Current protection circuit. Short circuit protection circuit
The integrated circuit (IC) KR142EN12A is an adjustable voltage stabilizer of the compensation type in the KT-28-2 package, which allows you to power devices with a current of up to 1.5 A in the voltage range of 1.2...37 V. This integrated stabilizer has thermally stable protection according to current and output short circuit protection.
Based on the KR142EN12A IC, you can build an adjustable power supply, the circuit of which (without a transformer and diode bridge) is shown in Fig.2. The rectified input voltage is supplied from the diode bridge to capacitor C1. Transistor VT2 and chip DA1 should be located on the radiator.
Heat sink flange DA1 is electrically connected to pin 2, so if DAT and transistor VD2 are located on the same heatsink, then they need to be isolated from each other.
In the author's version, DA1 is installed on a separate small radiator, which is not galvanically connected to the radiator and transistor VT2. The power dissipated by a chip with a heat sink should not exceed 10 W. Resistors R3 and R5 form a voltage divider included in the measuring element of the stabilizer. A stabilized negative voltage of -5 V is supplied to capacitor C2 and resistor R2 (used to select the thermally stable point VD1). In the original version, the voltage is supplied from the KTs407A diode bridge and the 79L05 stabilizer, powered from a separate winding of the power transformer.
For guard from closing the output circuit of the stabilizer, it is enough to connect an electrolytic capacitor with a capacity of at least 10 μF in parallel with resistor R3, and shunt resistor R5 with a KD521A diode. The location of the parts is not critical, but for good temperature stability it is necessary to use the appropriate types of resistors. They should be located as far as possible from heat sources. The overall stability of the output voltage consists of many factors and usually does not exceed 0.25% after warming up.
After switching on and warming up the device, the minimum output voltage of 0 V is set with resistor Rao6. Resistors R2 ( Fig.2) and resistor Rno6 ( Fig.3) must be multi-turn trimmers from the SP5 series.
Possibilities the current of the KR142EN12A microcircuit is limited to 1.5 A. Currently, there are microcircuits on sale with similar parameters, but designed for a higher current in the load, for example LM350 - for a current of 3 A, LM338 - for a current of 5 A. Recently on sale imported microcircuits from the LOW DROP series (SD, DV, LT1083/1084/1085) appeared. These microcircuits can operate at a reduced voltage between input and output (up to 1... 1.3 V) and provide a stabilized output voltage in the range of 1.25...30 V at a load current of 7.5/5/3 A, respectively . The closest domestic analogue in terms of parameters, type KR142EN22, has a maximum stabilization current of 7.5 A. At the maximum output current, the stabilization mode is guaranteed by the manufacturer at an input-output voltage of at least 1.5 V. The microcircuits also have built-in protection against excess current in the load of the permissible value and thermal protection against overheating of the case. These stabilizers provide output voltage instability of 0.05%/V, output voltage instability when the output current changes from 10 mA to a maximum value of no worse than 0.1%/V. On Fig.4 shows a power supply circuit for a home laboratory, which allows you to do without transistors VT1 and VT2, shown in Fig.2.
Instead of the DA1 KR142EN12A microcircuit, the KR142EN22A microcircuit was used. This is an adjustable stabilizer with a low voltage drop, which allows you to obtain a current of up to 7.5 A in the load. For example, the input voltage supplied to the microcircuit is Uin = 39 V, output voltage at the load Uout = 30 V, current at the load louf = 5 A, then the maximum power dissipated by the microcircuit at the load is 45 W. Electrolytic capacitor C7 is used to reduce output impedance at high frequencies, and also reduces noise voltage and improves ripple smoothing. If this capacitor is tantalum, then its nominal capacity must be at least 22 μF, if aluminum - at least 150 μF. If necessary, the capacitance of capacitor C7 can be increased. If the electrolytic capacitor C7 is located at a distance of more than 155 mm and is connected to the power supply with a wire with a cross-section of less than 1 mm, then an additional electrolytic capacitor with a capacity of at least 10 μF is installed on the board parallel to the capacitor C7, closer to the microcircuit itself. The capacitance of filter capacitor C1 can be determined approximately at the rate of 2000 μF per 1 A of output current (at a voltage of at least 50 V). To reduce the temperature drift of the output voltage, resistor R8 must be either wire-wound or metal-foil with an error of no worse than 1%. Resistor R7 is the same type as R8. If the KS113A zener diode is not available, you can use the unit shown in Fig.3. The author is quite satisfied with the protection circuit solution given in, as it works flawlessly and has been tested in practice. You can use any power supply protection circuit solutions, for example those proposed in. In the author’s version, when relay K1 is triggered, contacts K 1.1 are closed, short-circuiting resistor R7, and the voltage at the output of the power supply becomes equal to 0 V. The printed circuit board of the power supply and the location of the elements are shown in Fig. 5, the appearance of the power supply is shown in Fig.6.
