Simple protection of transistors from short circuits. How short circuit protection is designed and works. Video of the power supply protection circuit in action

The simplest short circuit protection is relevant for both experienced and novice radio amateurs, since no one is immune from errors. This article provides a simple but very original diagram that will help you protect your device from unwanted failure. The self-resetting fuse de-energizes the circuit, and the LEDs signal an emergency, quickly, reliably and simply.

Short circuit protection circuit:

The circuit shown in Figure No. 1 is a very easy-to-set up protection for amateur radio unit power supply or any other circuit.

Operation of short circuit protection circuit:

The scheme is very simple and understandable. Since the current flows along the path of least resistance while the fuse FU1 is intact, the output load Rн (Figure No. 2) is connected and current flows through it. In this case, the VD4 LED is constantly lit (preferably Green colour glow).

If the load current exceeds the maximum current permissible for the fuse, it trips, thereby breaking (bypassing) the load circuit, Figure No. 3. In this case, LED VD3 lights up (red) and VD4 goes out. In this case, neither your load nor the circuit suffers (of course, provided that the fuse trips in a timely manner).

Diodes VD1, VD5 and zener diode VD2 protect LEDs from reverse currents. Resistors R1, R2 limit the current in the protection circuit. For fuse FU1, I recommend using a self-resetting fuse. And you select the values ​​of all elements of the circuit depending on your needs.

This diagram represents simplest block power supply on transistors, equipped with short circuit protection (short circuit). Its diagram is shown in the figure.

Main parameters:

• Output voltage - 0..12V;
• The maximum output current is 400 mA.

The scheme works as follows. The input voltage of the 220V network is converted by a transformer to 16-17V, then rectified by diodes VD1-VD4. Filtering of rectified voltage ripples is carried out by capacitor C1. Next, the rectified voltage is supplied to the zener diode VD6, which stabilizes the voltage at its terminals to 12V. The remainder of the voltage is extinguished by resistor R2. Next, the voltage is adjusted by variable resistor R3 to the required level within 0-12V. This is followed by a current amplifier on transistors VT2 and VT3, which amplifies the current to a level of 400 mA. The load of the current amplifier is resistor R5. Capacitor C2 additionally filters output voltage ripple.

This is how protection works. In the absence of a short circuit at the output, the voltage at the terminals of VT1 is close to zero and the transistor is closed. The R1-VD5 circuit provides a bias at its base at a level of 0.4-0.7 V (voltage drop across the open p-n junction diode). This bias is enough to open the transistor at a certain collector-emitter voltage level. As soon as a short circuit occurs at the output, the collector-emitter voltage becomes different from zero and equal to the voltage at the output of the unit. Transistor VT1 opens, and the resistance of its collector junction becomes close to zero, and, therefore, at the zener diode. Thus, zero input voltage is supplied to the current amplifier; very little current will flow through transistors VT2, VT3, and they will not fail. The protection is turned off immediately when the short circuit is eliminated.

Details

The transformer can be any with a core cross-sectional area of ​​4 cm 2 or more. The primary winding contains 2200 turns of PEV-0.18 wire, the secondary winding contains 150-170 turns of PEV-0.45 wire. A ready-made frame scan transformer from old tube TVs of the TVK110L2 series or similar will also work. Diodes VD1-VD4 can be D302-D305, D229Zh-D229L or any for a current of at least 1 A and reverse voltage not less than 55 V. Transistors VT1, VT2 can be any low-frequency, low-power ones, for example, MP39-MP42. You can also use more modern silicon transistors, for example, KT361, KT203, KT209, KT503, KT3107 and others. As VT3 - germanium P213-P215 or more modern silicon high-power low-frequency KT814, KT816, KT818 and others. When replacing VT1, it may turn out that short-circuit protection does not work. Then you should connect another diode (or two, if necessary) in series with VD5. If VT1 is made of silicon, then it is better to use silicon diodes, for example, KD209(A-B).

In conclusion, it is worth noting that instead of those indicated in p-n-p scheme transistors can be used with similar parameters npn transistors(not instead of any of VT1-VT3, but instead of all of them). Then you will need to change the polarity of the diodes, zener diode, capacitors, and diode bridge. At the output, accordingly, the polarity of the voltage will be different.

List of radioelements

Almost everyone has experienced a short circuit in their life. But most often it happened like this: flash, clap and that’s it. This happened only because there was short circuit protection.

