Protection circuit for power supply and chargers. Power supply with short-circuit protection Short-circuit protection on relays

Good afternoon. In this note I would like to bring to your attention the power supply for an additional power amplifier for the Veda-FM portable radio station. Output voltage of the power supply is 24V, rated load current is 3.5A, short circuit protection current threshold is 5.5A, short circuit current is 0.06A.

The general view of the kit is shown in photo 1.

The power supply diagram is shown in Figure 1.

The power transformer of the unit is a rewound network transformer from an old TS-90-1 TV; all turns of the network winding of the transformer are used as the primary winding. The new secondary winding contains 2x65 turns of PETV-2 wire with a diameter of 1.25 mm. If there is no wire of this diameter, you can wind 130 turns of wire with a diameter of 0.9 mm on each of the coils. In this case, the coils are then connected in phase in parallel while maintaining the bridge rectifier circuit. If these coils are connected in series, then you can get rid of two diodes (Fig. 2).

The stabilizer circuit is assembled using a hinged installation (1 in photo 2). I have capacitors C3 and C4 in the power amplifier housing. The number two indicates an additional adjustable voltage stabilizer for powering the Veda-ChM, assembled on the KREN12A microcircuit. By changing the supply voltage of the radio station itself, you can change the output power of the amplifier within certain limits. The diagram of this stabilizer can be found in the section “Power supplies” - “Voltage stabilizer on KR142EN12A”. The overload indicator works as follows. The voltage on the rectifier filter capacitors C1 and C2 is approximately 37 volts, given that the output voltage is 24V, the voltage between points 1 and 2 will be in the region of 13 volts, which is not enough to break down the zener diodes VD5, VD6, since their total stabilization voltage is 15V . When “short”, the voltage between these points will increase, current will flow through the zener diodes and the HL1 LED will light up, and the HL2 LED will go out. Please note that on the “ground” there are collectors of powerful transistors, which is simply very convenient, placing the transistors directly on the body of the product. The power supply and power amplifier hang on the attic wall under the antenna, which significantly reduces power loss in the cable. Goodbye. K.V.Yu.

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 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 production 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 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.

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 higher values ​​were required, and technical progress made it possible to produce machines as 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 arises in the coil, which sets the machine’s release mechanism in motion. 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. It is installed in the circuit, which disconnects either the phase wire or the entire circuit 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, more powerful circuit breakers are installed... Alas, this is the most common cause of 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 openly. But what if they are hidden in the wall? New repairs are guaranteed to 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 fact is that nowadays 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 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.

When setting up various electrical and radio equipment, sometimes everything does not go as we would like and a short circuit (short circuit) occurs. A short circuit is dangerous both for the device and for the person installing it. To protect the equipment, you can use a device, the diagram of which is presented below.

Principle of operation

Relay P1 acts as a monitoring element against a short circuit; it is connected in parallel with the load. When voltage is applied to the input of the device, current flows through the relay winding, the relay connects the load, and the lamp does not light up. During a short circuit, the voltage at the relay will drop sharply, and it will turn off the load, while the lamp will light up and signal a short circuit. Resistor R1 is used to adjust the current threshold; its value is calculated using the formula

R1=U network /I additional

U mains – mains voltage, I additional – maximum permissible current.

For example, the network voltage is 220V, the current at which the relay will operate is 10A. We consider 220 V/10 A = 22 Ohm.

Relay power is calculated using the formula 0.2 * I add.

Resistor R1 should be taken with a power of 20 W or more.

That's all. If you have comments or suggestions regarding this article, please write to the site administrator.

List of used literature: V.G. Bastanov Moscow worker. "300 Practical Tips"