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 p-n junction’s own field, 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 amplifying 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 normal.
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 large 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 positive wire to the gate. 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.
h FE of a transistor is the current gain or amplification factor of a transistor.
h FE (which is also referred to as β) is the factor by which the base current is amplified to produce the amplified current of the transistor. The unamplified current is the base current, which then undergoes amplification by a factor of h FE to produce an amplified current which flows through the collector and emitter terminals.
A transistor works by feeding a current into the base of the transistor. The base current is then amplified by h FE to yield its amplified current. The formula is below:
I C = h FE I B =βI B
So if 1mA is fed into the base of a transistor and it has a h FE of 100, the collector current will be 100mA.
Every transistor has its own unique h FE. The h FE is normally seen to be a constant value, normally around 10 to 500, but it may change slightly with temperature and with changes in collector-to-emitter voltage.
Check the transistor's datasheet for the h FE value in its specifications.
Note that h FE may refer to DC or AC current gain. Many datasheets may just specify one value, such as the DC gain. The datasheets will normally specify whether the h FE value is for DC or AC current gain.
Also, note that as the h FE value is highly variable, many datasheets will specify a minimum and maximum h FE for the transistor. It is very hard for transistors to be produced with a precise h FE value during the manufacturing process. Therefore, manufacturers generally specify a range that h FE may be within.
Because h FE is so widely variable and unpredictable in nature, good transistor circuit design is important to give stable, predictable amplification for transistor circuits to account for this unpredictability.
The transistor is a ubiquitous and important component in modern microelectronics. Its purpose is simple: it allows you to control a much stronger one using a weak signal.
In particular, it can be used as a controlled “damper”: by the absence of a signal at the “gate”, block the flow of current, and by supplying it, allow it. In other words: this is a button that is pressed not by a finger, but by applying voltage. This is the most common application in digital electronics.
Transistors are available in different packages: the same transistor can look completely different in appearance. In prototyping, the most common cases are:
TO-92 - compact, for light loads
TO-220AB - massive, good heat dissipation, for heavy loads
The designation on the diagrams also varies depending on the type of transistor and the designation standard used in the compilation. But regardless of the variation, its symbol remains recognizable.
Bipolar transistors
Bipolar junction transistors (BJT, Bipolar Junction Transistors) have three contacts:
Collector - high voltage is applied to it, which you want to control
Base - a small amount is supplied through it current to unlock large; the base is grounded to block it
Emitter - current flows through it from the collector and base when the transistor is “open”
The main characteristic of a bipolar transistor is the indicator h fe also known as gain. It reflects how many times more current in the collector-emitter section the transistor can pass in relation to the base-emitter current.
For example, if h fe= 100, and 0.1 mA passes through the base, then the transistor will pass through itself a maximum of 10 mA. If in this case there is a component in the high current section that consumes, for example, 8 mA, it will be provided with 8 mA, and the transistor will have a “reserve”. If there is a component that draws 20 mA, it will only be provided with the maximum 10 mA.
Also, the documentation for each transistor indicates the maximum permissible voltages and currents at the contacts. Exceeding these values leads to excessive heating and reduced service life, and a strong excess can lead to destruction.
NPN and PNP
The transistor described above is a so-called NPN transistor. It is called that because it consists of three layers of silicon connected in the order: Negative-Positive-Negative. Where negative is a silicon alloy with an excess of negative charge carriers (n-doped), and positive is an alloy with an excess of positive charge carriers (p-doped).
NPNs are more effective and common in industry.
When designating PNP transistors, they differ in the direction of the arrow. The arrow always points from P to N. PNP transistors have an “inverted” behavior: current is not blocked when the base is grounded and blocked when current flows through it.
Field effect transistors
Field effect transistors (FET, Field Effect Transistor) have the same purpose, but differ in internal structure. A particular type of these components are MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) transistors. They allow you to operate with much greater power with the same dimensions. And the control of the “damper” itself is carried out exclusively using voltage: no current flows through the gate, unlike bipolar transistors.
