Published Date: 12/24/2017
Threshold voltage is the point at which an electrical device is configured to activate any of its operations. This usually occurs in a transistor that constantly monitors the power supply for changes, ignoring those that are weak or have inadvertently leaked through the system. Once the charge of the incoming electricity is sufficient to meet the specified standard, the threshold voltage is satisfied and is allowed to flow throughout the device to turn it on. Anything below a predefined threshold is contained and treated as a phantom charge.
While determining the threshold voltage in a single-circuit device may seem relatively simple and straightforward, modern electronics require a fairly complex mathematical formula to set and regulate the various thresholds. For example, an appliance such as a dishwasher can be programmed to perform 20 or more functions depending on the user's daily requirements, and each individual phase it enters is activated by an electrical charge. These subtle changes in power allow the device to know when to add more water, when to activate the drying mechanism, or how quickly to rotate the cleaning jets. Each of these actions is set to a different threshold voltage, so when multiple elements need to be activated at once, it requires a lot of planning to ensure proper operation. The equation for calculating the threshold voltage is the sum of the static voltage plus twice the volume potential and the oxide voltage.
The threshold voltage is usually created by a thin inversion layer that separates the insulating body from the actual transistor body. Tiny holes that are positively charged cover the surface of this area, and when electricity is applied, particles in these voids are repelled. Once the current within the inner and outer regions has been equalized, the transponder allows energy to be released to complete the circuit that activates the process. This entire process is completed within milliseconds, and the transistor constantly double-checks to ensure that the current flow is justified, sacrificing power when it is not.
Another term that is used when talking about transponders is metal oxide field effect transistor (MOSFET) threshold voltage. These conductive switches are designed with positive or negative charges, as in the example above, and they are the most common type of transistor in analog or digital devices. MOSFET transistors were originally proposed in 1925 and were built using aluminum until the 1970s, when silicon was discovered as a more viable alternative.
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During the life cycle of a building, renovation work is necessary at certain periods to update the interior. Modernization is also necessary when interior design or functionality lags behind modern times.
There are more than 100 million housing units in Russia, and most of them are “single-family houses” or cottages. In cities, suburbs and rural areas, own homes are a very common type of housing.
The practice of designing, constructing and operating buildings is most often a collective effort among various groups of professionals and professions. Depending on the size, complexity and purpose of a particular building project, the project team may include:
1. The real estate developer who provides financing for the project;
One or more financial institutions or other investors that provide financing;
2. Local planning and management bodies;
3. Service that carries out ALTA/ACSM and construction surveys throughout the project;
4. Building managers who coordinate the efforts of various groups of project participants;
5. Licensed architects and engineers who design buildings and prepare construction documents;
Voltage surges or surges still occur in power networks today. Nobody likes them, because when the voltage is too low, the bulbs begin to burn dimly, and when the voltage is too high, many electronic devices simply burn. This matter hits us in our pockets, and not in the pockets of network organizations. Has anyone tried to prove that the voltage was outside the norm and that’s why the TV burned out? Has anyone received compensation for burned equipment? I don't know such people.
Therefore, it is worth thinking about protecting your property from the harmful effects of unstable voltage in the network. To do this, you can use stabilizers or voltage relays. Here we will talk about second devices. Let's find out what types of voltage relays there are, how they are designed, how to use them and where to place them. You will also find here a visual diagram for connecting the voltage relay UZM-51M and RV-32A.
Today, these devices are produced by many manufacturers. These are RV-32A from EKF, UZM-51M from Meander, CM-PVE from ABB, RM17UAS15 from Schneider Electric and others. All of them are microprocessor devices designed to constantly monitor voltage and protect electrical installations from surges. If the mains voltage is within the set values on the device, then everything works. If the voltage in the network goes beyond these limits, the relay opens its power contacts, thus turning off the load.
Voltage relays are produced for monitoring both single-phase and three-phase networks. Depending on what type of network you have, choose the appropriate relay. They can be used with any grounding system in your home.
