Protection device for any power supply. Protection of power supply from KZ. Short Circuit Protection Scheme

The devices require a power supply (BP) in which there is an adjustment of the output voltage and the ability to regulate the level of response of protection against current over current over a wide limits. When the protection is triggered, the load (connected device) must automatically turn off.

The search on the Internet gave several suitable power supplies schemes. Stopped at one of them. The scheme is easy to manufacture and adjust, consists of available parts, executes the stated requirements.

The power supply proposed to be manufactured on the basis of the LM358 operating amplifier and it has the following characteristics:
Input voltage, in - 24 ... 29
Output stabilized stress, in - 1 ... 20 (27)
Current triggering, A - 0.03 ... 2.0

Photo 2. BP scheme

Description of the work of BP

The adjustable voltage stabilizer is assembled on the DA1.1 operating amplifier. The input of the amplifier (output 3) comes with an exemplary voltage from the engine of the variable resistor R2, for the stability of which the stabilitron VD1 corresponds to the inverting input (output 2), the voltage comes from the Emitter of the VT1 transistor through the voltage divider R10R7. Using a variable resistor R2, you can change the output voltage of BP.
The overcurrent protection block is made on the DA1.2 operating amplifier, it compares the voltages on the inputs of the OU. On the input 5 through the resistor R14, voltage from the load current sensor is R13 resistor. The inverting input (pin 6) comes with an exemplary voltage, for the stability of which the VD2 diode corresponds to the stabilization voltage of about 0.6 V.

While the voltage drop created by the load current on the R13 resistor, less than the exemplary, the output voltage (output 7) of the DA1.2 is close to zero. In the event that the load current exceeds the allowable set level, the voltage on the current sensor and the voltage at the OU DA1.2 output will increase almost to the supply voltage. This will include the HL1 LED, signaling the transistor VT2, shunting the R12 resistor by shunting the R12 resistor. As a result, the VT1 transistor closes, the output voltage of the BP will decrease almost to zero and the load will turn off. To turn on the load, click on the SA1 button. Adjusting the level of protection is performed using a variable resistor R5.

Manufacturing BP

1. The basis of the power supply, its output characteristics determines the current source - the transformer used. In my case, I found the use of a toroidal transformer from the washing machine. The transformer has two output windings on 8V and 15V. By connecting both windings consistently and adding a rectifier bridge on the hand-held diodes of the CD202M CD202M, received a constant voltage source of 23V, 2a for BP.

Photo 3. Transformer and rectifier bridge.

2. Another defining part of the BP is the body of the device. In this case, a children's diaper projector escaped in the garage was found. Removing excess and processing in front of the hole to install a microammmetry showing, the BP housing is prepared.

Photo 4. Billet BP

3. Installation of the electronic circuit is made on a universal mounting board with a size of 45 x 65 mm. The layout of parts on the board depends on the size found in the economy of components. Instead of resistors R6 (Currency current setting) and R10 (maximum output voltage limit) on the board installed trimming resistors with an increased by 1.5 times with a nominal value. At the end of the setting of the BP, they can be replaced with permanent.

Photo 5. mounting board

4. Assembling board and remote elements of the electronic circuit in full for testing, settings and adjusting output parameters.

Photo 6. BP control node

5. Production and fitting of the shunt and additional resistance to use a microamermeter as an ammeter or voltmeter of BP. Additional resistance consists of consistently connected constant and trimming resistors (on the photo from above). The shunt (in the photo below) is included in the main circuit of the current and consists of a wire with a low resistance. The wire section is determined by the maximum output current. When measuring the current, the device is connected parallel to the shunt.

Photo 7. Microammetter, Shunt and Additional Resistance

The adjustment of the length of the shunt and the magnitude of the additional resistance is performed with the corresponding connection to the device with control over the multimeter. Switching the device to an ammeter / voltmeter mode is performed by a toggle switch in accordance with the scheme:

Photo 8. Control mode switching scheme

6. Marking and processing of the front panel of BP, installation of remote parts. In this embodiment, a micro-vetermeter is deposited on the front panel (switching toggle switch A / V to the right of the device), output terminals, voltage regulators, and current, operation mode indicators. To reduce losses and due to frequent use, a separate stabilized 5 V output was additionally removed. For which the voltage, from the 8V transformer winding, is fed to the second rectifier bridge and the standard circuit on 7805 having built-in protection.

Photo 9. Facial panel

7. Build BP. All Elements of BP are installed in the housing. In this embodiment, the radiator of the control transistor VT1 serves as an aluminum plate with a thickness of 5 mm, fixed in the upper part of the housing cover, which serves as an additional radiator. The transistor is fixed on the radiator through an electrically insulating gasket.

