How to test a multimeter transistor: testing various types of devices
How to test a multimeter transistor: testing various types of devices
Before you start repairing an electronic device or assembling a circuit, you should make sure that all the elements that will be installed are in good condition. If new parts are used, it is necessary to make sure that they are working properly. The transistor is one of the main components of many circuits, so it should be rung first. How to check a transistor with a multimeter will be described in detail in this article.
Checking transistors is a mandatory step in the diagnosis and repair of chips
What is a transistor
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 transistor is one of the main components of microcircuits and electrical circuits
A bipolar transistor has three outputs: base, emitter and collector. A small current is applied 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 junction and corresponds to the polarity of the connection.
A transistor of this type is equipped with two p-n junctions. When electronic conductivity (n) prevails in the extreme region of the device, and hole conductivity (p) prevails in the middle region, then the transistor is called n—p-n (reverse conductivity). If the opposite is true, then the device is called a p-n-p type 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 (gate). In this case, the gate is affected by voltage, not current, which is typical for the bipolar type. An electric current passes between the source and the drain with a certain intensity, which depends on the signal. This signal is generated between the gate and the source or the gate and the drain. A transistor of this type can be with a control p-n junction or with an isolated gate. In the first case, the working pins are connected to a semiconductor wafer, which can be p- or n-type.
The principle of operation of a field-effect transistor
The main feature of field-effect transistors is that their control is provided not by current, but by voltage. The minimal use of electricity allows it to be used in radio components with quiet and compact power sources. Such devices may have different polarities.
How to check a transistor with a multimeter
Many modern testers are equipped with specialized connectors that are used to check the operability of radio components, including transistors.
To determine the operating state 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) that are counter-connected to the base. Hence, one semiconductor is formed by the terminals of the collector and base, and the other by the emitter and base.
Using a transistor to assemble a circuit board, it is necessary to clearly know the purpose of each pin. Incorrect placement of the element may cause it to burn out. With the help of the tester, you can find out the purpose of each output.
To determine the state of the transistor, it is necessary to test each of its elements
Important! This procedure is possible only for a serviceable transistor.
To do this, the device is switched to the resistance measurement mode at the maximum limit. The red probe should touch the left contact and measure the resistance at the right and middle terminals. For example, the display shows the values of 1 and 817 ohms.
Then the red probe should be moved to the middle, and with the help of the black one, the resistances on the right and left terminals should be measured. Here the result can be: infinity and 806 ohms. Transfer the red probe to the right contact and measure the remaining combination. Here, in both cases, the display will display the value of 1 ohm.
Making a conclusion from all measurements, the base is located on the right output. Now, to determine other conclusions, it is necessary to install a black probe on the base. On one pin, the value of 817 ohms appeared – this is the emitter junction, the other corresponds to 806 ohms, the collector junction.
The scheme of checking transistors using a multimeter
Important! The resistance of the emitter junction will always be greater than that of the collector junction.
How to ring a multimeter transistor
To make sure 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 the resistance measurement mode and set to the limit of 2000. Next, you should ring each pair of contacts in both directions. So six measurements are performed:
- the base-collector connection must conduct electric current in one direction;
- the base-emitter connection conducts electric current in one direction;
- the emitter-collector connection does not conduct electric current in any direction.
How to call a multimeter transistors whose conductivity is p-n-p (the emitter junction arrow is directed to the base)? To do this, you need to touch the base with a black probe, and touch the emitter and collector junctions alternately with red. If they are working properly, then the direct resistance of 500-1200 ohms will be displayed on the tester screen.
Transistor test points p-n-p
To check the reverse resistance, the red probe should touch the base, and the black one alternately to the emitter and collector terminals. Now the device should show a large resistance value at both transitions by displaying “1” on the screen. This means that both transitions are in good condition, and the transistor is not damaged.
This technique allows you to solve the question: how to check a transistor with a multimeter without soldering it out of the board. This is possible due to the fact that the transitions of the device are not shunted by 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 soldered out of the circuit.
Before checking the n-p-n transistor with a multimeter (the emitter junction arrow is directed from the base), the red probe of the tester is connected to the base to determine the direct resistance. The operability of the device is checked by the same method as a transistor with a p-n-p conductivity.
The malfunction of the transistor is indicated by the breakage of one of the transitions, where a large value of forward or reverse resistance is detected. If this value is 0, the junction is in a break and the transistor is faulty.