Remember that voltage relays do not replace circuit breakers, RCDs, RCBOs, SPDs).
Inside them there is a powerful relay with a control coil. The power contacts of the relay switch the phase circuit. The neutral conductor usually passes through the devices. This is implemented for ease of installation.
Below is a screenshot from the UZM-51M passport
There is also a schematic diagram on the RV-32A case, which shows that the neutral conductor simply passes through the device as a draft.
This device has two contacts on the top and bottom. One is connected to the “phase” and “zero” coming from the network, and to the other the “phase” and “zero” going to the load. They are signed on the case. Here you need to be careful, since for one manufacturer the input is connected to the lower contacts, and for another - to the upper.
For the UZM-51M relay, the input is connected to the upper contacts, and the load is connected to the lower ones.
For the RV-32A relay, the opposite is true. Its input is connected to the lower contacts, and the load is connected to the upper ones.
Such devices should be installed immediately after the input machine. This is necessary so that in a critical situation it can protect all electronic devices in the apartment.
Modern electricity meters are also electronic devices and overvoltage is also critical for them. Therefore, it is worth installing a voltage relay in front of the electrical energy meter. Only this point needs to be agreed upon with the network company, since the relay will also have to be sealed. As an option, you can install the input circuit breaker and voltage relay in a separate plastic box for sealing.
Also note that the power contacts of the UZM-51M are designed for a maximum load current of 63A, and the RV-32A only for 32A. Be sure to pay special attention to this. If you have an input circuit breaker rating greater than 32A, then the relay from EKF can no longer be used.
Both types of relays are mounted on a standard DIN rail and occupy two modules in the cabinet.
Below I post two visual diagrams that can help you understand the essence of the connection. In the first diagram, the UZM-51M is connected, and in the second, the RV-32A.
This device has two manual controls.
The upper regulator uses a screwdriver to set the upper voltage threshold. These are 240, 250, 255, 260, 265, 270, 275, 280, 285, 290 V. The error is ± 3V.
The lower regulator sets the lower cut-off voltage threshold. These are 100, 110, 120, 130, 140, 150, 160, 175, 190, 210 V. The error is ± 3V.
After power is applied to the relay, it first holds for 5 seconds and only then the green indicator begins to flash, which indicates that the specified turn-on time delay is counting. If the voltage is within the set thresholds, the yellow and green indicators light up and power is supplied to the load. You can also speed up the device’s startup by pressing the “Test” button.
If the relay was turned off due to the network voltage exceeding the thresholds, then it turns on automatically 10 seconds after the network voltage returns to the specified limits.
This relay has the ability to independently change the delay time for turning on the device. The delay can only be 10 seconds or 6 minutes. How to set it up? This is done like this:
Also note that when you press the "Test" button in emergency mode, the relay will not turn on the load.
When the mains voltage approaches the upper limit, the red indicator starts flashing. When the network goes beyond the limits, the load is disconnected, the yellow indicator goes out and the red indicator starts to light constantly.
When the mains voltage approaches the lower threshold, the green indicator starts blinking. When the voltage goes beyond the limits, the shutdown time delay starts counting and the red indicator starts flashing. When the delay time ends, the load is turned off, the yellow indicator goes out, and the red indicator starts to light every two seconds.
Alternate flashing of the red and green indicators means that you have forcibly disconnected the load from the network by pressing the "Test" button. Pressing it again and holding it for 2 seconds returns the device to working condition.
Now I think you won’t get confused by the blinking of these indicators.
There are already four manual controls.
The upper left regulator uses a small slotted screwdriver to set the upper cut-off voltage threshold. These are 225, 235, 245, 255, 265, 275 V.
The lower left regulator sets the lower cut-off voltage threshold. These are 165, 175, 185, 195, 205, 215 V.
The upper right knob sets the device response time delay when an emergency occurs. These are 0.1, 2, 4, 6, 8, 10 seconds.