Field effect transistors have three contacts:
Drain - high voltage is applied to it, which you want to control
Gate - voltage is applied to it to allow current to flow; the gate is grounded to block the current.
Source - current flows through it from the drain when the transistor is “open”
N-Channel and P-Channel
By analogy with bipolar transistors, field transistors differ in polarity. The N-Channel transistor was described above. They are the most common.
P-Channel when designated differs in the direction of the arrow and, again, has an “inverted” behavior.
Connecting transistors to drive high-power components
A typical task of a microcontroller is to turn a specific circuit component on and off. The microcontroller itself usually has modest power handling characteristics. So Arduino, with 5 V output per pin, can withstand a current of 40 mA. Powerful motors or ultra-bright LEDs can draw hundreds of milliamps. When connecting such loads directly, the chip can quickly fail. In addition, for the operation of some components, a voltage greater than 5 V is required, and Arduino cannot produce more than 5 V from the digital output pin.
But it is easily enough to control a transistor, which in turn will control a large current. Let's say we need to connect a long LED strip that requires 12 V and consumes 100 mA:
Now, when the output is set to logical one (high), the 5 V entering the base will open the transistor and current will flow through the tape - it will glow. When the output is set to logic zero (low), the base will be grounded through the microcontroller and current flow will be blocked.
Pay attention to the current limiting resistor R. It is necessary so that when control voltage is applied, a short circuit does not form along the route microcontroller - transistor - ground. The main thing is not to exceed the permissible current through the Arduino contact of 40 mA, so you need to use a resistor with a value of at least:
Here U d- this is the voltage drop across the transistor itself. It depends on the material from which it is made and is usually 0.3 – 0.6 V.
But it is absolutely not necessary to keep the current at the permissible limit. It is only necessary that the gain of the transistor allows you to control the required current. In our case it is 100 mA. Acceptable for the transistor used h fe= 100, then a control current of 1 mA will be enough for us
A resistor with a value from 118 Ohm to 4.7 kOhm is suitable for us. For stable operation on one side and light load on the chip on the other, 2.2 kOhm is a good choice.
If you use a field-effect transistor instead of a bipolar transistor, you can do without a resistor:
This is due to the fact that the gate in such transistors is controlled solely by voltage: there is no current in the microcontroller - gate - source section. And due to its high characteristics, a circuit using MOSFETs allows you to drive very powerful components.
Hi all! Today we will talk again about such a device as a multimeter. This device, also called a tester, is designed to measure the basic characteristics of an electrical circuit, electrical appliances, in cars - in general, wherever there is electricity. We have already talked a little about multimeters, today we will touch in more detail on what and how they can measure. Once upon a time, the multimeter was the domain of only electricians. However, now many people use it.
There are many different models of multimeters. There is a class of instruments for measuring only certain characteristics. Multimeters are conventionally reduced to two types:
- analog multimeters - data is displayed by an arrow. These are multimeters that are still used by people of the old school; they often cannot or do not want to work with modern instruments;
- digital multimeters – data is displayed in numbers. This type of tester has replaced the pointer tester; for example, I prefer to use such a device.
Since digital devices are now the most common, we will consider the description of this device using its example. Below are the main symbols that are found on almost any multimeter model.
If you examine the front panel of the multimeter, you can see eight blocks with different symbols:
What does the multimeter show when selecting different operating modes?
They are located around a round switch, with which you can set the desired mode. On the switch, the contact point is indicated by a dot or a raised triangle. Designations are divided into sectors. Almost all modern multimeters have a similar layout and a round switch.
sector OFF. If you set the switch to this position, the device is turned off. There are also models that automatically turn off after a while. This is very convenient, because for example, I forget to turn it off while working, and it’s not convenient when you measure, then solder, turn it off all the time. The battery lasts a long time.