The principle of operation of a bipolar transistor
This technique is suitable exclusively for bipolar transistors. Therefore, before checking, it is necessary to make sure that it does not belong to a composite or field device. Next, it is necessary to check the resistance between the emitter and the collector. There should be no closures here.
If it is necessary to use a transistor having an approximate current gain to assemble an electrical circuit, you can use a tester to determine the necessary element. To do this, the tester is switched to hFE mode. The transistor is connected to the appropriate connector for a specific type of device located on the device. The value of parameter h21 should be displayed on the multimeter screen.
How to check a thyristor with a multimeter? It is equipped with three p-n junctions, which is different from a bipolar transistor. Here the structures alternate with each other in the manner of a zebra. The main difference between it and 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. The use of a thyristor allows you to assemble more economical electrical circuits.
The scheme of checking the thyristor with a multimeter
The multimeter is set to a resistance measurement scale in the range of 2000 ohms. To open the thyristor, the black probe is attached to the cathode, and the red one to the anode. It should be remembered that the thyristor can be opened with 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 current of the control signal exceeds the retention threshold. If the current is less, the key will close.
How to check an IGBT transistor with a multimeter
A bipolar transistor with an isolated gate (IGBT) is a three-electrode power semiconductor device in which two transistors are connected in one structure according to the principle of cascade switching: field and bipolar. The first forms the control channel, and the second forms the power channel.
To test the transistor, the multimeter must be switched to the semiconductor test mode. After that, using probes, measure the resistance between the emitter and the gate in the forward and reverse directions to detect a short circuit.
IGBT transistors with collector-emitter voltage
Now connect the red wire of the device to the emitter, and touch the shutter briefly with the black one. The gate will be charged with a negative voltage, which will allow the transistor to remain closed.
Important! If the transistor is equipped with a built-in counter-parallel diode, which is connected by the anode to the emitter of the transistor, and by the cathode to the collector, then it must be rung accordingly.
Now you need to verify the functionality of the transistor. First, it is necessary to charge the gate-emitter input capacitance with a positive voltage. For this purpose, at the same time and for a short time, the red probe should touch the gate, and the black one should touch the emitter. Now you need to check the collector-emitter junction by connecting the black probe to the emitter and the red one to the collector. A slight voltage drop of 0.5-1.5 V should be displayed on the multimeter screen. This value should remain stable for several seconds. This indicates that there is no leakage in the input capacitance of the transistor.
Checking the transistor with a multimeter without soldering from the chip
Useful advice! If the voltage of the multimeter is not enough to open the IGBT transistor, then a constant 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 exhibit high sensitivity to static electricity, therefore, the organization of grounding is required beforehand.
Before you start checking the field-effect transistor, you should determine its cap. On imported devices, labels are usually applied that determine the device’s terminals. The letter S denotes the source of the device, the letter D corresponds to the drain, and the letter G is the gate. If there is no cap, then you need to use the documentation for the device.
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Before checking the serviceable condition of the transistor, it is worth considering that modern MOSFET-type radio components have an additional diode located between the source and drain, which is necessarily applied to the device circuit. The polarity of the diode depends entirely on the type of transistor.
Useful advice! To protect yourself from the accumulation of static charges, you can use an antistatic grounding bracelet that is worn on your hand, or touch the battery with your hand.
The device of a field-effect transistor with an N-channel
The main task of how to check a field-effect transistor with a multimeter without soldering it out of the board consists of the following actions:
Step-by-step verification of a field-effect transistor with a multimeter
It is possible to talk about the serviceable condition of the transistor based on how it has the ability to open and close with the help of a constant voltage from the tester. Due to the fact that the field-effect transistor has a large input capacity, it will take some time to discharge it. This characteristic is important when the transistor is first opened using the voltage generated by the tester (see paragraph 6), and measurements are carried out for a small amount of time (see paragraphs 7 and 8).
The multimeter checks the working state of the p-channel field-effect transistor using the same method as the n-channel one. You should only start measuring by connecting the red probe to the minus, and the black one to the plus, i.e. change the polarity of the connection of the tester wires to the reverse.
The serviceability of any transistor, regardless of the type of device, can be checked with a simple multimeter. To do this, you should clearly know the type of element and determine the marking of its conclusions. Next, in the mode of ringing diodes or measuring resistance, find out the forward and reverse resistance of its transitions. Based on the results obtained, to judge the serviceable condition of the transistor.