The lower right knob sets the delay time for power supply to the load after the mains voltage returns to the established limits. These are 0.3, 6, 12, 18, 24, 30 seconds.
This relay has an error of 3% of the set threshold.
RV-32A indicator signaling:
Now I think you won’t get confused by the blinking indicators of this voltage relay either.
Do you use a voltage relay at home?
Let's smile:
A man was admitted to the hospital with a broken jaw in three places. When he recovered and was able to speak, the surgeon asked what happened.
- I work as an excavator operator. On Friday evening, leaving work, I noticed an open hatch near a construction site. To prevent any passers-by from falling into it, he brought an excavator and covered the hatch with a bucket. On Monday I come to work, start the excavator, lift the bucket, and three electricians climb out of the hatch...
Field-effect transistor. Definition. Designation. Classification (10+)
Field-effect transistor
A field-effect transistor (FET) is an electronic device that allows you to regulate current by changing the control voltage. As I wrote earlier, to design electronic circuits there is no need to have an understanding of the physical principles of operation and the design of an electronic device. It is enough to know that it is a black box with certain characteristics. Nothing will change if they suddenly invent a new technology that makes it possible to make devices with characteristics similar to field-effect transistors, but based on different principles. We will put them in the same schemes and call them field workers.
A field-effect transistor is a device with four terminals: Source, Drain, Gate, Substrate. Control voltage is applied between Gate and Source. In most cases, the substrate inside the package is connected to the source, so that three leads stick out. Some types of field-effect transistors do not have a substrate (p-n junction transistors).
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Field-effect transistors (FETs) are increasingly being used in amateur radio designs, especially in VHF equipment circuits. But many refuse to assemble them, although the circuits are simple, time-tested, since they use PTs, which have special requirements for describing the circuits. Many PT devices and testers are described in magazines and the Internet (5,6), but they are complex, because at home it is difficult to measure the basic parameters of PT. Devices for testing PTs are very expensive and there is no point in buying them for the sake of selecting two or three PTs.Test circuit for field effect transistors (reduced)
At home, it is possible to measure approximately the main parameters of the PT and select them. To do this, you must have at least two instruments, one of which measures current, and the other voltage, and two power sources. Having assembled the circuit (1, 2), you first need to set the zero voltage on the gate VT1 with resistor R1, the R1 slider in the lower position with resistor R2 set the drain-source voltage Usi VT1 according to the reference book, for the transistor being tested, usually 10-12 volts. Then connect the PA2 device, switched to the current measurement mode, to the drain circuit and take a reading, Ic.init is the initial drain current, it is also called the DC saturation current at a given drain-source voltage and zero gate-source voltage. Then, slowly moving the R1 slider behind the PA2 reading and as soon as the current drops to almost zero (10-20 μA), measure the voltage between the gate and source, this voltage will be the cutoff voltage Uots..
To measure the slope of the SmA/V DC characteristic, you must again set the zero voltage U with resistor R1, PA2 will show Is.start. Resistor R1 also slowly increases the voltage Uzi to one volt across PA1, to simplify the calculation, PA2 will show a lower current Ic.measurement. If we now divide the difference between the two readings PA2 by the voltage Uzi, the resulting result will correspond to the slope of the characteristic:
SmA/B=Is.beginning - Is.measurement/Uzi.
This is how transistors with a control p-n junction and a p-type channel are checked; for an n-type PT, you need to reverse the switching polarity Upit.
There are also insulated gate field effect transistors. There are two types of MOS transistors with induced and built-in channels.
Transistors of the first type can only be used in enrichment mode. Transistors of the second type can operate in both channel depletion and channel enrichment modes. Therefore, insulated gate field effect transistors are often called MOS transistors or MOS transistors (metal oxide semiconductor).