2 and 8– two sectors with the designation V, this symbol indicates voltage in volts. If just a symbol V– then the DC voltage is measured if V~, AC voltage is measured. The numbers next to them show the range of the measured voltage. Moreover, constant is measured from 200m (millivolts) to 1000 volts, and variable is measured from 100 to 750 volts.
3 and 4– two sectors for measuring direct current. Only one range is highlighted in red for measuring current up to 10 amperes. The remaining ranges are: from 0 to 200, 2000 microamps, from 0 to 20, 200 milliamps. In ordinary life, ten amperes are enough; when measuring current, the multimeter is connected to the circuit by connecting the probes to the desired socket, specially designed for measuring current. One day I tried to measure the current in an outlet for the first time with my first simple tester model. I had to replace the probes with new ones - the standard ones were burnt out.
5 (fifth) sector. The icon looks like WiFi. 🙂 Setting the switch in this position allows you to conduct an audible test of a circuit, such as a heating element.
6 (sixth) sector – setting the switch to this position checks the serviceability of the diodes. Checking diodes is a very popular topic among motorists. You can check the serviceability of, for example, the diode bridge of a car generator:
7 - symbol Ω . Here resistance is measured from 0 to 200, 2000 Ohm, from 0 to 20, 200 or 2000 kOhm. This is also a very popular mode. In any electrical circuit there are the most resistance elements. It happens that by measuring resistance you quickly find a fault:
What is HFE mode on a multimeter?
Let's move on to more advanced functions. The multimeter has the following type of measurements: HFE. This is a test of transistors, or the current transfer coefficient of a transistor. There is a special connector for this measurement. Transistors are an important element; perhaps only the light bulb does not have them, but even there they will probably appear soon. The transistor is one of the most vulnerable elements. They burn out most often due to power surges, etc. I recently replaced two transistors in my car battery charger. To check, I used a tester and unsoldered the transistors.
The connector pins are marked with letters such as "E, B and C". This means the following: "E" is emitter, "B" is base, and "C" is collector. Typically all models have the ability to measure both types of transistors. With inexpensive models of multimeters, it can be very inconvenient to check soldered transistors because of their short, cut legs. And the new ones are the best :):). Let's watch a video on how to check the serviceability of a transistor using a tester:
The transistor, depending on its type (PNP or NPN), is inserted into the corresponding connectors and, according to the readings on the display, it is determined whether it is working or not. If there is a fault, the display shows 0 . If you know the current transfer coefficient of the transistor being tested, you can check it in the HFE by checking the tester readings and the transistor data sheet
How is resistance indicated on multimeters?
One of the main measurements taken by a multimeter is resistance. It is indicated by a horseshoe symbol: Ω, Greek Omega. If there is only such an icon on the multimeter body, the device measures the resistance automatically. But more often there is a range of numbers nearby: 200, 2000, 20k, 200k, 2000k. Letter " k" after the number denotes the prefix "kilo", which in the measurement system SI corresponds to the number 1000.
Why is there a hold button in a multimeter and what is it for?
Button Data hold, which the multimeter has, is considered useless by some, while others, on the contrary, use it often. It means data retention. If you press the hold button, the data displayed on the display will be fixed and will be displayed continuously. When pressed again, the multimeter will return to operating mode.
This function can be useful when, for example, you have a situation where you alternately use two devices. You have carried out some kind of standard measurement, displayed it on the screen, and continue to measure with another device, constantly checking with the standard. This button is not available on all models; it is intended for convenience.
Designations of direct current (DC) and alternating current (AC)
Measuring direct and alternating current with a multimeter is also its main function, as is measuring resistance. You can often find the following symbols on the device: V And V~ — DC and AC voltage respectively. On some devices, constant voltage is designated DCV, and alternating voltage ACV.
Again, it is more convenient to measure current in automatic mode, when the device itself determines how many volts, but this function is available in more expensive models. In simple models, direct and alternating voltage during measurements must be measured with a switch depending on the range being measured. Read about this in detail below.
