Short circuit protection. How short circuit protection works and works. Power supply circuits with protection
I think every radio amateur who regularly designs electronic devices has an regulated power supply at home. The thing is really convenient and useful, without which, once you try it in action, it becomes difficult to do without. Indeed, if we need to check, for example, an LED, we will need to accurately set its operating voltage, since if the voltage supplied to the LED is significantly exceeded, the latter may simply burn out. Also with digital circuits, we set the output voltage on the multimeter to 5 volts, or any other voltage we need and go ahead.
Many novice radio amateurs first assemble a simple regulated power supply, without adjusting the output current and without short circuit protection. So it was with me, about 5 years ago I assembled a simple power supply with only adjustable output voltage from 0.6 to 11 volts. Its diagram is shown in the figure below:
But a few months ago I decided to upgrade this power supply and add a small short circuit protection circuit to its circuit. I found this diagram in one of the issues of Radio magazine. Upon closer examination, it turned out that the circuit is in many ways reminiscent of the above circuit diagram of the power supply I assembled earlier. If there is a short circuit in the powered circuit, the short circuit LED goes out, signaling this, and the output current becomes equal to 30 milliamps. It was decided to take part of this scheme and supplement it with my own, which is what I did. The original diagram from Radio magazine, which includes an addition, is shown in the figure below:
The following picture shows the part of this circuit that will need to be assembled.
The value of some parts, in particular resistors R1 and R2, needs to be recalculated upward. If anyone still has questions about where to connect the output wires from this circuit, I will provide the following figure:
I’ll also add that in the assembled circuit, regardless of whether it is the first circuit or the circuit from the Radio magazine, you must place a 1 kOhm resistor at the output, between plus and minus. In the diagram from Radio magazine this is resistor R6. All that remains is to etch the board and assemble everything together in the power supply case. Mirror boards in the program Sprint Layout no need. Short circuit protection circuit board drawing:
About a month ago I came across a diagram of an output current regulator attachment that could be used in conjunction with this power supply. I took the diagram from this site. Then I assembled this set-top box in a separate case and decided to connect it as needed to charge batteries and similar actions where monitoring the output current is important. Here is the diagram of the set-top box, the transistor KT3107 in it was replaced with KT361.
But later the idea came to me to combine, for convenience, all this in one building. I opened the power supply case and looked, there was not enough space left, the variable resistor would not fit. The current regulator circuit uses a powerful variable resistor, which has rather large dimensions. Here's what it looks like:
Then I decided to simply connect both cases with screws, making the connection between the boards with wires. I also set the toggle switch to two positions: output with adjustable current and unregulated. In the first case, the output from the main board of the power supply was connected to the input of the current regulator, and the output of the current regulator went to the clamps on the power supply case, and in the second case, the clamps were connected directly to the output from the main board of the power supply. All this was switched with a six-pin toggle switch in 2 positions. Here is a drawing of the current regulator printed circuit board:
In the figure of the printed circuit board, R3.1 and R3.3 indicate the first and third terminals of the variable resistor, counting from the left. If anyone wants to repeat it, here is a diagram for connecting a toggle switch for switching:
Printed circuit boards of the power supply, protection circuits and current control circuits are attached in the archive. Material prepared by AKV.
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 an amateur radio power supply or any other circuit.
Figure No. 1 – Short circuit protection 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).
Figure No. 2 - Operation of the circuit with a full fuse 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).
Figure No. 3 – The fuse has tripped 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.
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 |
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. 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.
Implementing a protection circuit is not difficult, especially since it is very important for protecting all your devices from short circuits and overloads. If a short circuit occurs in the device for any reason, this can lead to irreparable consequences for it. To protect you from unnecessary costs and the device from burnout, it is enough to make a small modification according to the diagram below.
It is important to note that the entire circuit is built on a complementary pair of transistors. To understand, let’s decipher the meaning of the phrase. Transistors with the same parameters, but different directions of p-n junctions are called a complementary pair.
Those. all parameters of voltage, current, power and other transistors are absolutely the same. The difference only appears in the type of transistor p-n-p or n-p-n. We will also provide examples of complementary pairs to make your purchase easier. From the Russian nomenclature: KT361/KT315, KT3107/KT3102, KT814/KT815, KT816/KT817, KT818/KT819. BD139/BD140 are perfect for imported ones. The relay must be selected for an operating voltage of at least 12 V, 10-20 A.
Operating principle:
When a certain threshold is exceeded (the threshold is set by a variable resistor, experimentally), the switches of the complementary pair of transistors are closed. The voltage at the device output disappears and the LED lights up, indicating that the device’s protective system has activated.
The button between the transistor allows you to reset the protection (in a stationary state it is closed, i.e. it works to open). You can reset the protection in another way, simply turn the unit off and on. Protection is relevant for power supplies or battery charges.
