A semiconductor type diode refers to those electronic devices that are characterized by conductivity in only one direction.
Users are often faced with the question of how to test a diode. In order to check whether the diode is functioning normally, it is best to use the method of monitoring its condition using a digital multimeter. All diodes have two outputs. One of them - the anode - has a plus sign, and the other - the cathode - has a minus sign.
From a physical point of view, any diode- This is a p-n type transition device. You should know that devices with a semiconductor system can have several such transitions (the dinistor has 3 transitions). Meanwhile, a conventional semiconductor diode is the most basic electronic device in existence, based on one such junction. It should also be remembered that a diode with a semiconductor system can fully exhibit its physical properties only after it is turned on at full strength.
Turning on at full strength implies the fact that the anode of a particular diode was connected to a voltage with a plus sign, and the cathode to a voltage with a minus sign. Only then does the diode fully open and its transition begins to conduct the electrical dock. If you do everything the other way around and connect a negative voltage to the anode of the diode, and a positive voltage to the cathode, then this diode will be considered closed and will not allow electric current to pass through it. This process will last until the voltage in the device reaches its maximum level, which will entail the destruction of the crystalline base of the semiconductor. Thus, the principle of operation of the diode - conductivity in one direction - is confirmed.
Answer to the question: “How to test a diode with a multimeter?”- very simple. In most cases, any modern digital tester (multimeter) that can now be found on sale is equipped with a function for checking the physical health of diodes. This property can be used in situations where you need to check the functionality of the transistor.
When checking the functionality of the device, it is not the value of the junction resistance that appears on the screen, but the so-called “breakdown” voltage in the diode. This means: if this threshold is exceeded, the junction will open and the diode will start working. As a rule, the value of this indicator is in the range from one hundred to eighty millivolts. They will be displayed on the device monitor. If you swap the multimeter leads (from negative to positive and vice versa), then the monitor should not show anything. This will be evidence that the diode does not pass current in the other direction, and therefore functions normally.
In order to facilitate the verification process, it is advisable to have a breadboard with you. First of all, you should make sure that you do not touch the diode outputs and tester probes with both hands. You can’t do this, because then your body will also affect the measurement results - its resistance will be added. Therefore, everything must be held with only one hand - then only the elements necessary for this will be included in the measurement chain.
This feature should not be forgotten when measuring other devices, for example, capacitors or resistors. You should start by checking during a direct connection. To do this, the positive probe of the multimeter (it is red) must be connected to the anode of the diode, and the negative probe (it is black) must be connected to the cathode. The cathode output is located on the side of the device on which the ring is painted with white paint.
This is how the cathode output is noted in most modern diodes. If everything went well and the monitor displayed a normal voltage value, then you can check the diode by swapping the contacts. It is worth noting that diodes still pass electric current in the opposite direction, but in such small quantities that this indicator is never taken into account in calculations. So if you connect a black probe to the anode and a red probe to the cathode, the display should show the value “one”. This will indicate that the diode is functioning absolutely normally.
Semiconductor diodes, as a rule, are characterized by two types of faults: junction breakdown and junction breakage. The following is worth knowing about them:
The breakdown voltage of most germanium diodes is in the range of three hundred to four hundred millivolts. For example, the often used diode model D9, which is also used as a detector in radio receivers, is characterized by this indicator in the amount of four hundred millivolts.
Here are the main types of diodes and the voltages that correspond to them:
Using this technique, you can not only check how well the diode functions, but also approximately find out what material served as the raw material for its manufacture. This can be determined by finding out the breakdown voltage.
If you have concerns that you will not be able to independently check the health of the diode using a multimeter, it would be best to contact a specialist. Using the services of the Yudu platform, you can order the services of a specialist to check the diode with a multimeter in just ten minutes.
This can be done in the following ways:
On the Yudu platform, you will not be limited in choosing a specialist and will be able to use the services of the specialist whom you consider the most qualified. All Yudu performers underwent a special check during registration on the site and can guarantee the high quality of the work performed.
