Power supply with protection. Homemade power supply with a short-circuit protection system The simplest short-circuit protection

A protection design for any type of power supply is presented. This protection circuit can work together with any power supply - mains, switching and DC batteries. The schematic decoupling of such a protection unit is relatively simple and consists of several components.

Power supply protection circuit

The power part - a powerful field-effect transistor - does not overheat during operation, therefore it does not need a heat sink either. The circuit is at the same time a protection against power overload, overload and short circuit at the output, the protection operation current can be selected by selecting the resistance of the shunt resistor, in my case the current is 8 Amperes, 6 resistors of 5 watts 0.1 Ohm connected in parallel were used. The shunt can also be made from resistors with a power of 1-3 watts.

The protection can be more accurately adjusted by selecting the resistance of the trimming resistor. Power supply protection circuit, current limit regulator Power supply protection circuit, current limit regulator

~~~In the event of a short circuit and overload of the unit output, the protection will instantly operate, turning off the power source. An LED indicator will indicate that the protection has been triggered. Even if the output short-circuits for a couple of tens of seconds, the field-effect transistor remains cold

~~~The field-effect transistor is not critical; any switches with a current of 15-20 Amps or higher and an operating voltage of 20-60 Volts will do. Keys from the IRFZ24, IRFZ40, IRFZ44, IRFZ46, IRFZ48 line or more powerful ones - IRF3205, IRL3705, IRL2505 and the like are ideal.

~~~This circuit is also great for protecting a charger for car batteries; if the connection polarity is suddenly reversed, then nothing bad will happen to the charger; the protection will save the device in such situations.

~~~Thanks to the fast operation of the protection, it can be successfully used for pulsed circuits; in the event of a short circuit, the protection will operate faster than the power switches of the switching power supply have time to burn out. The circuit is also suitable for pulse inverters, as current protection. If there is an overload or short circuit in the secondary circuit of the inverter, the inverter's power transistors instantly fly out, and such protection will prevent this from happening.

Comments
Short circuit protection, polarity reversal and overload are assembled on a separate board. The power transistor was used in the IRFZ44 series, but if desired, it can be replaced with a more powerful IRF3205 or with any other power switch that has similar parameters. You can use keys from the IRFZ24, IRFZ40, IRFZ46, IRFZ48 line and other keys with a current of more than 20 Amps. During operation, the field-effect transistor remains icy. therefore it does not need a heat sink.

The second transistor is also not critical; in my case, a high-voltage bipolar transistor of the MJE13003 series was used, but there is a large choice. The protection current is selected based on the shunt resistance - in my case, 6 0.1 Ohm resistors in parallel, the protection is triggered at a load of 6-7 Amps. You can set it more precisely by rotating the variable resistor, so I set the operating current to around 5 Amps.

The power of the power supply is quite decent, the output current reaches 6-7 Amps, which is quite enough to charge a car battery.
I chose shunt resistors with a power of 5 watts, but 2-3 watts is also possible.

If everything is done correctly, the unit starts working immediately, close the output, the protection LED should light up, which will light up as long as the output wires are in short-circuit mode.
If everything works as it should, then we proceed further. Assembling the indicator circuit.

The circuit is copied from a battery screwdriver charger. The red indicator indicates that there is output voltage at the power supply output, the green indicator shows the charging process. With this arrangement of components, the green indicator will gradually go out and finally go out when the voltage on the battery is 12.2-12.4 Volts; when the battery is disconnected, the indicator will not light up.

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.

The devices require a power supply unit (PSU), which has adjustable output voltage and the ability to regulate the level of overcurrent protection over a wide range. When the protection is triggered, the load (connected device) should automatically turn off.

An Internet search yielded several suitable power supply circuits. I settled on one of them. The circuit is easy to manufacture and set up, consists of accessible parts, and fulfills the stated requirements.

The power supply proposed for manufacture is based on the LM358 operational amplifier and has the following characteristics:
Input voltage, V - 24...29
Output stabilized voltage, V - 1...20 (27)
Protection operation current, A - 0.03...2.0

Photo 2. Power supply circuit

Description of the power supply

The adjustable voltage stabilizer is assembled on the DA1.1 operational amplifier. The amplifier input (pin 3) receives a reference voltage from the motor of the variable resistor R2, the stability of which is ensured by the zener diode VD1, and the inverting input (pin 2) receives the voltage from the emitter of the transistor VT1 through the voltage divider R10R7. Using variable resistor R2, you can change the output voltage of the power supply.
The overcurrent protection unit is made on the DA1.2 operational amplifier; it compares the voltages at the op-amp inputs. Input 5 through resistor R14 receives voltage from the load current sensor - resistor R13. The inverting input (pin 6) receives a reference voltage, the stability of which is ensured by diode VD2 with a stabilization voltage of about 0.6 V.

As long as the voltage drop created by the load current across resistor R13 is less than the exemplary value, the voltage at the output (pin 7) of op-amp DA1.2 is close to zero. If the load current exceeds the permissible set level, the voltage at the current sensor will increase and the voltage at the output of op-amp DA1.2 will increase almost to the supply voltage. At the same time, the HL1 LED will turn on, signaling an excess, and the VT2 transistor will open, shunting the VD1 zener diode with resistor R12. As a result, transistor VT1 will close, the output voltage of the power supply will decrease to almost zero and the load will turn off. To turn on the load you need to press the SA1 button. The protection level is adjusted using variable resistor R5.

PSU manufacturing

1. The basis of the power supply and its output characteristics are determined by the current source - the transformer used. In my case, a toroidal transformer from a washing machine was used. The transformer has two output windings for 8V and 15V. By connecting both windings in series and adding a rectifier bridge using medium-power diodes KD202M available at hand, I obtained a constant voltage source of 23V, 2A for the power supply.