Today, my article will be solely theoretical character, or rather in it will not be "iron" as in previous articles, but do not be discouraged - it has not become less useful. The fact is that the problem of protecting electronic nodes directly affects the reliability of devices, their resource, and therefore, on your important competitive advantage - ability to give a long-term product warranty. The implementation of protection concerns not only my favorite power electronics, but also any device in principle, so even if you design IoT-crafts and you have a modest 100 mA - you still need to understand how to ensure the trouble-free operation of your device.

Current protection or short-circuit protection (KZ) is probably the most common type of protection because the neglect of this issue causes destructive consequences in the literal sense. For example, I propose to look at the voltage stabilizer, which has become sad from the resulting KZ:

The diagnosis here is simple - an error occurred in the stabilizer and in the circuit began to flow ultrahigh currents, according to good protection was to turn off the device, but something went wrong. After familiarization with the article, I think you yourself will be able to assume what the problem could be.

As for the load itself ... If you have an electronic device with a matchbox, there are no such currents, then do not think that you cannot become as sad as the stabilizer. Surely you do not want to burn the packs of chips on 10-1000 $? If so, I invite you to get acquainted with the principles and methods of combating short circuits!

The purpose of the article

I focus on people for whom electronics are hobbies and beginner developers, so everything will tell "on the fingers" for a more meaningful understanding of what is happening. For those who want academics - we go and read any university tutorials on electrical engineering + "Classic" Horowitsa, Hill "Art of Chairchiki".

Separately, I wanted to say that all solutions will be hardware, that is, without microcontrollers and other perversions. In recent years, it has become quite fashionable to program where it is necessary and not necessary. Often I observe the "protection" on the current, which is implemented by a banal measurement of the ADC voltage of any arduino or a microcontroller, and then the devices still fail. I strongly do not advise you to do the same! I'll tell you more about this problem in more detail.

A little about short circuit currents

In order to start inventing protection methods, you must first understand with what we are struggling at all. What is a "short circuit"? Here we will help our favorite law of Oma, consider the ideal case:

Simply? Actually this scheme is an equivalent circuit of almost any electronic device, that is, there is a source of energy, which gives it to the load, and he heats and does something else or does not.

We agree that the source power allows the voltage to be permanent, that is, "do not see" under any load. In normal operation, the current acting in the chain will be equal to:

Now imagine that Uncle Vasya dropped the wrench on the wires going to the light bulb and our load decreased 100 times, that is, instead of R it became 0.01 * R and with the help of simple calculations we get a current 100 times more. If the bulb consumed 5a, now the current from the load will be selected about 500A, which is enough to melt the uncle Wasi key. Now a small conclusion ...

Short circuit - A significant reduction in load resistance, which leads to a significant increase in current in the chain.

It is worth understanding that the currents of the KZ are usually hundreds and thousands of times more than the current nominal and even a short period of time enough so that the device fails. Here, many will surely remember the electromechanical devices of protection ("automata" and others), but here everything is very prosaic ... Typically, the household socket is protected by an automatic with a rated current 16a, that is, the shutdown will occur at 6-7 to a fold current, which is already about 100a. The laptop power supply has a power of about 100 W, that is, the current is less than 1a. Even if the KZ occurs, the automatic will not be noticed for a long time and turn off the load only when everything is already burning. It is rather protection against fire, not the protection of technology.

Now let's consider another, often occurring case - through current. I'll show it on the example of the DC / DC converter with a synchronous BUCK topology, all MPPT controllers, many LED drivers and powerful DC / DC circulars on boards built according to it. We look at the converter scheme:

The diagram indicates two options for exceeding the current: green Path For the "classic" KZ, when the load resistance decreased ("snot" between roads after soldering, for example) and orange Path. When can the current flow over the orange path? I think many know that the resistance of the open channel of the field transistor is very small, in modern low-voltage transistors it is 1-10 MΩ. Now imagine that the keys simultaneously came the PWM with a high level, that is, both keys opened, for the source "VCCIN - GND" is equivalent to the load connection with resistance of about 2-20 MΩ! Apply the Great and Mighty Ohm Law and get even when nutrition 5V current value of more than 250a! Although do not worry, there will be no such current - the components and conductors on the printed circuit board will burn earlier and rupture the chain.

This error very often occurs in the power system and especially in the power electronics. It may occur for various reasons, for example, due to a management error or long transition processes. In the latter case, even "dead time" (DeadTime) in your converter will not save.

I think the problem is understandable and many of you are familiar, now it is clear what you need to fight and only come up with how. This will go further a story.

Principle of operational protection

Here it is necessary to apply the usual logic and see the causal relationship:
1) the main problem is a lot of current values \u200b\u200bin the chain;
2) How to understand what value of the current? -\u003e Measure it;
3) measured and obtained -\u003e comparing it with a given permissible value;
4) If you exceeded the value -\u003e we turn off the load from the current source.