In MOS transistors with a built-in channel a conducting channel, manufactured technologically, is formed when the voltage at the gate is equal to zero. The drain current can be controlled by changing the value and polarity of the voltage between the gate and source. At some positive gate-source voltage of a transistor with a p-channel or a negative voltage of a transistor with an n-channel, the current in the drain circuit stops. This voltage is called cut-off voltage (Uots). A MOS transistor with a built-in channel can operate both in the mode of enrichment and in the mode of depletion of the channel by the main charge carriers.
Operation of a p-channel induced MOSFET. In the absence of bias (Usi = 0; Usi = 0), the near-surface layer of the semiconductor is usually enriched with electrons. This is explained by the presence of positively charged ions in the dielectric film, which is a consequence of the previous oxidation of silicon and its photolithographic processing.
The gate voltage at which the channel is induced is called the threshold voltage Unop. Since the channel appears gradually as the gate voltage increases, to eliminate ambiguity in its definition, a certain value of the drain current is usually set, above which it is considered that the gate potential has reached the threshold voltage Unop.
When a negative voltage is applied, the channel expands and the current increases. Thus, MOS transistors with built-in channels operate in both depletion and enrichment modes.
Sometimes the MOSFET structure has a built-in diode between the source and drain. The diode does not affect the operation of the transistor, since it is connected in the reverse direction to the circuit. Recent generations of power MOSFETs use a built-in diode to protect the transistor.
The main parameters of field-effect transistors are considered to be;
1 . Initial drain current Is.init - drain current when the voltage between the gate and source is zero. Measured at a given value of constant voltage Uc for a transistor of a given type.
2 . Residual drain current Is.res. - drain current when the voltage between the gate and source exceeds the cut-off voltage.
3 . Gate leakage current Iz.ut - gate current at a given voltage between the gate and the remaining terminals, closed to each other.
4 . Reverse current of the gate-drain transition Iзс.о - current flowing in the gate-drain circuit at a given reverse voltage between the gate and drain and the remaining open terminals.
5 . Reverse current of the gate-source transition Izi.o - current flowing in the gate-source circuit at a given reverse voltage between the gate and source and the remaining open terminals.
6 . Cut-off voltage Uots - the voltage between the gate and source of a p-n junction or insulated gate transistor operating in depletion mode, at which the drain current reaches a specified low value (usually 10 μA).
7 . Threshold voltage of field effect transistor Upor - the voltage between the gate and source of an insulated gate transistor operating in the enrichment mode at which the drain current reaches a specified low value (typically 10 μA).
8
. The slope of the field-effect transistor characteristics
S
- the ratio of the change in drain current to the change in gate voltage during an AC short circuit at the output of the transistor in a circuit with a common source.
For these measurements, it is also necessary to introduce a voltage polarity switch between the gate and source. By switching with this switch the polarity supplied to the gate of the transistor being tested, the parameters of the PT are measured. The procedure is quite long, but what if there is only one tester. And in this case, it is possible to check the field-effect transistor, the verification process is the same as described above, but only even longer, since a lot of switching and other operations will need to be done. This method for checking and selecting PTs is not suitable when purchasing in stores and radio markets.
As you know, assembling a DC voltmeter is much easier than a milliammeter, having the same head, and every radio amateur, even beginners, have combined instruments. By assembling the device according to the diagram shown in the figure, you can significantly simplify the procedure for checking the PT many times over. This device can be made even by novice radio amateurs who have no experience working with PT. The device is powered by 9 volts from a stabilized voltage converter assembled according to the circuit from the Radio magazine (3).
The principle of measuring PT parameters. Having set switches SA1-SA3, SB2 in the desired position, depending on the type and channel of the PT being tested, connect any tester, pointer or digital (preferred), to sockets XS1, XS2, switched to DC measurement mode, connect to sockets XS3 in accordance with a PT base and turn on the device with switch SA4.
All components of the device are installed in a suitable housing, the size of which depends on the size of the components and the PA1 head used. On the front side there are PA1, SA1-SA3, XS1-XS2, R1, R2 with corresponding inscriptions indicating functions. The converter is installed in the device body, from which there is a connector for connecting to the GB1 battery.