It's sad, but you need to start with theory. You will have to study the types of diodes, area and purpose of application. Without delving into the physical foundations of electronics, let's go over the search queries. It is important to understand that all diodes are united by the ability to pass current in one direction, blocking the movement of particles in the opposite direction, forming a kind of valves. Then we will discuss how to test a diode with a multimeter.
So, diodes pass current in the forward direction and block it in the reverse direction. On electrical diagrams, diodes are indicated by black arrows bounded by a crossbar. The symbol shows the direction of the current in the physical sense - the directional movement of positive particles. To create a forward current, a negative potential is applied to the end of the arrow, and a positive potential is applied to the beginning. Otherwise, the diode will be in a “locked” state.
When electrons move due to the imperfection of the molecular lattice, heat is lost, which entails a voltage drop in the forward direction. Silicon diodes have a higher direct potential, germanium diodes have a lower one. Schottky diodes are characterized by a smaller potential drop due to the replacement of one semiconductor layer with a metal one, i.e. there is no p-n junction. The loss current increases, and the voltage drop across the open switch in the forward direction is record low.
The effect is not typical in all voltage ranges. Schottky diodes are most effective at voltages equal to tens of volts. They are used in output filters of switching power supplies. Remember: the voltage ratings of the system unit are 5, 12, 3 V. The method of constructing circuits using a Schottky diode is typical.
A popular type of diode is a zener diode. Its working area is the breakdown area. Where a conventional diode fails, a zener diode protects the equipment. The process is characterized by an increase in voltage to nominal and a sharp stabilization. Through zener diodes, sensitive and weak microcircuits of switching power supply controllers are powered from high-voltage lines so that they cut the voltage into pulses of large amplitude. Without zener diodes, powering microcircuits is solved using extremely complex methods.
When evaluating a Zener diode using a multimeter, take into account that the working area is the reverse branch. Technically, the breakdown voltage for testing is obtained from batteries connected in series, then the presence of stabilization is checked. Direct connection of a zener diode is used extremely rarely; ringing in the traditional way is a bad idea. Zener diodes also include an avalanche diode, where the impact ionization effect is used to stabilize the current.
It happens that the specifics of the device are not clear. The printed circuit boards are marked - each element has a strictly defined designation, and the powerful diodes of the rectifier bridge cannot be confused with a tiny glass zener diode. The worst option is a tangle of conductors with unknown elements: either a diode, or an unusual type of resistor, or an exotic capacitor.
When faced with a similar situation, they carefully take an enlarged photo, then search the Internet using the image. Although the markings of zener diodes are illegible, it is possible to find information on the Internet. This step greatly speeds up the process of identifying and assessing the performance of the device.
An infrared diode is checked with a multimeter in the same way: we remove the forward voltage, then make sure that there is no reverse current flowing. To check the glow, use the viewfinder of a night video camera. It directly registers the infrared radiation of objects. A working IR diode is visible in the viewfinder - like a star. They check the glow with thermal imagers and night vision devices, being careful: the radiation power of light and IR diodes is high, comparable to the power of laser radiation.
The inscription inside the printer about the presence of a laser cannot be considered a joke. And neglect her. Keep your retina away from the infrared diode.
To test diodes, multimeters are equipped with a special scale marked with the corresponding icon - a schematic designation of a diode. When the mode is turned on, low resistances turn on the buzzer, high ones are characterized by the nominal value or the voltage dropping across it. Based on the readings, they judge the characteristics of the diode, for example, the resistance of direct connection.
To correctly interpret the readings, it is important to take into account the characteristics of the tester: constant voltage and low nominal voltage used for evaluation. Example: when measuring resistance, the tester passes current through it, applying a certain voltage to the probes. Any multimeter model is characterized by unique parameters. The voltage is recognized by the charge of the capacitor: turn the multimeter into the ringing or diode testing mode, after a short time a potential difference will form on the capacitor plates. Measured using the standard scale of the tester. The value ranges from hundreds of millivolts (fractions of a volt) to units of volts.
