conductive layer that acts as a channel through which electric current flows.

These devices are represented by modifications of the “p” or “p”-channel type. Channels can be located vertically or horizontally, and their configuration can be volumetric or near-surface. The latter option is also divided into inversion layers containing enriched and depleted ones. The formation of all channels occurs under the influence of an external electric field. Devices with near-surface channels have a metal-insulator-semiconductor structure, which is why they are called MOS transistors.

Checking a bipolar transistor with a multimeter

The functionality of the bipolar transistor can be checked using a digital multimeter. This device measures direct and alternating currents, as well as voltage and resistance. Before starting measurements, the device must be properly configured. This will allow you to more effectively solve the problem of how to test a bipolar transistor with a multimeter without desoldering.

Modern multimeters can operate in a special measurement mode, which is why a diode icon is displayed on the body. When the question of how to check a bipolar transistor with a tester is decided, the device switches to the semiconductor testing mode, and one should be displayed on the display. The device terminals are connected in the same way as in resistance measurement mode. The black wire is connected to the COM port, and the red wire is connected to the output that measures resistance, voltage and frequency.

Older multimeters may not have a diode and transistor test function. In such cases, all actions are carried out in the resistance measurement mode set to maximum. The multimeter battery must be charged before use. In addition, you need to check the serviceability of the probes. To do this, their tips are connected to each other. The squeak of the device and the zeros shown on the display indicate that the probes are working properly.

Checking a bipolar transistor with a multimeter is performed in the following order:

• First of all, you need to correctly connect the leads of the multimeter and the transistor. To do this, you need to determine exactly where the base, collector and emitter are located. To determine the base, a black probe is connected to the first electrode, which is presumably considered the base. Another red probe is alternately connected first to the second and then to the third electrode. The probes are swapped until the device detects a voltage drop. After this, the bipolar transistor is finally checked with a multimeter and the pairs are determined: “base-emitter” or “base-collector”. The emitter and collector electrodes are determined using a digital multimeter. In most cases, the voltage drop and resistance at the emitter junction are higher than at the collector junction.
• Definition of a base-to-collector pn junction: the red probe is connected to the base, and the black probe is connected to the collector. This connection operates in diode mode and allows current to flow in only one direction.
• Definition of a base-emitter pn junction: the red probe remains connected to the base, and the black probe must be connected to the emitter. Just as in the previous case, with this connection, current flows only when connected directly. This is confirmed by verification npn transistor multimeter
• Definition of an emitter-collector pn junction: if this junction is in good working order, the resistance in this section will tend to infinity. This is indicated by the unit shown on the display.
• The multimeter is connected to each pair of contacts in two directions. That is transistors p-p-p types are checked by connecting back to the probes. In this case, a black probe is connected to the base. After measurements, the results obtained are compared with each other.
• After checking the pnp transistor with a multimeter, the performance of the bipolar transistor is confirmed when, when measuring one polarity, the multimeter shows the final resistance, and when measuring the reverse polarity, one is obtained. This check does not require desoldering parts from the common board.

Many people are trying to solve the question of how to test a transistor without a multimeter using light bulbs and other devices. This is not recommended, since the element is highly likely to fail.

Checking the functionality of the field-effect transistor

Field-effect transistors are widely used in audio and video equipment, monitors and power supplies. The functioning of most electronic circuits. Therefore, in case of any malfunctions, these elements are checked different ways, including checking transistors without desoldering them from the circuit with a multimeter.

A typical field-effect transistor circuit is shown in the figure. The main terminals - gate, drain and source - can be located differently, depending on the brand of transistor. If there is no marking, it is necessary to clarify the reference data regarding a particular model.

The main problem encountered during repairs electronic equipment with field-effect transistors, is to check the transistor with a multimeter without desoldering. As a rule, faults concern high-power field-effect transistors, which are used in power supplies. In addition, these devices are very sensitive to static discharges. Therefore, before deciding how to test a transistor on a board with a multimeter, you should wear a special antistatic bracelet and familiarize yourself with the safety rules when performing this procedure.

Testing using a multimeter involves the same actions as for bipolar transistors. A working field-effect transistor has an infinitely large resistance between the terminals, regardless of the test voltage applied to it.

However, solving the question of how to ring a transistor with a multimeter has its own characteristics. If the positive probe of the multimeter is applied to the gate, and the negative probe to the source, then in this case the gate capacitance will be charged and the junction will open. When measuring between the drain and the source, the device shows the presence of a small resistance. Sometimes electrical engineering in the absence practical experience, may consider this to be a malfunction, which is not always true. This may be important when testing a horizontal transistor with a multimeter. Before testing the drain-source channel, it is recommended to short-circuit all terminals of the field-effect transistor to discharge the junction capacitances. After this, their resistance will increase again, after which you can re-test the transistors with a multimeter. If this procedure does not give positive result, this means that the element is inoperative.

In field-effect transistors used for high-power switching power supplies, internal diodes are often installed at the drain-source junction. That's why this channel during testing it exhibits the properties of a conventional semiconductor diode. Therefore, to exclude an error, before checking the serviceability of the transistor with a multimeter, you should make sure that the internal diode is present. After the first check, the multimeter probes need to be swapped. After this, a unit will appear on the screen, indicating infinite resistance. If this does not happen, there is a high probability of a malfunction of the field-effect transistor. Using the device, you can not only check, but also measure the transistor with a multimeter.

How to test a compound transistor with a multimeter

A compound transistor or Darlington transistor is a circuit that combines two or more bipolar transistors. This allows you to significantly increase the current gain. Such transistors are used in circuits designed to operate with high currents, for example, in voltage stabilizers or output stages of power amplifiers. They are necessary when you need to provide a large input impedance, that is, total complex resistance.

The general conclusions of a composite transistor are the same as those of the bipolar model. The NPN transistor is checked in exactly the same way with a multimeter. In this case, a technique similar to testing a conventional bipolar transistor is used.