The amateur radio circuits presented below for protecting power supplies or chargers can work together with almost any source - mains, pulse and rechargeable batteries. The circuitry implementation of these designs is relatively simple and can be repeated even by a novice radio amateur.
The power part is made of a powerful field-effect transistor. It does not overheat during operation, so there is no need to use a heat sink. The device at the same time provides excellent protection against overvoltage, overload and short circuit in the output circuit, the operating current can be selected by selecting a shunt resistor, in our case it is 8 Amperes, 6 parallel-connected resistances with a power of 5 watts 0.1 Ohm are used. A shunt can also be made from a resistance with a power of 1-3 watts.
The protection can be adjusted more accurately by adjusting the resistance of the trimming resistor. In the event of a short circuit and overload at the output, the protection will work almost immediately, turning off the power supply. The LED will indicate that the protection has been triggered. Even when the output is closed for 30-40 seconds, the field worker remains almost cold. Its type is not critical; almost any power switches with a current of 15-20 Amps and an operating voltage of 20-60 Volts are suitable. Transistors from the IRFZ24, IRFZ40, IRFZ44, IRFZ46, IRFZ48 or more powerful series are perfect.
This version of the circuit will be useful to car enthusiasts as a protection for the charger for lead batteries; if you suddenly reverse the polarity of the connection, then nothing bad will happen to the charger.
Thanks to the fast response of the protection, it can be perfectly used for pulsed circuits; in the event of a short circuit, the protection will operate much faster than the power switches of the switching power supply will burn out. The design is also suitable for pulse inverters, as current protection.
MOSFET short circuit protection |
If your power supplies and chargers use a field-effect transistor (MOSFET) to switch the load, then you can easily add short-circuit or overload protection to such a circuit. In this example, we will use an internal resistance RSD, which produces a voltage drop proportional to the current flowing through the MOSFET.
The voltage following through the internal resistor can be detected using a comparator or even a transistor that switches at a voltage level of 0.5 V, i.e., you can abandon the use of a current-sensing resistance (shunt), which usually produces excess voltage. The comparator can be monitored using a microcontroller. In case of short circuit or overload, you can programmatically start PWM control, alarm, emergency stop). It is also possible to connect the output of the comparator to the gate of the field-effect transistor, if, when a short circuit occurs, you need to immediately turn off the field-effect transistor.
Power supply with short circuit protection system |
Short circuits occur in any electrical installation, regardless of its complexity. Even if the electrical wiring is new, the lamps and sockets are in working order, and the electrical equipment is produced by world-famous manufacturers, no one is immune from short circuits. And you need to protect yourself from them.
Network emergency protection devices
Fuses are the simplest protection devices. Previously, only they were used to eliminate emergency conditions in household electrical wiring. In some devices, fuses are still used today. The reason is that they have high performance and are indispensable for protecting semiconductor devices.
After tripping, the fuse is either replaced with a new one, or the fuse inside it is changed. Inserts for the same fuse body are available for different current ratings. But the need to keep a supply of fuse links on site or in an apartment for quick replacement is a disadvantage of fuses.
The most common fuse in Soviet times was the “plug”.
Fuse - “plug”They were replaced by automatic traffic jams like STEAM, produced for currents of 10, 16 and 25 A. They were screwed into place of the plugs, were reusable and had two protective elements called releases. One protected from short circuits and was triggered instantly, the second - from overloads and triggered with a time delay.
All have the same releases circuit breakers, which replaced fuses. The instantaneous release is called electromagnetic, because its operation is based on the principle of retracting the coil rod when the rated current is exceeded. The rod hits the latch and the spring opens the contact system of the switch.
A release that operates with a time delay is called thermal. It works on the principle of a thermostat in an iron or electric heater. When a current passes through it, the bimetallic plate heats up and slowly bends to the side. The greater the current through it, the faster the bending occurs. Then it acts on the same latch, and the machine turns off. If the influence of the current has stopped, the plate cools down, returns to its original position, and no shutdown occurs.
In old electrical panels, automatic switches in carbolite housings of types A-63, A3161, or more modern AE1030 are still preserved. But all of them no longer meet modern requirements.
They are worn out and their mechanical parts are either rusty or slow. And not all of them have instantaneous short circuit protection. In some devices, only a thermal release was installed. And the response speed of the electromagnetic release in the machines of these series is lower than in the modular ones.
Therefore, such protective devices need to be replaced with modern ones, before they do anything wrong through their inaction.
Principles of defense design
In apartment buildings, machines are installed in a panel on the landing. This is enough to protect apartments. But if, when replacing electrical wiring, you installed a personal switchboard, then it is better to install a personal automatic switch in it for each group of consumers. There are several reasons for this.
- When replacing an outlet, you will not need to turn off the lights in the apartment and use a flashlight.
- To protect some consumers, you will reduce the rated current of the machine, which will make their protection more sensitive.