Knowing the voltage applied to the diode, the accuracy of the reading is verified using its current-voltage characteristic. Enter a search query on Yandex and get acquainted with the full technical documentation for the element being studied. Then they place an abscissa ruler at the right place on the scale to find the output current. Using Ohm's formula, the open state resistance is calculated: R = U/I, where U is the auxiliary voltage generated by the tester. Compare the value found from the graph with that indicated on the display.
This is one of many techniques. It is important to know how to find the right paths, analyze and compare data. The first step is to search for generalized information: what diodes are, their characteristics (primarily current-voltage), the intricacies of the operation of a particular device. Knowing the theoretical foundations, it is easy to operate with information and draw correct conclusions from research results.
Let's move on to a real-life example: let's examine a diode bridge from a car generator!
A car needs electricity - for air conditioning systems (along with engine energy), wipers, exterior and interior lighting. Constantly loading the battery, which is done while parked, is not economical. The problem is solved by connecting a synchronous alternating current generator to the motor shaft. Previously we used a collector circuit. But the brushes do not tolerate shaking, and there was a need for frequent maintenance.
Three-phase generators are now being installed. Because the revolutions constantly fluctuate, the constancy of the output characteristics is maintained by changing the rotor feeding current. As a result, the strength of the alternating magnetic field of the stator monitors every change in the operation of the motor. The price to pay is instability of the output voltage. It is rectified and filtered using a Larionov diode bridge circuit.
Deep technical details are redundant, we will limit ourselves to light knowledge:
From the diagram you can see:
Having created the correct electrical connection layout, they begin checking each diode individually. The branch going to ground is tested from the generator side, the other from the load side. The direction is known from Larionov's scheme. We check the diode bridge with a multimeter, touching the base of the black arrow (see figure) of each element with a red probe, and the tip of the same element with a black probe. At the same time, check the insulation of contacts with crescent-shaped planes, incl. neighboring. Based on the data obtained, the need to continue troubleshooting is assessed.
Conclusion: the diode, without desoldering, is checked with a multimeter on a rough structure like a car generator bridge. Ringing an electronic board is more difficult. Any check is carried out with specially shaped probes. For rough designs, use crocodile grips, and check the motherboard with thin needle-shaped probes. In the latter case, there is a chance to test the diode with a multimeter on the board under voltage with the risk of burning the tester.
We hope that the reader now understands how to test a diode with a multimeter.
Among home craftsmen and craftsmen, there is periodically the need to determine the performance of a thyristor or triac, which are widely used in household appliances to change the speed of electric motor rotors, in power regulators for lighting devices and in other devices.
How a diode and thyristor work
Before describing the testing methods, let us recall the design of a thyristor, which is not for nothing called a controlled diode. This means that both semiconductor elements have almost the same structure and operate in exactly the same way, except that the thyristor has the limitation of being controlled through an additional electrode by passing an electric current through it.
The thyristor and diode pass current in one direction, which in many Soviet diode designs is indicated by the direction of the triangle angle on a mnemonic symbol located directly on the case. In modern diodes in a ceramic case, the cathode is usually marked by applying a ring strip near the cathode.
You can check the performance of the thyristor by passing load current through them. To do this, it is allowed to use an incandescent light bulb from old flashlights, the filament of which glows from a current of about 100 mA or less. When current passes through the semiconductor, the light bulb will light up, but if there is no current, it will not.
Read more about how diodes and thyristors work here:
How to check the health of the diode
Usually, to assess the health of the diode, they use an ohmmeter or other devices that have the function of measuring active resistance. By applying voltage to the diode electrodes in the forward and reverse directions, the resistance value is determined. When the p-n junction is open, the ohmmeter will show a value equal to zero, and when closed, it will show infinity.
If there is no ohmmeter, then the health of the diode can be checked using a battery and a light bulb.
Before testing a diode in this way, its power must be taken into account. Otherwise, the load current may destroy the internal structure of the crystal. To evaluate low-power semiconductors, it is recommended to use an LED instead of a light bulb and reduce the load current to 10-15 mA.