Photo 3. Transformer and rectifier bridge.

2. Another defining part of the power supply is the device body. In this case, a children's slide projector hanging around in the garage found use. By removing the excess and processing the holes in the front part for installing an indicating microammeter, a blank power supply housing was obtained.

Photo 4. PSU body blank

3. The electronic circuit is mounted on a universal mounting plate measuring 45 x 65 mm. The layout of the parts on the board depends on the sizes of the components found on the farm. Instead of resistors R6 (setting the operating current) and R10 (limiting the maximum output voltage), trimming resistors with a value increased by 1.5 times are installed on the board. After setting up the power supply, they can be replaced with permanent ones.

Photo 5. Circuit board

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

Photo 6. Power supply control unit

5. Fabrication and adjustment of a shunt and additional resistance for using a microammeter as an ammeter or power supply voltmeter. Additional resistance consists of permanent and trimming resistors connected in series (pictured above). The shunt (pictured below) is included in the main current circuit and consists of a wire with low resistance. The wire size is determined by the maximum output current. When measuring current, the device is connected in parallel to the shunt.

Photo 7. Microammeter, shunt and additional resistance

Adjustment of the length of the shunt and the value of additional resistance is carried out with the appropriate connection to the device with control for compliance using a multimeter. The device is switched to the Ammeter/Voltmeter mode using a toggle switch in accordance with the diagram:

Photo 8. Control mode switching diagram

6. Marking and processing of the front panel of the power supply unit, installation of remote parts. In this version, the front panel includes a microammeter (toggle switch for switching the A/V control mode to the right of the device), output terminals, voltage and current regulators, and operating mode indicators. To reduce losses and due to frequent use, a separate stabilized 5 V output is additionally provided. Why is the voltage from the 8V transformer winding supplied to the second rectifier bridge and a typical 7805 circuit with built-in protection.

Photo 9. Front panel

7. PSU assembly. All power supply elements are installed in the housing. In this embodiment, the radiator of the control transistor VT1 is an aluminum plate 5 mm thick, fixed in the upper part of the housing cover, which serves as an additional radiator. The transistor is fixed to the radiator through an electrically insulating gasket.

Power supply protection circuit

Many homemade units have the disadvantage of lacking protection against power reverse polarity. Even an experienced person can inadvertently confuse the polarity of the power supply. And there is a high probability that after this the charger will become unusable.

This article will discuss 3 options for reverse polarity protection, which work flawlessly and do not require any adjustment.

Option 1

This protection is the simplest and differs from similar ones in that it does not use any transistors or microcircuits. Relays, diode isolation - that’s all its components.

The scheme works as follows. The minus in the circuit is common, so the positive circuit will be considered.

If there is no battery connected to the input, the relay is in the open state. When the battery is connected, the plus is supplied through the diode VD2 to the relay winding, as a result of which the relay contact closes and the main charging current flows to the battery.

At the same time, the green LED indicator lights up, indicating that the connection is correct.

And if you now remove the battery, then there will be voltage at the output of the circuit, since the current from the charger will continue to flow through the VD2 diode to the relay winding.

If the connection polarity is reversed, the VD2 diode will be locked and no power will be supplied to the relay winding. The relay will not work.

In this case, the red LED will light up, which is intentionally connected incorrectly. It will indicate that the polarity of the battery connection is incorrect.

Diode VD1 protects the circuit from self-induction that occurs when the relay is turned off.

If such protection is introduced into , it’s worth taking a 12 V relay. The permissible current of the relay depends only on the power . On average, it is worth using a 15-20 A relay.

This scheme still has no analogues in many respects. It simultaneously protects against power reversal and short circuit.

The operating principle of this scheme is as follows. During normal operation, the plus from the power source through the LED and resistor R9 opens the field-effect transistor, and the minus through the open junction of the “field switch” goes to the output of the circuit to the battery.

When a polarity reversal or short circuit occurs, the current in the circuit increases sharply, resulting in a voltage drop across the “field switch” and across the shunt. This voltage drop is enough to trigger the low-power transistor VT2. Opening, the latter closes the field-effect transistor, closing the gate to ground. At the same time, the LED lights up, since power for it is provided by the open junction of transistor VT2.

Due to its high response speed, this circuit is guaranteed to protect for any problem at the output.

The circuit is very reliable in operation and can remain in a protected state indefinitely.

This is a particularly simple circuit, which can hardly even be called a circuit, since it uses only 2 components. This is a powerful diode and fuse. This option is quite viable and is even used on an industrial scale.

Power from the charger is supplied to the battery through the fuse. The fuse is selected based on the maximum charging current. For example, if the current is 10 A, then a 12-15 A fuse is needed.

The diode is connected in parallel and is closed during normal operation. But if the polarity is reversed, the diode will open and a short circuit will occur.

And the fuse is the weak link in this circuit, which will burn out at the same moment. After this you will have to change it.

The diode should be selected according to the datasheet based on the fact that its maximum short-term current was several times greater than the fuse combustion current.

This scheme does not provide 100% protection, since there have been cases when the charger burned out faster than the fuse.

Bottom line

From an efficiency point of view, the first scheme is better than the others. But from the point of view of versatility and speed of response, the best option is scheme 2. Well, the third option is often used on an industrial scale. This type of protection can be seen, for example, on any car radio.

All circuits, except the last one, have a self-healing function, that is, operation will be restored as soon as the short circuit is removed or the polarity of the battery connection is changed.

Attached files:

How to make a simple Power Bank with your own hands: diagram of a homemade power bank