Measure the current -\u003e learn whether the allowable current -\u003e disable the load

Absolutely any protection, not only for the current, is based on this way. Depending on the physical quantity on which protection is built, there will be different technical problems on the implementation of the implementation and methods of their solution, but the essence is unchanged.

Now I propose in order to go through the entire chain of building protection and solve all the technical problems that arise. Good protection is the defense that provided in advance and it works. It means without modeling we can not do, I will use popular and free Multisim Blue.which is actively moving to the MouseR. You can download it there. - Link. Also, I will say in advance that within the framework of this article I will not delve into the schemechnical sizes and score to you too much at this stage things, just know that everything is a bit more difficult in real gland.

Measurement of current

This is the first point in our chain and probably the easiest to understand. You can measure the current in the chain in several ways and each has its own advantages and disadvantages, which one to apply specifically in your task - to solve only you. I will tell you, relying on my experience, about these most advantages and disadvantages. Some of them are "generally accepted", and some of my world interests, I ask you to notice that some kind of truth is not even trying to apply.

1) Current shunt. The basis of the foundation, "works" everything is on the same and mighty law of Ohm. The easiest, cheapest, fastest and most general way, but with a number of shortcomings:

BUT) No galvanic junction. You will have to be implemented separately, for example, using a high-speed optter. It is not difficult to implement, but requires an additional place on the board, unleashed by DC / DC and other components that cost money and add dimensional sizes. Although the galvanic isolation is not always needed, of course.

B) On large currents speeding global warming. As I previously wrote, "works" is all on the law of Ohm, which means heats up and heats the atmosphere. This leads to a decrease in the efficiency and the need to cool the shunt. There is a way to minimize this disadvantage - to reduce the resistance of the shunt. Unfortunately, it is impossible to reduce it impossible and in general i would not recommend to reduce it less than 1 mΩIf you still have little experience, because there is a need to combat interference and increase the requirements for the installation of the printed circuit board.

In your devices, I love to use these shunts PA2512FKF7W0R002E:

The current measurement occurs by measuring the voltage drop on the shunt, for example, when flowing current 30A on Shunts will fall:

That is, when we get a drop of 60 mV on the shunt - this will mean that we have achieved the limit and if the drop will increase, then you will need to turn off our device or load. Now let's consider how much heat will stand out on our shunt:

Not enough, right? This moment must be considered, because The maximum power of my shunt is 2 W and it is impossible to exceed it, it is not necessary to solder the shunts with a low-melting point - it can be disappeared, I saw it.

Use shunts when you have a lot of voltage and not very big current
Watch the amount of heat allocated on shunt
Use shunts where you need to maximize the speed
Use shunts only from special materials: Constantane, Manganin and Similar

2) Current sensors on the Hall effect. Here I will make my own classification that quite reflects the essence of various solutions on this effect, namely: cheap and expensive.

BUT) Cheap, for example, ACS712 and the like. Of the advantages, I can note the ease of use and availability of galvanic junction, on this advantages end. The main disadvantage is extremely unstable behavior under the influence of RF interference. Any DC / DC or a powerful reactive load is noise, that is, in 90% of cases, these sensors are useless, for "go crazy" and show the weather rather on Mars. But not in vain make them?

Do they have a galvanic junction and can measure high currents? Yes. Do not love interference? Also yes. Where to put them? That's right, the monitoring system is low responsible and to measure the current consumption with batteries. I have in inverters for the SES and WES for a qualitative assessment of current consumption with battery, which allows to extend the life cycle of batteries. Data sensors look like this:

B) Expensive. Have all the advantages of cheap, but do not have their minuses. An example of such a LEM LTS 15-NP sensor:

What we have in the end:
1) high speed;
2) galvanic junction;
3) ease of use;
4) large measured currents regardless of voltage;
5) high measurement accuracy;
6) even "evil" Amy do not interfere with work and not; affect accuracy.

But what then minus? Those who discovered the link above it once saw it - this is the price. 18 $, Karl! And even on the 1000+ series, the price does not fall below $ 10, but the real purchase will be 12-13 $. In BP for a couple of bucks, it does not put this, but I would like ... Summarize:

A) this is the best solution in principle for measuring the current, but expensive;
b) apply these sensors in severe operating conditions;
c) apply these sensors in the responsible nodes;
d) Apply them if your device costs a lot of money, for example, the UPS by 5-10 kW, there it will definitely justify itself, because the price of the device will be several thousand $.

3) Current transformer. Standard solution in many devices. Minus two - do not work with constant current and have nonlinear characteristics. Pros - cheap, reliably and can be measured simply the huge currents. It is on current transformers that automatics and protection systems in RU-0.4, 6, 10, 35 kV in enterprises were built, and there are a normal phenomenon.