Short circuit protection device

The device may be electronic, electromechanical, or a simple fuse. Electronic devices are mainly used in complex electronic devices, and we will not consider them in this article. Let's focus on fuses and electromechanical devices. Fuses were first used to protect household electrical circuits. We are used to seeing them in the form of “plugs” in the electrical panel.

There were several types, but all the protection boiled down to the fact that inside this “plug” there was a thin copper wire that burned out when a short circuit occurred. It was necessary to run to the store, buy a fuse, or store at home a supply of fuses that might not be needed soon. It was inconvenient. And automatic switches were born, which at first also looked like “traffic jams”.

It was a simple electromechanical circuit breaker. They were produced for different currents, but the maximum value was 16 amperes. Soon more were needed high values, and technical progress has made it possible to produce machines the way we now see them in most electrical panels of our homes.

How does a machine gun protect us?

It has two types of protection. One type is based on induction, the second on heating. A short circuit is characterized by a large current that flows through the short-circuited circuit. The machine is designed in such a way that current flows through a bimetallic plate and an inductor. So, when a large current flows through the machine, a strong magnetic flux, which drives the release mechanism of the machine. Well, the bimetallic plate is designed to carry the rated current. When current flows through wires, it always causes heat. But we often don’t notice this, because the heat has time to dissipate and it seems to us that the wires are not heating up. A bimetallic strip consists of two metals with different properties. When heated, both metals deform (expand), but as one metal expands more than the other, the plate begins to bend. The plate is selected in such a way that when the nominal value of the machine is exceeded, due to bending, it activates the release mechanism. Thus, it turns out that one protection (inductive) works on short-circuit currents, and the second on currents flowing for a long time through the cable. Since short circuit currents are rapid in nature and flow in the network for a short period of time, the bimetallic plate does not have time to heat up to such an extent as to deform and turn off the circuit breaker.

Short circuit protection circuit

In fact, there is nothing complicated in this scheme. A circuit is installed that turns off either phase wire, or the entire chain at once. But there are nuances. Let's look at them in more detail.

1. You cannot install separate machines in the phase circuit and the zero circuit. For one simple reason. If suddenly, due to a short circuit, the zero circuit breaker turns off, then the entire electrical network will be energized, because the phase circuit breaker will remain on.
   
2. You cannot install a wire with a smaller cross-section than the machine allows. Very often in apartments with old wiring, in order to increase power, they install more powerful machines... Alas, this is the most common cause short circuits. This is what happens in such cases. Suppose, for clarity, there is a copper wire with a cross-section of 1.5 sq. mm, which is capable of withstanding a current of up to 16 A. A 25A machine is placed on it. We connect a load to this network, say 4.5 kW, and a current of 20.5 amperes will flow through the wire. The wire will start to get very hot, but the machine will not turn off the network. As you remember, the machine has two types of protection. The short circuit protection does not work yet because there is no short circuit, and the rated current protection will operate at a value greater than 25 amps. So it turns out that the wire gets very hot, the insulation begins to melt, but the machine does not work. In the end, an insulation breakdown occurs and a short circuit appears and the machine finally trips. But what do you get? The line can no longer be used and must be replaced. This is not difficult if the wires are laid open method. But what if they are hidden in the wall? New renovation provided for you.
3. If the aluminum wiring is more than 15 years old, and the copper wiring is more than 25 years old, and you are going to make repairs, definitely replace it with new wiring. Despite the investment it will save you money. Imagine that you have already made a repair, and there is a bad contact in some junction box? This is if we talk about copper wire (in which, as a rule, only the insulation ages or the joints oxidize or weaken over time, then begin to heat up, which leads to the destruction of the twist even faster). If we talk about aluminum wire, then everything is even worse. Aluminum is a very ductile metal. With temperature fluctuations, the compression and expansion of the wire is quite significant. And if there was a microcrack in the wire (manufacturing defect, technological defect), then over time it increases, and when it becomes quite large, which means the wire in this place is thinner, then when current flows, this area begins to heat up and cool down, which only speeds up the process . Therefore, even if it seems to you that everything is fine with the wiring: “It worked before!”, it’s better to change it anyway.
4. Junction boxes. There are articles about this, but I will briefly go through them here. NEVER DO SCROLLS!!! Even if you make them well, it's a twist. Metal tends to shrink and expand under the influence of temperature, and the twist weakens. Avoid using screw terminals for the same reason. Screw terminals can be used in open wiring. Then at least you can periodically look into the boxes and check the condition of the wiring. Screw clamps of the “PPE” type or terminal connections of the “WAGO” type are best suited for this purpose; screw clamps of the “Nut” type are best suited for power wiring (such clamps have two plates that are held together with four screws, in the middle there is another plate, i.e. using such clamps you can connect copper and aluminum wires). Leave a reserve of at least 15 cm of stripped wire. This serves two purposes: if the twist contact is poor, the wire has time to dissipate heat, and you have the opportunity to redo the twist if something happens. Try to place the wires in such a way that there is no overlap between the phase and neutral wires with the ground wire. The wires can cross, but not lie on top of each other. Try to place the twists so that the phase wire is on one side, and the neutral and ground wires are on the other.