- If there is damage to the electrical wiring, you can quickly turn off the emergency section and leave the rest in operation.
In private houses, two-pole switches are used as input switches. This is necessary in case of erroneous switching at substations or lines, as a result of which the phase will be at zero. The use of two single-pole switches for this purpose is unacceptable, since the one at zero may turn off, but the phase will remain.
It is not practical to use a three-pole switch as the equivalent of three single-pole ones. Removing the strip connecting the three poles will not help. There are rods inside the switch that disconnect the remaining poles when one of them trips.
When using an RCD, be sure to protect the same line with a circuit breaker. The RCD protects against leakage currents, but does not protect against short circuits and overloads. The functions of leakage protection and emergency operating modes are combined in a differential automatic machine.
Selection of circuit breakers
When replacing an old circuit breaker, set the new one to the same rated current. According to the requirements of Energosbyt, the rated current of the circuit breaker is taken based on the maximum permitted load.
The distribution network is designed in such a way that as you approach the power source, the rated currents of the protection devices increase. If your apartment is connected through a single-phase 16 A circuit breaker, then all apartments in the entrance can be connected to a three-phase 40 A circuit breaker and evenly distributed among the phases. If your machine does not turn off during a short circuit, after some time the overload protection at the driveway will work. Each subsequent protective device backs up the previous one. Therefore, you should not overestimate the rated current of the circuit breaker. It may not work (there is not enough current) or it may turn off along with a group of consumers.
Modern modular circuit breakers are available with characteristics “B”, “C” and “D”. They differ in the multiplicity of cut-off currents.
Be careful when using machines with characteristics “D” and “B”.
And remember: if a short circuit is not turned off, it will cause a fire. Make sure your protection is in good working order and live in peace.
The term “short circuit” in electrical engineering refers to the emergency operation of voltage sources. It occurs when technological processes for transmitting electricity are disrupted, when the output terminals of an operating generator or chemical element are short-circuited (short-circuited).
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 of static electricity of various signs and values 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 under normal conditions ensures the operation of the circuit, for example, the glow 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 magnitude of the electromotive force has not changed, the value of the current 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:
the use of single-phase or three-phase network diagrams of 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 via overhead power lines, power supply systems can use different neutral connection schemes:
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 options for assembling an electrical circuit and the possibility of short-circuit currents occurring in them are taken into account when creating a 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.
The graph in brown shows the 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;
induction of 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 successfully operate in electronic devices, electrical equipment of automobiles, household appliances, and 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. In addition to them, digital technologies based on the operation of static relays have recently begun to be actively introduced in industry and especially in the energy sector. 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 of a high accuracy class. 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. When emergency situations occur, the device logic issues a command to the actuating shutdown 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 a large number of functions, settings and capabilities, including recording the pre-emergency state of the network and its shutdown mode.
This is an incredibly useful device that will protect your home from short circuits when testing any appliances being tested. There are times when it is necessary to check an electrical device for the absence of a short circuit, for example, after repair. And in order not to expose your network to danger, to play it safe and avoid unpleasant consequences, this very simple device will help.
Will need
- Overhead socket.
- Key switch, overhead.
- Incandescent light bulb 40 - 100 W with socket.
- Two-core wire in double insulation 1 meter.
- The fork is removable.
- Self-tapping screws.
All parts will be attached to a wooden square made of chipboard or other material.
It is better to use a wall socket for a light bulb, but if you don’t have one, we make a clamp for the girth from thin sheet metal.
And we roll out a square of thick wood.
It will be attached like this.
Assembling a socket with short circuit protection
Diagram of the entire installation.
As you can see, all elements are connected in series.
First of all, we assemble the plug by connecting the wire to it.
Since the socket and switch are wall-mounted, use a round file to make cuts on the side for the wire. This can be done with a sharp knife.
We screw the wooden square to the base with self-tapping screws. Choose ones that won't go right through.
We screw the lamp socket with a bracket to a wooden square.
We disassemble the socket and switch. Screw it to the base with self-tapping screws.
We connect the wires to the socket.
For complete reliability, all wires are soldered. That is: we clean it, bend the ring, solder it with a soldering iron with solder and flux.
We fix the power cord with nylon ties.
The circuit is assembled, the installation is ready for testing.
To test, insert a cell phone charger into the outlet. We press the switch - the lamp does not light. This means there is no short circuit.
Then we take a more powerful load: a power supply from a computer. Turn it on. The incandescent lamp first flashes and then goes out. This is normal, since the unit contains powerful capacitors, which initially become infected.
We simulate a short circuit - insert tweezers into the socket. Turn it on, the lamp lights up.
This is such a wonderful and very necessary device.
This installation is suitable not only for low-power devices, but also for powerful ones. Of course, the washing machine or electric stove will not work, but by the brightness of the glow you can understand that there is no short circuit.
Personally, I have been using a similar device almost my entire life, testing all newly assembled ones on it.