How to check the health of a thyristor
The performance of a thyristor can be assessed using several methods. Let's look at three of the most common and available at home.
Battery and light bulb method
When using this method, one should also evaluate the current load of 100 mA created by the light bulb on the internal circuits of the semiconductor and apply it for a short time, especially for the control electrode circuits.
The figure does not show checking that there is no short circuit between the electrodes. This malfunction practically does not occur, but to be completely sure of its absence, you should try passing current through each pair of all three electrodes of the thyristor in the forward and reverse directions. This will only take a few seconds.
When assembling the circuit according to the first option, the semiconductor junction of the device does not pass current, and the light bulb does not light up. This is its main difference in operation from a conventional diode.
To open the thyristor, it is enough to apply a positive source potential to the control electrode. This option is shown in the second diagram. A working device will open its internal circuit and current will flow through it. This will be indicated by the glow of the light bulb filament.
The third diagram shows turning off power from the control electrode and passing current through the anode and cathode. This occurs due to the excess holding current of the internal junction.
The holding effect is used in power control circuits when a short-term current pulse from a phase-shifting device is applied to the control electrode to open the thyristor that controls the amount of alternating current.
The light coming on in the first case or its absence in the second indicates a malfunction of the thyristor. But the loss of glow when the voltage is removed from the control electrode contact can be caused by a current flowing through the anode-cathode circuit that is less than the holding limit value.
An open circuit through the anode or cathode causes the thyristor to turn off.
Test method using a homemade device
The risk of damage to the internal circuits of semiconductor junctions when testing low-power thyristors can be reduced by selecting the current values through each chain. To do this, it is enough to assemble a simple electrical circuit.
The figure shows a device designed to operate from 9-12 volts. When using other supply voltages, the values of resistances R1-R3 should be recalculated.
Rice. 3. Diagram of a device for testing thyristors
A current of about 10 mA is enough to pass through the HL1 LED. If you frequently use the device to connect the electrodes of the VS thyristor, it is advisable to make contact sockets. The SA button allows you to quickly switch the control electrode circuit.
The LED lighting before pressing the SA button or its absence is a clear sign of damage to the thyristor.
Method using tester, multimeter or ohmmeter
The presence of an ohmmeter simplifies the process of checking the thyristor and resembles the previous circuit. In it, the device’s batteries serve as the current source, and instead of the LED glow, the needle deflection for analog models or digital readings on the display for digital devices are used. When the resistance readings are high, the thyristor is closed, and when the resistance is low, it is open.
Here the same three test steps are evaluated with the SA button disabled, pressed for a short time and disabled again. In the third case, the thyristor will most likely change its behavior due to the small value of the current being tested: it will not be enough to hold it.
Low resistance in the first case and high in the second indicate violations of the semiconductor transition.
The ohmmeter method allows you to check the health of semiconductor junctions without unsoldering the thyristor from most circuit boards.
The design of a triac can be conventionally represented as consisting of two thyristors connected opposite to each other. Its anode and cathode do not have strict polarity like a thyristor. They work with alternating electric current.
The quality of the triac state can be assessed using the verification methods described above.
To determine the health of the diode, you can use the following method for checking it with a digital multimeter.
But first, let's remember what a semiconductor diode is.
A semiconductor diode is an electronic device that has the property of unidirectional conductivity.
The diode has two terminals. One is called the cathode, which is negative. The other output is the anode. It is positive.
At the physical level, a diode is a single p-n junction.
Let me remind you that semiconductor devices can have several p-n junctions. For example, the dinistor has three of them! A semiconductor diode is essentially the simplest electronic device based on just one p-n junction.
Let us remember that the operating properties of the diode appear only when connected directly. What does direct connection mean? This means that a positive voltage is applied to the anode terminal ( + ), and to the cathode – negative, i.e. ( - ). In this case, the diode opens and through its p-n junction current begins to flow.