   

5. Do not connect copper and aluminum wires directly. Either use WAGO terminal blocks or Walnut clamps. This is especially true for wires intended for connecting electric stoves. Usually, when they make repairs and move a stove socket, they extend the cable. Very often these are aluminum wires that are extended with copper.
6. A little special. Do not skimp on switches and sockets (especially for electric stoves). The point is that in present time It’s quite difficult to find good sockets for electric stoves (I’m talking about small towns), so it’s best to either use the “Nut” U739M clamps or find a good socket.
7. When tightening the terminals on sockets, do it more tightly, but do not break the thread; if this happens, it is better to change the socket immediately, do not rely on “maybe”.
8. When laying a new electrical route, use the following standards: 10-15 cm from corners, ceilings, walls (along the floor), jambs, window frames, floor (along the wall). This will protect you when installing, for example, suspended ceilings or baseboards, which are secured using dowels for which you need to punch a hole. If the wire is located in the corner between the floor and the wall, it is very easy to get caught in the wire. All wires must be positioned strictly horizontally or vertically. This will make it easier for you to understand where you can make a new hole if you suddenly need to hang a shelf or a picture or a TV.
9. Do not daisy chain (from one to another) more than 4 sockets. In the kitchen, I generally do not recommend connecting more than two, especially where you plan to use an oven, kettle, dishwasher and microwave in one place.
10. It is best to lay a separate line for the oven or connect it to the line from which the hob is powered (because very often they consume about 3 kW.) Not every outlet can withstand such a load, and even if another powerful consumer is connected to it ( for example, a kettle), you risk getting a short circuit due to the strong heating of the connection in the socket by the cable.
11. Avoid using extension cords to power high-power electrical appliances, such as oil heaters, or use extension cords from reputable manufacturers rather than Chinese "no name" brands. Read carefully what power a given extension cord can handle, and do not use it if it has less power than you need to power. When using an extension cord, try to avoid stranded wire. If the wire just lies there, it has time to dissipate heat. If the wire is twisted, the heat does not have time to dissipate and the wire begins to heat up noticeably, which can also lead to a short circuit.
12. Do not connect several powerful consumers to one outlet (through a tee or an extension cord with several outlets). A load of 3.5 kW can be connected to a good outlet, and up to 2 kW to a not-so-good outlet. In houses with aluminum wiring, no more than 2 kW in any socket, and even better, do not include more than 2 kW in a group of sockets powered by one circuit breaker.
13. Before installing a heater in each room, make sure that the rooms are powered from different machines. As they say: “And sometimes a stick can shoot,” the same is with machine guns: “And sometimes a machine gun can fail to work,” and the consequences of this are quite cruel. Therefore, protect yourself and your loved ones.
14. Handle heating devices carefully, making sure that the wire does not come into contact with the heating elements.

Short circuit circuit breaker

Why did I make this a separate point? It's simple. It is the machine that provides short circuit protection. If you install, then you must install an automatic machine next, or install it immediately (this is a two-in-one device: an RCD and an automatic machine). Such a device turns off the network in case of a short circuit, and when the rated current value is exceeded, and when there is a leakage current, when, for example, you are under voltage and electric current begins to flow through you. Let me remind you again: the RCD DOES NOT PROTECT FROM SHORT CIRCUIT, the RCD protects you from damage electric shock. Of course, it may be that the RCD will turn off the network in the event of a short circuit, but it is not intended for this. The operation of an RCD during a short circuit is completely random. And all the wiring may burn out, everything may be in flames, but the RCD will not turn off the network.

Similar materials.