PA1 - microammeter type M4200 with a current of 300 μA, with a scale of 15 V, it is possible to use others, the size of the case will depend on its dimensions, when selecting R3, R4 when setting up, R1, R2 - SP4-1, SPO-1 with a resistance of 4, 7 kOhm to 47 kOhm, R3, R4 - MLT-0.25, S2-23 and others. Switches SA1 - 3P12NPM, 12P3N,PG2, PG3, P2K, SB1 - P2K. Toggle switches SA2 - SA4 - MT-1, P1T-1-1 and others.
Transformer TP1 in the converter is made of a ferrite armored magnetic core with an outer diameter of 30 and a height of 18 mm. Winding I contains 17 turns of PEL 1.0 wire, winding II contains 2x40 turns of PEL 0.23 wire. It is possible to use a different core with appropriate recalculation.
Transistors VT1 - KT315, KT3102, VT2, VT3 - KT801A, KT801B, VT4 - KT805B and others, diodes VD1, VD2 - KD522, KD521, VD4-VD7 - KD105, KD208, KD209 or diode bridge KTs407, DD microcircuit 1 - K555LN1, K155LN1 .
As XS3, a crib for microcircuits is used, installed on a printed circuit board and soldered to the PT type (pin layout) in order not to bend the PT leads or other connector soldered accordingly. The installation is extensive. The converter board is installed on the bottom (back cover).
Setting up the device is practically not required. A correctly assembled converter, made from serviceable parts, starts working immediately, the output voltage of 15 V is set with trimming resistor R4, monitoring the voltage with a voltmeter.
Then the sliders of resistors R1, R2 are set to the lowest position according to the diagram, which corresponds to zero voltage. Switch SA3 is moved to the 1.5 V position, and SA2 to the Uzi position. Having connected the control voltmeter to the R1 engine, move it by monitoring the reading of PA1 on the control voltmeter and if it differs, select the resistance of the resistor R3. After selecting resistor R3, switch SA3 to position 15 V and then move the R3 slider, controlling the voltage and if it also does not match, select R4. In this way, the internal voltmeter of the device is adjusted. After all the settings, close the back cover, the device is ready for use.
As practice shows, the following provisions are important for a radio amateur:
1. Check the serviceability of the PT. To do this, it is usually enough to make sure that its parameters are stable, do not “float” and are within the reference data.
2. Based on certain characteristics, select from the few PT copies available to the radio amateur those that are more suitable for use in the assembled circuit. Usually the qualitative principle “more is less” works here.
For example, you need a field-effect transistor with a higher S or lower cutoff voltage. And from several copies, the one with the best (more or less) selected indicator is selected. Thus, high accuracy of measured parameters in practice is often not as important as one might think.
Nevertheless, the proposed device makes it possible to check the performance and most important characteristics of the PT with fairly high accuracy.
Insert the field-effect transistor into connector XS3 in accordance with the base of the PT being tested. By turning on the device, resistor R2 sets the drain-source voltage Usi specified in the reference book for this transistor. Move SA2 to the Uzi position, and SA3 to 1.5 V. Press the SB1 “Measure” button. in this case, the PA2 tester will show some value, for example 0.8 mA at the limit of 1 mA, this value indicates the initial drain current Is.init. Record this value for a given PT. Then the R1 “Uzi” slider is slowly moved while controlling the gate voltage across PA1, the Uzi voltage is increased until the drain current Ic measured by the PA2 tester decreases to the minimum specified, usually 10-20 µA, switching PA2 to lower limits. As soon as the current decreases to the specified value, a reading is taken from PA1 (for example, 0.9 V), this voltage is the DC cutoff voltage Uots., it is also recorded.
To measure the slope of the SmA/B characteristic, set the PA2 tester to the limit that was originally set for this transistor and reduce Uzi to zero, PA2 will show Is.beginning. Resistor R1 slowly increases Uzi to 1 V according to PA1, PA2 will show a lower current Ic.measurement. If we now subtract Is.measurement from Is.initial, this will correspond to the numerical value of the slope of the SmA/V DC characteristic. A digital tester with automatic limit changes is preferable.