Honestly, I try not to use them, because I do not like, but in various control cabinets and other systems on alternating current, I still put it, because They cost a couple of $ and give a galvanic junction, and not 15-20 $ as lem-s and their task in the 50 Hz network perfectly. They usually look like this, but they are on all sorts of EFD cores:

Perhaps with current measurement methods can be finished. I talked about the main, but not about all. To expand your own horizons and knowledge, I advise additionally at least google yes to watch various sensors on the same Digikey.

Strengthening the measured voltage drop

Further construction of the protection system will go on the Schunts Base as a current sensor. Let's build a system with a previously voiced current value in 30a. On Shunts, we get a drop of 60 mV and here 2 technical problems arise:

A) Measure and compare the signal with an amplitude of 60 mV is uncomfortable. The ADCs usually have a measurement range 3.3V, that is, with 12 bit bits, we get a quantization step:

This means that on the range of 0-60 mV, which corresponds to 0-30a. We will get a small number of steps:

We get that the dimension of measurement will be only:

It is worth understanding that this is an idealized figure and in reality they will be widely worse, because The ADC itself has an error, especially in the zero area. Of course, the ADC will not use the ADC to protect, but to measure the current from the same shunt to build a control system will have to. Then the task was to be clearly explained, but it is also relevant for comparators, which in the region of the Potential of the Earth (0V usually) work quite unstable, even Rail-to-Rail.

B) if we want to drag the signal with an amplitude of 60 mV on the board, then after 5-10 cm, nothing will remain from it because of the interference, and at the time of the KZ it is not necessary to count on it, because Amy will increase. Of course, you can hang a protection scheme directly on the foot of the Shunts, but we will not get rid of the first problem.

To solve these problems, we will need an operational amplifier (OU). To talk about how he works will not - the topic googles perfectly, but we will talk about critical parameters and choosing OU. First, let's decide on the scheme. I said that there will be no special grace here, therefore we will cover the OU negative feedback (OOS) and we obtain an amplifier with the known gain factors. This action I simulate in Multisim (picture clickable):

You can download the file for simulation.

The voltage source V2 acts as our shunt, or rather, it simulates the voltage drop on it. For clarity, I chose a fall value equal to 100 mV, now we need to enhance the signal so as to transfer it to a more convenient voltage, usually between 1/2 and 2/3 V REF. This will make it possible to obtain a large number of quantization steps in the range of currents + Leave the stock for measurements to evaluate how bad and count the current increase time is important in complex jet load control systems. The gain in this case is:

In this way, we have the opportunity to strengthen the signal from our signal to the required level. Now consider what parameters it is worth paying attention to:

OU should be Rail-to-Rail to work adequately with signals near the Potential of the Earth (GND)
It is necessary to choose an OU with a high speed of increasing output. In my favorite OPA376, this parameter is 2B / μs, which allows you to achieve the maximum output value of the CCC 3.3B for only 2 μs. This speed is enough to save any converter or load with frequencies up to 200 kHz. These parameters should be understood and turn on the head when choosing OU, otherwise there is a chance to put OU for $ 10 where there would be enough and an amplifier for $ 1
The bandwidth selected by the OU should be at least 10 times more than the maximum load switching frequency. Again, look for the "golden middle" in the ratio "Price / TTX", everything is good in moderation

In most of your projects, I use OPA from Texas Instruments - OPA376, its TTH has enough for the implementation of protection in most tasks and a price tag at $ 1 is quite good. If you need cheaper, then look at the S solutions from ST, and if even cheaper, then on Microchip and Micrel. I use only Ti and Linear for religious reasons, for I like it and sleep so calmer.

Add realism to the protection system

Let's now add a shunt, load, power supply, and other attributes that will bring our model to reality. The result obtained is as follows (picture clickable):

Download the simulation file for Multisim is possible.

Here we already see our shunt R1 with resistance all the same 2 MΩ, the power source I chose 310V (straightened network) and the load for it is a resistor 10.2 ohms, which again according to the Ohm law gives us a current:

At Shunts, as you see fall, previously counted, 60 mV and we enhance them with the gain ratio:

At the output, we obtain a reinforced signal with an amplitude of 3.1V. You must agree, it is already on the ADC, and to the comparator and drag on the board of 20-40 mm without any concerns and deterioration of the stability of work. With this signal we will continue to work.

Comparison of signals using a comparator

Comparator - This is a diagram that takes on the input 2 of the signal and if the signal amplitude at the direct input (+) is greater than in the inverse (-), the log appears at the output. 1 (VCC). Otherwise log. 0 (GND).