When turned back on, when a negative voltage is applied to the anode ( - ), and to the cathode is positive ( + ), then the diode is closed and does not pass current.
This will continue until the voltage on the reverse-connected diode reaches a critical value, after which damage to the semiconductor crystal occurs. This is the main property of the diode - one-way conductivity.
The vast majority of modern digital multimeters (testers) have the ability to test a diode in their functionality. This function can also be used to test bipolar transistors. It is indicated in the form of a diode symbol next to the marking of the multimeter mode switch.
A little note! It is worth understanding that when checking diodes in direct connection, the display does not show the transition resistance, as many people think, but its threshold voltage! It is also called voltage drop across the p-n junction. This is the voltage above which the p-n junction completely opens and begins to pass current. If we draw an analogy, this is the amount of effort aimed at opening the “door” for electrons. This voltage ranges from 100 to 1000 millivolts (mV). This is what the device display shows.
In reverse connection, when a negative one is connected to the anode ( - ) tester output, and to the cathode positive ( + ), then no values should be shown on the display. This indicates that the junction is working properly and does not allow current to flow in the opposite direction.
In the documentation (datasheets) for imported diodes, the threshold voltage is referred to as Forward Voltage Drop(abbreviated V f), which literally translates as " voltage drop in direct connection".
The voltage drop across the pn junction itself is undesirable. If we multiply the current flowing through the diode (direct current) by the magnitude of the voltage drop, then we get nothing more than power dissipation - the power that is uselessly spent on heating the element.
You can find out more about the diode parameters.
To make it more clear, let’s check the 1N5819 rectifier diode. This is a Schottky diode. We will soon see this.
I draw your attention to the fact that during measurements you cannot hold the leads of the element being tested and the metal probes with both hands. This is a big mistake. In this case, we measure not only the parameters of the diode, but also the resistance of our body. This can significantly affect the result of the test.
You can hold the probes and terminals of the element with only one hand! In this case, only the measuring device itself and the element being tested are included in the measuring circuit. This recommendation is also valid when measuring the resistance of resistors, as well as when checking capacitors. Don't forget this important rule!
So, let's check the diode in direct connection. In this case, the positive probe ( red) connect the multimeter to the anode of the diode. Negative probe ( black) connect to the cathode. In the photo shown earlier, you can see that the cylindrical body of the diode has a white ring on one edge. It is on this side that it has a cathode terminal. This is how the cathode terminal of most imported diodes is marked.
As you can see, the threshold voltage value for 1N5819 appeared on the display of the digital multimeter. Since this is a Schottky diode, its value is small - only 207 millivolts (mV).
Now let's check the diode in reverse connection. We remind you that when switched in reverse, the diode does not allow current to pass through. Looking ahead, we note that in the reverse connection, a small current still flows through the pn junction. This is the so-called reverse current ( I arr.). But it is so small that it is usually not taken into account.
Let's change the connection of the diode to the multimeter's test leads. Red connect the probe to the cathode, and black to the anode.
The display will show " 1 " in the high order of the display. This indicates that the diode does not pass current and its resistance is high. Thus, we checked the 1N5819 diode and it turned out to be fully operational.
Many people ask the question: “Is it possible to test a diode without desoldering it from the board?” Yes, you can. But in this case, it is necessary to remove at least one of its pins from the board. This must be done in order to exclude the influence of other parts that are connected to the diode being tested.
If this is not done, then the measuring current will flow through everything, including through the elements connected to it. As a result of testing, the multimeter readings will be incorrect!
In some cases, this rule can be neglected, for example, when it is clearly visible that there are no parts on the printed circuit board that could affect the test result.
The diode has two main faults. This breakdown transition and its break.
Breakdown. During a breakdown, the diode turns into an ordinary conductor and freely passes current, either in the forward direction or in the reverse direction. In this case, as a rule, the multimeter buzzer beeps, and the display shows the value of the junction resistance. This resistance is very small and amounts to several ohms, or even zero.