The term “short circuit” in electrical engineering refers to the emergency operation of voltage sources. It occurs when there is a violation technological processes transmission of electricity when the generator is in operation or chemical element The output terminals are short-circuited (shorted).

In this case, the entire power of the source is instantly applied to the short circuit. Huge currents flow through it, which can burn equipment and cause electrical injuries to nearby people. To stop the development of such accidents, special protections are used.

What are the types of short circuits?

Natural electrical anomalies

They appear during lightning discharges accompanied by.

The sources of their formation are high potentials static electricity of various signs and sizes accumulated by clouds when they are moved by the wind over vast distances. As a result of natural cooling when rising to altitude, moisture vapor inside the cloud condenses, forming rain.

A humid environment has low electrical resistance, which creates a breakdown of the air insulation for the passage of current in the form of lightning.

An electrical discharge jumps between two objects with different potentials:

• on approaching clouds;

• between a thundercloud and the ground.

The first type of lightning is dangerous for aircraft, and a discharge to the ground can destroy trees, buildings, industrial facilities, and overhead power lines. To protect against it, lightning rods are installed, which consistently perform the following functions:

1. receiving, attracting lightning potential to a special catcher;

2. passing the resulting current through the current conductor to the grounding loop of the building;

3. discharging the high-voltage discharge with this circuit to the ground potential.

Short circuits in DC circuits

Galvanic voltage sources or rectifiers create a difference of positive and negative potentials at the output contacts, which normal conditions ensure the operation of the circuit, for example, the lighting of a light bulb from a battery, as shown in the figure below.

The electrical processes occurring in this case are described by a mathematical expression.

The electromotive force of the source is distributed to create a load in the internal and external circuits by overcoming their resistances “R” and “r”.

In emergency mode, a short circuit with very low electrical resistance occurs between the battery terminals “+” and “-”, which practically eliminates the flow of current in the external circuit, rendering this part of the circuit inoperable. Therefore, in relation to the nominal mode, we can assume that R=0.

All current circulates only in the internal circuit, which has low resistance, and is determined by the formula I=E/r.

Since the value electromotive force has not changed, the current value increases very sharply. Such a short circuit flows through the shorted conductor and the internal circuit, causing enormous heat generation inside them and subsequent structural failure.

Short circuits in AC circuits

All electrical processes here are also described by Ohm’s law and occur according to a similar principle. Features on their passage are imposed:

application of single-phase or three-phase network diagrams various configurations;

presence of a ground loop.

Types of short circuits in alternating voltage circuits

Short circuit currents can occur between:

phase and ground;

two different phases;

two different phases and ground;

three phases;

three phases and earth.

To transmit electricity over overhead power lines, power supply systems can use different scheme Neutral connections:

1. isolated;

2. solidly grounded.

In each of these cases, short circuit currents will form their own path and have different magnitudes. Therefore, all of the listed assembly options electrical diagram and the possibility of short circuit currents occurring in them are taken into account when creating the current protection configuration for them.

A short circuit can also occur inside electrical consumers, such as an electric motor. In single-phase structures, the phase potential can break through the insulation layer to the housing or neutral conductor. In three-phase electrical equipment, a fault may additionally occur between two or three phases or between their combinations with the frame/ground.

In all these cases, as with a short circuit in DC circuits, a very large short circuit current will flow through the resulting short circuit and the entire circuit connected to it up to the generator, causing an emergency mode.

To prevent it, protection is used that automatically removes voltage from equipment exposed to high currents.

How to choose the operation limits of short circuit protection

All electrical appliances are designed to consume a certain amount of electricity in their voltage class. It is customary to evaluate the workload not by power, but by current. It is easier to measure, control and create protection on it.

The picture shows graphs of currents that can occur in different operating modes of the equipment. The parameters for setting up and adjusting protective devices are selected for them.

On the chart brown shows a sine wave of the nominal mode, which is selected as the initial one when designing an electrical circuit, taking into account the power of electrical wiring, and selecting current protective devices.

The frequency of an industrial sinusoid in this mode is always stable, and the period of one complete oscillation occurs in 0.02 seconds.

The operating mode sine wave in the picture is shown in blue. It is usually less than the nominal harmonic. People rarely fully use all the reserves of power allocated to them. As an example, if there is a five-arm chandelier hanging in a room, then for lighting they often turn on one group of light bulbs: two or three, and not all five.