In this way, it will be possible to select PTs with similar parameters from the same batch with the same or different letter indices, because different indices only indicate the spread of PT parameters, so KP303A have Uots. - 0.3-3.0 V, SmA/V - 1-4, and KP303V Uots. - 1.0 - 4.0 V, SmA/V - 2-4, but some PTs with different indexes may have the same values for a given drain-source voltage Usi. which is quite important when selecting PT.
Measurement of parameters of MOSFETs with a built-in channel, depletion mode. Switch SA1 sets the type of channel, SB2 is set to depletion mode, resistors R1, R2 are set to zero positions, connect to sockets XS1 and XS2 a tester switched to the mode for measuring current to the limit specified in the reference book for this PT. Move SA2 to the Uс position, and SA3 to the 15 V position. Insert the PT into connector XS3 in accordance with the base of the PT being tested. For double-gate or with PT substrate, the second gate, the substrate is connected to the housing contact “K” of the XS3 connector. Resistor R2 sets the drain-source voltage Usi specified in the reference book for this transistor. Then switch SA2 to the Uzi position, and SA3 to the 1.5 V position. PA2 is switched to the minimum current measurement mode. After turning on the device, press the SB1 button, the PA2 microammeter will show some current, this will be the initial drain current Is.init.
As the voltage Ui increases, the drain current Ic will decrease and at a certain value it will become minimal, about 10 μA; the readings taken from PA2 will be the cutoff voltage Uots.
To check the transistor in enrichment mode, switch SB2 is moved to the “Enrichment” position and the gate voltage Uzi is increased, while the drain current Ic will increase.
As mentioned above, induced channel MOSFETs can only operate in enrichment mode. Measuring the parameters of MOS-type field-effect transistors with an induced channel. Switch SA1 sets the type of channel, SB2 is set to enrichment mode, resistors R1, R2 are set to zero positions, connect to sockets XS1 and XS2 a tester switched to the mode for measuring current to the limit specified in the reference book for this PT. Move SA2 to the Uс position, and SA3 to the 15 V position. Insert the PT into connector XS3 in accordance with the base of the PT being tested.
For double-gate or with PT substrate, the second gate, the substrate is connected to the housing contact “K” of the XS3 connector. Resistor R2 sets the drain-source voltage Usi specified in the reference book for this transistor. Then switch SA2 to the Uzi position, and SA3 to the 1.5 V position. PA2 is switched to the minimum current measurement mode. After turning on the device, press the SB1 button. When Uzi = 0, drain current Ic = 0.
By increasing the voltage Ui, monitor the change in the drain current Ic and at a certain voltage Ui, the drain current will begin to increase; this will be the threshold voltage Uthr. With its further increase, the drain current Ic will increase.
This device can measure the parameters Is.init, Uots., S ma/V DC of medium and high power by applying the required voltage to the external connector XP1, according to the reference books for this DC, adding the necessary measurement limits with the internal voltmeter PA1, adding the required number of resistors to switch SA3. Diodes VD5, VD6 protect the converter from external voltage.
If you do not need to measure the exact values of Is.init and Uots., but only select PTs with similar parameters, instead of PA2 you can include indicators used in household appliances to monitor signal levels, M4762, M68501, M4248, M4223 and the like, adding to these indicators a switch and shunts for different currents. All other measurements are made according to the method described above. I have been using this device for more than six years. It is very helpful in the design of field-effect transistor equipment, where special requirements apply to them.