Break. When broken, the diode does not pass current either in forward or reverse connection. In any case, the device display shows " 1 ". With such a defect, the diode is an insulator. "Diagnosis" - a break can be accidentally made to a working diode. This is especially easy to do when the tester probes are quite worn out and damaged. Make sure that the measuring probes are in good working order; their wires are oh so “thin” and with frequent use they tear easily.
And now a few words about how the value of the threshold voltage (voltage drop at the junction - Forward Voltage Drop ( V f)) you can roughly judge the type of diode and the material from which it is made.
Here is a small selection made up of specific diodes and their corresponding values V f, which were obtained when testing them with a multimeter. All diodes were previously checked for serviceability.
Diode brand |
diode bridge. Germanium diodes have a forward voltage drop of 300 - 400 millivolts. For example, the D9 germanium point diode we tested, which was previously used as a detector in radio receivers, has a threshold voltage of about 400 millivolts. Schottky diodes have V f in the region of 100 – 250 mV; For germanium diodes V f, as a rule, is equal to 300 – 400 mV; Silicon diodes have the largest voltage drop across the junction equal to 400 – 1000 mV. Thus, using the described technique, you can not only determine the serviceability of the diode, but also approximately find out what material and what technology it is made of. This can be determined by the size V f. Perhaps after reading this technique you will have a question: “How to check the diode bridge?” It's actually very simple. I have already talked about this. |
In this article we will explain how to test a diode with a multimeter. A semiconductor diode, as a component of an electronic circuit, quite often fails for various reasons, for example, exceeding the maximum permissible forward current, reverse voltage, and the like. There are two types of diode malfunction - breakdown and short circuit.
The effect of a diode, as a semiconductor device with a p-n junction, is that it passes electric current only in one direction (from the anode to the cathode), while no current flows in the opposite direction (from the cathode to the anode).
Knowing this property of the diode, you can easily check it for malfunction using a regular one.
Conventional diodes, as well as zener diodes, can be checked using a multimeter. To test this semiconductor device using a digital multimeter, set the multimeter's switch to diode test mode, usually this mode has a diode icon:
It should be noted that when testing in this mode, the multimeter displays direct voltage, and not resistance, when the diode is simply rung in resistance mode.
If the multimeter readings are different, it can be argued that the diode being tested is faulty.
If your multimeter is not equipped with a diode test mode, you can check the diode using the simple diagram shown below.
During this test, the multimeter must be switched to constant voltage measurement mode. If a working diode is connected as indicated in the diagram, the voltmeter will show the forward voltage on the diode. If the diode leads are now swapped, it will not conduct current, and the voltmeter will indicate the supply voltage (in this case, 5 volts).
You can also ring the diode and determine its general condition by measuring resistance, both in the forward and reverse directions.
To do this, you need to switch the multimeter to resistance measurement mode, range up to 2 kOhm. When connecting a diode in the forward direction (red to the anode, black to the cathode), the measuring device will show a resistance of several hundred Ohms; in the opposite direction, the device will show an open circuit symbol, which indicates a very high resistance.
Before moving on to the issue of checking the diode bridge, we will briefly describe it. A diode bridge is an assembly of four diodes connected in such a way that the alternating voltage (AC) supplied to two of the four terminals of the diode bridge turns into a direct voltage (DC) taken from its other two terminals.
Thus, the purpose of the diode bridge is to rectify alternating voltage in order to obtain a constant voltage.
A diode (rectifier) bridge consists of four rectifier diodes connected according to a specific circuit:
Since the diode bridge is designed to rectify alternating voltage (sinusoid), during the first half-wave of alternating voltage one pair of diodes is involved in the operation:
and at the next half-wave another pair of rectifier diodes operates:
Checking a diode bridge is no different from checking a regular diode. You just need to decide which terminals to connect the multimeter to. Conventionally, we number the rectifier terminals from 1 to 4:
It follows that to check the diode bridge we only need to ring 4 diodes:
When checking, you must rely on the readings of the multimeter, as when checking conventional diodes.