In order for electrical appliances to operate reliably at rated load, a small current reserve is created for setting up protections. The amount of current at which they are set to turn off is called the setting. When it is reached, the switches remove voltage from the equipment.

In the range of sinusoid amplitudes between the nominal mode and the set point, the electrical circuit operates in a slight overload mode.

The possible time characteristic of the fault current is shown in black on the graph. Its amplitude exceeds the protection setting, and the oscillation frequency has changed sharply. Usually it is aperiodic in nature. Each half-wave varies in magnitude and frequency.

Any short circuit protection includes three main stages of operation:

1. constant monitoring of the state of the controlled current sinusoid and determining the moment when a malfunction occurs;

2. analysis of the current situation and issuance of a command by the logical part to the executive body;

3. Relieve voltage from equipment using switching devices.

Many devices use another element - introducing a time delay for operation. It is used to ensure the principle of selectivity in complex, branched circuits.

Since the sinusoid reaches its amplitude in 0.005 seconds, at least this period is necessary for its measurement by protections. The next two stages of work also do not happen instantly.

For these reasons, the total operating time of the fastest current protections is slightly less than the period of one harmonic oscillation of 0.02 seconds.

Design features of short circuit protection

Electric current passing through any conductor causes:

thermal heating of the conductor;

guidance magnetic field.

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

Protection based on the principle of thermal influence of current

The thermal effect of current, described by the scientists Joule and Lenz, is used for protection by fuses.

Fuse protection

It is based on installing a fuse-link inside the current path, which optimally withstands the rated load, but burns out when it is exceeded, breaking the circuit.

The higher the magnitude of the emergency current, the faster a circuit break is created - voltage relief. If the current is slightly exceeded, shutdown may occur after a long period of time.

Fuses work successfully in electronic devices, electrical equipment of cars, household appliances, industrial devices up to 1000 volts. Some of their models are used in high-voltage equipment circuits.

Protection based on the principle of electromagnetic influence of current

The principle of inducing a magnetic field around a current-carrying conductor has made it possible to create a huge class of electromagnetic relays and circuit breakers that use a trip coil.

Its winding is located on a core - a magnetic circuit, in which the magnetic fluxes from each turn are added up. The moving contact is mechanically connected to the armature, which is the swinging part of the core. It is pressed against a permanently fixed contact by spring force.

A nominal current passing through the turns of the trip coil creates a magnetic flux that cannot overcome the spring force. Therefore, the contacts are constantly in a closed state.

When emergency currents occur, the armature is attracted to the stationary part of the magnetic circuit and breaks the circuit created by the contacts.

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

It uses:

automatic shutdown of emergency modes;

electric arc extinguishing system;

manual or automatic activation.

Digital short circuit protection

All the protections discussed above work with analog values. Besides them in Lately In industry and especially in the energy sector, digital technologies based on the operation of static relays are beginning to be actively introduced. The same devices with simplified functions are produced for household purposes.

The magnitude and direction of the current passing through the protected circuit is measured by a built-in step-down current transformer high class accuracy. The signal measured by it is digitized by superposition using the principle of amplitude modulation.

Then it goes to the logical part of the microprocessor protection, which works according to a certain, pre-configured algorithm. Whenever emergency situations The device logic issues a command to the actuator disconnecting mechanism to remove voltage from the network.

To operate the protection, a power supply is used that takes voltage from the network or autonomous sources.

Digital short circuit protection has big amount functions, settings and capabilities up to recording the pre-emergency state of the network and its shutdown mode.

To protect the power supply when designing various circuits, it is recommended to add an overcurrent protection unit to the power supply output. Simple scheme The device is built using a thyristor as a voltage protection control element.

As long as the supply voltage at the input is within normal limits, the zener diode and thyristor are closed, and current flows into the load. When the supply voltage exceeds 15.2V, the zener diode opens, followed by the thyristor, since there is a potential difference between its cathode and the control electrode sufficient to unlock it. Thyristor VS1 connected in parallel to the output of the power supply, when overloaded, breaks the fuse within a few microseconds if the output voltage is above the permissible value. The threshold for opening the thyristor, namely, triggering the protection, depends on the technical data of the zener diode. If the fuse blows, a piezo sound emitter with a built-in generator will turn on, which will signal an external fault, which also indicates a possible short circuit in the load. The alarm will sound until it is turned off. general nutrition or load device.

Video of the power supply protection circuit in action