Formally, any OU can be included as a comparator, but such a decision on TTX will give up the comparator for the speed and the ratio of the price / result. In our case, the higher the speed, the higher the likelihood that the defense will have time to work and save the device. I love to apply a comparator, again from Texas Instrumets - LMV7271. What you should pay attention to:

A trigger delay, on fact this is the main speed limiter. At the above comparator this time is about 880 ns, which quickly and in many tasks are somewhat excessive at $ 2 and you can pick up a more optimal comparator
Again, I advise you to use a Rail-to-Rail comparator, otherwise you will have no 5V, and less. Make sure the simulator will help you, choose something not Rail-to-Rail and experiment. The signal from the comparator is usually fed to the driver of the drivers (SD) and it would be nice to have a steady TTL signal there.
Choose a comparator with a Push-Pull output, not Open-Drain and others. It is convenient and we have predicted TTX on exit

Now let's add a comparator to our project in the simulator and look at its work in the mode when the protection has not worked and the current does not exceed the emergency (clickable):

Download the simulation file in Multisim is possible.

What we need ... It is necessary in case of exceeding the current of more than 30a, so that the exit of the comparator was log. 0 (GND), this signal will be supplied to the SD or EN driver input and turn it off. In the normal state at the output there should be a log. 1 (5V TTL) and enable the operation of the power key driver (for example, "folk" IR2110 and less ancient).

Return to our logic:
1) measured the current at Shunts and received 56.4 mV;
2) strengthened our signal with a coefficient of 50.78 and received 2.88V at the OU output;
3) On the direct input of the comparator we feed the reference signal with which we will compare. We specify it using a divider on R2 and exposes 3.1V - this corresponds to the current in about 30a. This resistor is regulated by the protection threshold!
4) Now the signal from the OU output is submitted to the inverse and compare two signals: 3.1V\u003e 2.88V. In direct entry (+), the voltage is higher than in the inverse input (-), it means that the current is not exceeded and at the log output. 1 - Drivers work, and our LED1 LED does not burn.

Now we increase the current to the value\u003e 30a (twist R8 and reduce resistance) and look at the result (clickable picture):

Let's restore the items from our "logic":
1) measured the current at Shunts and received 68.9 mV;
2) strengthened our signal with a coefficient of 50.78 and received 3.4B at the OU output;
4) Now the signal from the OU output is submitted to the inverse and compare two signals: 3.1V< 3.4В. На прямом входу (+) напряжение НИЖЕ, чем на инверсном входе (-), значит ток превышен и на выходе лог. 0 - драйвера НЕ работают, а наш светодиод LED1 горит.

Why hardware?

The answer to this question is simple - any programmable solution on the MK, with external ADC, etc., may simply "hang up" and even if you are a fairly competent soft spider and included a watchdog timer and other protection against freezing - until it processes your device to scorit.

Hardware protection allows you to implement a system with speed within a few microseconds, and if the budget allows you to within 100-200 ns, which is sufficiently enough for any task. Also, the hardware protection will not be able to "hang" and save the device, even if for some reason your control microcontroller or DSP "depended". Protection will disable the driver, your control circuit will safely restart, tested the hardware and either give an error, for example, in Modbus or starts if everything is fine.

It is worth noting that in specialized controllers to build power converters there are special inputs that allow hardware to disable the generation of PWM signal. For example, in all the favorite STM32 there is a BKIN input for this.

Separately, it is worth saying about such a thing as CPLD. In essence, it is a set of high-speed logic and in reliability it is comparable to a hardware solution. Completely common sense will put a small CPLD on the fee and implement in it and hardware protection, and deadtime and other charms, if we talk about DC / DC or some control cabinets. CPLD allows you to make such a solution very flexible and convenient.

Epilogue

This is probably all. I hope you were interested to read this article and she will give you some new knowledge or refreshing old. Always try to think in advance what modules in your device are to implement hardware, and which software. Often the implementation of the hardware to orders is easier to implement the program, and this leads from saving time on the development and, accordingly, its value.

The format of the article without "iron" for me is new and ask you to express your opinion in the survey.

This scheme is the simplest power supply on transistors, equipped with short-circuit protection (KZ). Its scheme is presented in the picture.

Main settings:

Output voltage - 0..12V;
Maximum output current - 400 mA.

The scheme works as follows. The input voltage of the 220V network is converted by a transformer in 16-17V, then straightened by VD1-VD4 diodes. The pulsation filtering of the straightened voltage is carried out by the C1 condenser. Next, the straightened voltage enters the VD6 stabilitron, which stabilizes the voltage at its conclusions up to 12V. The remainder of the voltage is quenched on the R2 resistor. Next, the voltage is adjusted by the variable resistor R3 to the required level in the range of 0-12V. Then follows the current amplifier on the transistors VT2 and VT3, which enhances the current to 400 mA. The load amplifier load is R5 resistor. Capacitor C2 additionally filters output voltage ripples.