Decoding the symbols 20k and 20m on a multimeter
Next to the numbers indicating the measurement range, you can see letters such as µ, m, k, M. These are so-called prefixes, which indicate the multiplicity and fractionality of units of measurement.
- 1µ (micro) – (1*10-6 = 0.000001 from unit);
- 1m (millies) – (1*10-3 = 0.001 from unit);
- 1k (kilo) – (1*103 = 1000 units);
- 1M (mega) – (1*106 = 1,000,000 units);
For example, to check the same heating elements, it is better to take a tester with a megometer function. I had a case where a malfunction of the heating element in a dishwasher was detected only by this function. For radio amateurs, of course, more complex devices are suitable - with the function of measuring frequencies, capacitor capacity, and so on. Nowadays there is a very large selection of these devices; the Chinese don’t do anything.
The bipolar transistor is one of the oldest but most famous type of transistor, and is still used in modern electronics. A transistor is indispensable when you need to control a fairly powerful load for which the control device cannot provide sufficient current. They come in different types and capacities, depending on the tasks performed. Basic knowledge and formulas about transistors can be found in this article.
Introduction
Before starting the lesson, let's agree that we are discussing only one type of way to turn on a transistor. A transistor can be used in an amplifier or receiver, and typically each transistor model is manufactured with certain characteristics to make it more specialized to perform better in a particular application.
The transistor has 3 terminals: base, collector and emitter. It is impossible to say unambiguously which of them is the input and which is the output, since they are all connected and influence each other in one way or another. When a transistor is turned on in switch mode (load control), it acts like this: the base current controls the current from the collector to the emitter or vice versa, depending on the type of transistor.
There are two main types of transistors: NPN and PNP. To understand this, we can say that the main difference between these two types is the direction of the electric current. This can be seen in Figure 1.A, where the direction of the current is indicated. In an NPN transistor, one current flows from the base into the transistor and the other current flows from the collector to the emitter, but in a PNP transistor the opposite is true. From a functional point of view, the difference between these two types of transistors is the voltage across the load. As you can see in the picture, the NPN transistor provides 0V when it is turned on, and the PNP provides 12V. You'll understand later why this affects transistor selection.
For simplicity, we will only study NPN transistors, but all this applies to PNP, taking into account that all currents are reversed.
The figure below shows the analogy between a switch (S1) and a transistor switch, where it can be seen that the base current closes or opens the path for current from the collector to the emitter:
By knowing exactly the characteristics of a transistor, you can get the most out of it. The main parameter is the DC gain of the transistor, which is usually denoted H fe or β. It is also important to know the maximum current, power and voltage of the transistor. These parameters can be found in the documentation for the transistor, and they will help us determine the value of the base resistor, which is described below.
Using an NPN transistor as a switch
The figure shows the inclusion of an NPN transistor as a switch. You will encounter this inclusion very often when analyzing various electronic circuits. We will study how to run a transistor in the selected mode, calculate the base resistor, transistor current gain and load resistance. I propose the simplest and most accurate way to do this.
1. Assume that the transistor is in saturation mode: In this case, the mathematical model of the transistor becomes very simple, and we know the voltage at point V c. We will find the value of the base resistor at which everything will be correct.
2. Determination of collector saturation current: The voltage between collector and emitter (V ce) is taken from the transistor documentation. The emitter is connected to GND, respectively V ce = V c - 0 = V c. Once we know this value, we can calculate the collector saturation current using the formula:
Sometimes, the load resistance R L is unknown or cannot be as accurate as the relay coil resistance; In this case, it is enough to know the current required to start the relay.
Make sure that the load current does not exceed the maximum collector current of the transistor.