Literature:
1. The simplest ways to check the serviceability of electrical radio elements in repair and amateur conditions, p. 70, 300 practical tips. Bastanov V.G. - Moscow worker 1986
2. Measurement of parameters and application of field-effect transistors, - "Radio", 1969, No. 03, pp. 49-51
3. Stabilized voltage converter - Radio No. 11 1981 p. 61 (abroad).
4. Entertaining experiments: some possibilities of a field-effect transistor - "Radio", number 11, 1998. B. Ivanov
5. Attachment for testing transistors. Radio No. 1 – 2004, pp. 58-59.
6. Tester of field-effect transistors - A.P. Kashkarov, A.L. Butov - Circuits for home radio amateurs pp. 242-246, MRB-1275 2008
7. Measuring the parameters of field-effect transistors, - "Radio", 2007, No. 09, pp. 24-26.
8. Meerson A.M. Radio measurement technology (3rd ed.). MRB - Issue 0960 pp. 363-367. (1978)
The design was sent to the competition by: Alexander Vasilievich Slinchenkov, Ozersk, Chelyabinsk region.
Semiconductor diodes are commercially available electronic circuit components. It is on them that rectifiers are built. The range of diodes is extremely extensive. To use them correctly in rectifiers, you need to know and understand the meaning of their basic technical characteristics.
The main static characteristics of semiconductor diodes are discussed below.
The threshold voltage U pg is the voltage value at the junction, starting from which the semiconductor diode conducts current. At forward voltages below the threshold, the diode practically does not conduct current. It is generally accepted that the threshold voltage is 0.7V for silicon devices and 0.3V for germanium devices. As noted above, the actual voltage drop between the diode terminals U d is always greater than the threshold U pg (Fig. 10, a).
U For silicon devices, the actual voltage drop is
1 V. The threshold voltage varies from sample to sample, even for devices of the same type (Fig. 10, b). For discrete diodes, this difference can reach 0.1V. For diodes manufactured using integrated technology, it does not exceed 0.01V. Therefore, the direct branches of the current-voltage characteristics of semiconductor devices do not coincide.
The threshold voltage of semiconductor diodes also depends on temperature. It decreases at a rate of – 2.5 mV/0 C with increasing transition temperature. This means that even if the direct branches of the characteristics of two diodes initially coincided (Fig. 10, c), then when, for example, diode 1 is heated to a temperature exceeding the temperature of diode 2, the direct branch of the current-voltage characteristic of the 1st diode will shift to the left ( dotted line in Fig. 10, c).
By rated we mean the maximum direct current that can flow through the diode for an arbitrarily long time without destroying the device. The concept of rated current is related to the concept of permissible power dissipation in the diode.
When current I pr flows through the device due to a finite voltage drop U pr across it, power P in =U pr I pr is released in the device. This leads to heating of the junction, i.e., its temperature Tp exceeding the ambient temperature T0. The latter causes the outflow of heat from the transition into the environment, that is, power dissipation. The higher the junction temperature Tp compared to the ambient temperature T0, the greater the power dissipation. Obviously, at P in =const, an increase in the scattering power P races, caused by an increase in the transition temperature, can lead to thermal equilibrium P in =P races, observed at a certain transition temperature. The relationship between the dissipation power P races and the temperature difference T = T p –T 0 is assumed to be linear for small temperature differences T . This relationship is usually written in the form of the relation T=R T P races similar to Ohm’s law for resistive electrical circuits. The coefficient R T is called the thermal resistance of the transition-medium section. R T is determined practically by the surface area of the diode body. Since the diode housings are unified, each specific type of diode corresponds to a very specific value of R T.
As is known, the temperature of p-n junctions is limited to a certain permissible value T p dp, exceeding which means failure of the device. For silicon devices T p dp ≈ (175÷ 200) ° C, and for germanium
niev T p dp ≈ (125÷ 150) ° C.
It follows that at room temperature, for each specific type of diode there is a concept of permissible power dissipation
T pdp − T 0 P dis.dp(T pdp) R T .
Thus, under conditions of thermal equilibrium, the power released in the device is limited:
T dp − T 0 |
|||||
Taking into account the approximate constancy of the forward voltage drop across semiconductor diodes
P ex dp = I d dpU p = I d dp const ≈ I d dp 1B = | I d dp |.