Protection works like this. In the absence of KZ at the output, the voltage at the outputs VT1 is close to zero and the transistor is closed. The R1-VD5 circuit provides a bias at its base at 0.4-0.7 V (voltage drop in the open P-N diode transition). This offset is sufficient to open the transistor at a certain level of voltage collector-emitter. As soon as the output takes a short circuit, the voltage collector-emitter becomes different from zero and equal voltage at the outlet of the block. The VT1 transistor opens, and the resistance of its collector transition becomes close to zero, and, it means, in Stabilon. Thus, a zero input voltage comes to the current amplifier, through the transistors VT2, VT3 will be a very small current, and they will not fail. Protection turns off immediately when eliminating KZ.

Details

The transformer can be any with a core section 4 cm 2 or more. The primary winding contains 2200 turns of the wire PEV-0.18, the secondary - 150-170 turns of the wire PEV-0.45. A ready-made transformer of frame expandment from old TWK110L2 series TVs or similar is suitable. VD1-VD4 diodes can be D30-D305, D229G-D229L or any at least 1 A and the reverse voltage of at least 55 V. Transistors VT1, VT2 can be any low-frequency low-power, for example, MP39-MP42. Silicon more modern transistors can be used, for example, CT361, CT203, CT209, KT503, KT3107 and others. As VT3 - Germany P213-P215 or more modern silicon powerful low-frequency KT814, KT816, KT818 and others. When replacing VT1, it may be that the protection against KZ does not work. Then it follows sequentially with the VD5 to include another diode (or two, if necessary). If VT1 is silicon, then the diodes are better to use silicon, for example, KD209 (A-B).

In conclusion, it is worth noting that instead of the transistors specified in the P-N-P scheme, the transistors of the N-P-N (not instead of any of the VT1-VT3, and instead of all of them) can be applied. Then it will be necessary to change the polarity of the inclusion of diodes, stabilion, capacitors, a diode bridge. At the output, respectively, the polarity of the voltage will be different.

List of radio elements

Designation	A type	Nominal	number	Note	My notebook
VT1, VT2.	Bipolar transistor	MP42B	2	MP39-MP42, KT361, KT203, KT209, KT503, kt3107	In notebook
VT3.	Bipolar transistor	P213B	1	P213-p215, kt814, kt816, kt818	In notebook
VD1-VD4.	Diode	D242B	4	D302-d305, d229zh-d229l	In notebook
VD5	Diode	KD226B	1		In notebook
VD6.	Stabilirton	D814d	1		In notebook
C1.		2000 μF, 25 in	1		In notebook
C2.	Electrolytic condenser	500 μF. 25 B.	1		In notebook
R1	Resistor	10 com	1		In notebook
R2.	Resistor	360 Oh.	1		In notebook
R3	Variable resistor	4.7 com	1		In notebook
R4, R5	Resistor

The term "short circuit" in electrical engineering is called the emergency mode of operation of voltage sources. It occurs with violations of technological transmission processes of electricity transmission, when the output terminals are closed on the current generator or chemical element (curls).

In this case, all the power of the source is instantly applied to the spin. Huge currents flow through it, capable of burning equipment and cause electrical injuries to near-locked people. To stop developing such accidents, special protection is used.

What types of short circuits

Natural electric anomalies

They manifest themselves during thunderstorms accompanied by.

The sources of their formation are the high potentials of static electricity of various signs and values \u200b\u200baccumulated by clouds when they move the wind to huge distances. As a result of natural cooling when lifting the height of a pair of moisture inside the clouds condensed, forming rain.

The wet medium has a low electrical resistance, which creates a test of air insulation to pass the current in the form of lightning.

Electric discharge skips between two objects with different potentials:

on the approaching clouds;

between the thunderstorm clouds and land.

The first type of lightning is dangerous for aircraft, and the discharge on the ground is able to destroy trees, buildings, industrial facilities, air lines. To protect it from it, lightning tracks are installed, which consistently perform functions:

1. Admission, attraction of lightning potential per special catcher;

2. transmitting the resulting current to the tokend to the contour of the building of the building;

3. High-voltage discharge discharge by this circuit on the potential of the Earth.

Short circuits in DC circuits

Electroplating voltage sources or rectifiers create a difference in positive and negative potentials on weekend contacts, which in normal conditions provide the operation of the circuit, for example, the glow of the battery light bulb, as shown in the figure below.

The electrical processes occurring at the same time describes the mathematical expression.

The electromotive power of the source is distributed to the creation of a load in the internal and external circuits by overcoming their resistances "R" and "R".