3. Calculation of the required base current: Knowing the collector current, you can calculate the minimum required base current to achieve that collector current using the following formula:
It follows from it that:
4. Exceeding permissible values: After you have calculated the base current, and if it turns out to be lower than that specified in the documentation, then you can overload the transistor by multiplying the calculated base current, for example, by 10 times. Thus, the transistor switch will be much more stable. In other words, the transistor's performance will decrease if the load increases. Be careful not to exceed the maximum base current stated in the documentation.
5. Calculation of the required value of R b: Considering an overload of 10 times, the resistance R b can be calculated using the following formula:
where V 1 is the transistor control voltage (see Figure 2.a)
But if the emitter is connected to ground, and the base-emitter voltage is known (about 0.7V for most transistors), and assuming that V 1 = 5V, the formula can be simplified to the following: 
It can be seen that the base current is multiplied by 10 taking into account the overload.
When the value of Rb is known, the transistor is "set" to operate as a switch, also called "saturation and cutoff mode", where "saturation" is when the transistor is fully open and conducting current, and "cutting" is when it is closed and not conducting current .
Note: When we say , we are not saying that the collector current must be equal to . This simply means that the transistor's collector current can rise to this level. The current will follow Ohm's laws, just like any electrical current.
Load calculation
When we considered that the transistor was in saturation mode, we assumed that some of its parameters did not change. This is not entirely true. In fact, these parameters were changed mainly by increasing the collector current, and therefore it is safer for overload. The documentation indicates a change in transistor parameters during overload. For example, the table in Figure 2.B shows two parameters that change significantly:
H FE (β) varies with collector current and voltage V CEsat. But V CEsat itself changes depending on the collector and base current, as shown in the table below.
The calculation can be very complex, since all the parameters are closely and complexly interrelated, so it is better to take the worst values. Those. the smallest H FE, the largest V CEsat and V CEsat.
Typical application of a transistor switch
In modern electronics, a transistor switch is used to control electromagnetic relays, which consume up to 200 mA. If you want to control a relay with a logic chip or microcontroller, then a transistor is indispensable. In Figure 3.A, the resistance of the base resistor is calculated depending on the current required by the relay. Diode D1 protects the transistor from the pulses that the coil generates when turned off.
2. Connecting an open collector transistor:
Many devices, such as the 8051 family of microcontrollers, have open-collector ports. The base resistor resistance of the external transistor is calculated as described in this article. Note that the ports can be more complex, and often use FETs instead of bipolar ones and are called open-drain outputs, but everything remains exactly the same as in Figure 3.B
3. Creating a logical element OR-NOT (NOR):
Sometimes you need to use a single gate in a circuit and you don't want to use a 14-pin 4-gate chip either due to cost or board space. It can be replaced with a pair of transistors. Note that the frequency characteristics of such elements depend on the characteristics and type of transistors, but are usually below 100 kHz. Reducing the output resistance (Ro) will increase power consumption but increase the output current.
You need to find a compromise between these parameters.
The figure above shows a NOR gate built using 2 2N2222 transistors. This can be done with PNP 2N2907 transistors, with minor modifications. You just have to consider that all the electrical currents then flow in the opposite direction.
Finding errors in transistor circuits
When a problem occurs in circuits containing many transistors, it can be quite difficult to know which one is bad, especially when they are all soldered in. I give you some tips that will help you find the problem in such a scheme quickly:
1. Temperature: If the transistor gets very hot, there is probably a problem somewhere. It is not necessary that the problem is a hot transistor. Usually the defective transistor does not even heat up. This temperature increase may be caused by another transistor connected to it.
2. Measuring V CE of transistors: If they are all the same type and all work, then they should have approximately the same VCE. Finding transistors that have different V CE is a quick way to detect defective transistors.
3. Measuring the voltage across the base resistor: The voltage across the base resistor is quite important (if the transistor is turned on). For a 5V NPN transistor driver, the voltage drop across the resistor should be more than 3V. If there is no voltage drop across the resistor, then either the transistor or the transistor control device is defective. In both cases, the base current is 0.