It follows: I ddp = T ddp − T 0 . Due to the constancy of U p = 1V power
The power released in the diode is determined by the average current through the diode.
Then I d dp = I av dp.
For this reason, the average current through the diode, specified in the technical documentation, is the permissible value of the average current at room temperature. As the ambient temperature increases, this current must decrease accordingly to avoid diode failure. An increase in I avg dp is possible due to a decrease in R T. This means that it is necessary to increase the heat-dissipating surface of the diode, that is, add a heat sink to it.
As follows from the above, I av dp is a measure of the permissible power dissipation in the diode. So a diode with an average current of 1A is able to dissipate a power approximately equal to 1 W at room temperature.
Thus, for each specific type of device there is a concept of current that is permissible at room temperature, the excess of which leads to burnout of the diode. The rated current, as a current that guarantees reliable operation of the diode, is selected less than permissible.
The rated current through the diode decreases with increasing ambient temperature. It can also be increased by decreasing R T. This is achieved by increasing the heat-removing surface of the diode - a special structural element called a heat sink is attached to the diode body.
Peak or maximum currents through a diode can significantly exceed their rated values. The issue of peak currents is more complex than that of rated currents. The permissible values of peak currents in diodes depend not only on the magnitude, but also on the duration, as well as on the frequency of their repetition. Thus, at a frequency of about 50 Hz, peak currents lasting 5 ms can exceed the nominal currents by 10–20 times. When the duration is reduced to 2 ms, current pulses can exceed the rated current by 50–100 times. Most often, the actual characteristics of pulsed currents in electrical circuits are difficult to determine. For this reason, it is better not to exceed their official permissible values.
The reverse current at room temperature is negligible in silicon devices, but significant in germanium devices. Unfortunately, this current
increases exponentially with increasing transition temperature. It can be roughly estimated by the formula
I o (T 1) = I o (T 0) 2(T 1 − T 0)/10,
where Iо (T 1 ) is the reverse current at the transition temperature T 1 ; Iо (T 0 ) – reverse current measured at the transition temperature T 0 . Naturally, the assessment of the current using this formula is the more reliable, the smaller T = T 1 – T 0.
Reverse voltage U rev, as a technical characteristic of the diode, is put in correspondence with its breakdown voltage. Naturally, it is less than the breakdown voltage, because in the breakdown mode the diode loses the property of one-way conductivity - it ceases to be a diode. Usually U about is determined with some margin.
In addition to the listed static technical characteristics of the diode, there are also dynamic ones. The most significant ones are discussed below.
Since at U pr >0.1 V the direct branch of the current-voltage characteristic of the diode is determined by relation (2), the dynamic resistance of the device - its resistance to increments of forward current through the junction - can be determined by a simple procedure:
∂i |
/ϕ T |
I pr |
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or r = |
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∂u |
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An ideal diode connected in series with a resistive load (Fig. 11, a) passes current only in the forward direction. When the sign of the voltage in the circuit U c changes, the reverse current through the diode stops.
appears (Fig. 11, b and c).
In real semiconductor diodes, opening the circuit when the sign of the circuit voltage instantly changes from direct to reverse does not occur immediately. The fact is that when passing through a crystal, direct current saturates it with the main carriers. Their concentration in the crystal is proportional to the magnitude of the forward current. In order for the diode to open the circuit so that the crystal becomes non-conducting, it is necessary to remove the main current carriers from the crystal, that is, to create a depletion zone at the boundary of the contact of the layers of the p and n semiconductor. This process takes time. During this time - the carrier resorption time t r - the diode conducts current in the reverse direction, as well as in the forward direction (Fig. 12).
U c |
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U c |
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At the end of the resorption process, a slow decline in the reverse current through the diode occurs to the value I 0 (Fig. 12, a). The resorption time and the decay time in total form the turn-off time of the diode. The diode turn-off time t off is a technical characteristic of the diode.
U c |
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t on |
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U c |
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