In emergency mode between the battery terminals "+" and "-", a short of a very low electrical resistance occurs, which practically eliminates the flow of current in the outer chain, withdrawing this part of the scheme from work. Therefore, with respect to the nominal regime, it can be considered that R \u003d 0.

The entire current circulates only in the inner contour, which has a small resistance, and is determined by the formula I \u003d E / R.

Since the magnitude of the electromotive force has not changed, the value of the current increases very sharply. Such a short circuit flows along a short conductor and an inner contour, causes a huge heat release and subsequent disruption.

Short circuits in alternating circuits

All electrical processes here are also described by the action of the Ohm law and occur according to a similar principle. Features of their passage impose:

application of single-phase or three-phase networks of various configurations;

the presence of grounding circuit.

Types of short circuits in alternating voltage schemes

COPs kZ may arise between:

phase and land;

two different phases;

two different phases and land;

three phases;

three phases and land.

To transmit electricity over an air power transmission power supply system, the power supply system can use a different neutral connection scheme:

1. Isolated;

2. Plug-free.

In each of these cases, short circuit currents will form their own way and have a different value. Therefore, all the listed options for assembling the electrical circuit and the possibility of short circuits in them are taken into account in creating current protection configurations for them.

Inside the consumers of electricity, for example, a short circuit may also occur. At single-phase structures, the phase potential may break through the insulation layer on the housing or zero conductor. In three-phase electrical equipment, an additional fault may arise between two or three phases or between their combinations with the housing / ground.

In all these cases, as with the CW in the DC circuits, through the resulting twirling and all the scheme connected to it to the generator will flow a short circuit current of a very large value that causes the alarm mode.

To prevent it, protects, which automatically remove the voltage from the equipment undergoing the action of increased currents.

How to choose a short circuit protection borders

All electrical devices are designed to consume a certain amount of electricity in its voltage class. The workload is not appreciated by no power, and the current. It is easier to measure, control and create protection on it.

The picture presents graphs of currents that may arise in different modes of equipment operation. The parameters of the settings and setup of protective devices are selected.

A brown graph shows the sinusoid of the nominal mode, which is selected as the original when designing the electrical circuit, accounting for the power of the electrical wiring, selection of current protective devices.

The frequency of industrial sinusoids is always stable at the same time, and a period of one complete oscillation occurs in time 0.02 seconds.

The sinusoid of the working mode in the picture is shown in blue. It is usually less than the nominal harmonic. People rarely fully use all reserves of power allotted. As an example, if a five-growing chandelier hangs in the room, then for lighting, it often includes one bulb group: two or three, and not all five.

In order for the electrical appliances to work reliably at rated load, create a small current for the protection current to configure. The value of the current to which they are configured to shut down are called the setpoint. When it is reached, the switches remove the voltage from the equipment.

In the interval, the amplitude of the sinusoid between the nominal regime and the setpoint of the electric hammer operates in a small overload mode.

The possible time characteristic of the emergency current is shown in black graphics. She has an amplitude exceeds the setpoint of protection, and the frequency of oscillations has changed dramatically. Usually it has aperiodic character. Each half-wave varies in size and frequency.

Any short circuit protection includes three main stages of work:

1. Permanent monitoring of the state of the monusoids of the controlled current and determining the moment of malfunction;

2. Analysis of the situation and issuing a logical part of the team to the executive body;

3. Removal of voltage with equipment with switching devices.

In many devices, another element is used - entering time delay on triggering. It is used to ensure the principle of selectivity in complex, branched schemes.

Since the sinusoid reaches its amplitude for the time 0.005 seconds, then this period, at least, it is necessary to measure protection. The following two stages of work are also not performed instantly.

The total working time of the fastest current protection for these reasons is slightly less than a period of one harmonic oscillation of 0.02 seconds.

Constructive Features Protection against Short Circuit

Electric current passing through any conductor calls:

thermal heating of the conductor;

magnetic field guidance.

These two actions are taken as the basis for the design of protective apparatuses.

Protection based on the principle of thermal exposure

The thermal effect of the current described by scientists Joule and Lenz is used to protect the fuses.

Protection fuses

It is based on the installation within the flow of the insert, which optimally maintains the nominal load, but burns out when it is exceeded, breaking the circuit.

The higher the emergency current, the faster the circuit is created - the removal of the voltage. With a slight excess current, the shutdown may occur after a long period of time.

Fuses successfully operate in electronic devices, electrical equipment of cars, household appliances, industrial devices up to 1000 volts. Separate models are operated in high-voltage equipment chains.

Protection based on the principle of electromagnetic exposure

The principle of targeting the magnetic field around the conductor with the current allowed us to create a huge class of electromagnetic relays and protective automata using a shutdown coil.

Its winding is located on the core - the magnetic lines, in which the magnetic flows from each turn are folded. Movable contact is mechanically connected with an anchor, which is a swinging part of the core. It is pressed against the stationary attachment of the springs with force.

The nominal value current passing through the coils of the shutdown coil creates a magnetic stream that cannot overcome the spring force. Therefore, contacts are constantly in a closed state.

If emergency currents occur, the anchor is attracted to the stationary part of the magnetic pipeline and breaks the chain created by the contacts.

One of the types of automatic switches operating on the basis of electromagnetic removal of voltage from the protected circuit is shown in the picture.

It uses:

automatic shutdown of emergency modes;

electrical arc valve system;

manual or automatic inclusion in the work.

Digital short circuit protection

All the protection covered above work with analogue values. In addition to them, recently in the industry and especially in the energy sector, digital technology based on work and static relays are actively introduced. The same instruments with simplified functions are available for household purposes.

Measurement and direction of current passing through the protected scheme performs a built-in lowering current transformer of high accuracy. The measurement signal is exposed to digitization by applying on the principle of amplitude modulation.

Then it enters the logical part of the microprocessor protection, which works on a certain, pre-tuned algorithm. In case of emergency situations, the logic of the device issues the command to the executive disconnecting mechanism for removing the voltage from the network.

To work protection, use the power supply tension or autonomous sources.

Digital protection against short circuits have a large number of functions, settings and capabilities up to the register of the pre-emergency status of the network and the mode of its disconnection.

Presented design protection for the power supply of any type. This protection scheme can work together with any power supply - network, pulse and DC batteries. A schematic disunity of such a protection block is relative and consists of several components.

Power Supply Scheme

The power part is a powerful field transistor - during the work does not overheat, therefore does not need it in the heat sink either. The scheme is simultaneously protected from supplying the power supply, overload and KZ at the output, the trigger current can be selected by selecting the resistance of the shunt resistor, in my case the current is 8 amps, 6 resistors are 5 watts 0.1 ohms parallel to connected. The shunt can also be made of resistors with a capacity of 1-3 watts.

More precise protection can be adjusted by selecting the resistance of the trim resistor. Power supply protection circuit, current limit control circuit protection diagram, current limit control

~~~ with a CW and overloading of the block output, the protection instantly will work, turning off the power supply. The LED indicator will inform about the protection of protection. Even with a CZ exit for a couple of dozen seconds, the field transistor remains cold

~~~ The field transistor is not critical, any keys with a current of 15-20 and above ampere and with a working voltage of 20-60 volts are suitable. The keys from the IRFZ24, IRFZ46, IRFZ44, IRFZ46, IRFZ48 or more powerful - IRF3205, IRL3705, IRL2505 and them are similar are excellent.

~~~ This scheme is also great as the protection of a charger for automotive batteries, if the polarity of the connection suddenly was confused, then nothing terrible with the charger will occur, the defense will save the device in such situations.

~~~ Thanks to rapid protection, it can be successfully applied for pulse schemes, with a short-circuit protection will work faster than they will have to burn the power keys of the pulse power supply. Schematics will also be suitable for pulse inverters, as current protection. When overloading or KZ in the secondary chain of the inverter, the power transistors of the inverter fly through the moment, and such protection will not give it to happen.

Comments
Short circuit protection, surprise polarnosi and overload is assembled on a separate board. The power transistor was used by the IRFZ44 series, but if desired, it can be replaced with a more powerful IRF3205 or any other power key that has close parameters. You can use the keys from the IRFZ24 line, IRFZ40, IRFZ46, IRFZ48 and other keys with a current of more than 20 amps. During the work, the field transistor remains ice,. Therefore, the heat sink does not need.

The second transistor is also not critical, in my case a high-voltage bipolar transistor of the MJE13003 series is used, but the choice is large. The protection current is selected based on the resistance of the shunt - in my case 6 resistors in 0.1Ω parallel, the protection is triggered with a load of 6-7 amps. You can more accurately configure the rotation of the variable resistor, so I set up a trigger current in the region of 5 amps.

The power of the power supply is quite decent, the output current comes to 6-7 amps, which is enough to charge the car battery.
Schunts resistors chose 5 watts with a capacity, but it is possible for 2-3 watts.

If everything is done correctly, the unit begins to work immediately, closure the output, the LED indicator of the protection will light up, which will be lit until the output wires are in the KZ mode.
If everything works as needed, then proceed on. We collect the scheme of the indicator.

The scheme is drawn from the battery charger. The red indicator suggests that there is an output voltage at the output of the BP, the green indicator shows the charging process. With such a layout of the components, the green indicator will gradually swell and will finally go out when the voltage on the battery will be 12.2-12.4 volts, when the battery is disabled, the indicator will not burn.