This circuit is the simplest transistor power supply equipped with short circuit protection (SC). Its diagram is shown in the figure.

Main settings:

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

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

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

Details

The transformer can be any with a core cross-sectional area of \u200b\u200b4 cm 2 or more. The primary winding contains 2200 turns of wire PEV-0.18, the secondary - 150-170 turns of wire PEV-0.45. A ready-made frame scan transformer from old tube TVs of the TVK110L2 series or similar is also suitable. Diodes VD1-VD4 can be D302-D305, D229ZH-D229L or any for a current of at least 1 A and reverse voltage not less than 55 V. Transistors VT1, VT2 can be any low-frequency low-power, for example, MP39-MP42. You can also use more modern silicon transistors, for example, KT361, KT203, KT209, KT503, KT3107 and others. As VT3 - germanium P213-P215 or more modern silicon powerful low-frequency KT814, KT816, KT818 and others. When replacing VT1, it may turn out that the short circuit protection does not work. Then one more diode should be connected in series with VD5 (or two, if necessary). If VT1 is silicon, then it is better to use silicon diodes, for example, KD209 (AB).

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

List of radioelements

Power Good signal

When we turn on, the output voltages do not immediately reach the desired value, but after about 0.02 seconds, and in order to exclude the supply of undervoltage to the PC components, there is a special "power good" signal, also sometimes called "PWR_OK" or simply "PG", which energized when the voltages at the outputs + 12V, + 5V and + 3.3V reach the correct range. To supply this signal, a special line is allocated on the ATX power connector connected to (No. 8, gray wire).

Another consumer of this signal is the undervoltage protection circuit (UVP) inside the PSU, which will be discussed later - if it is active from the moment it is turned on on the PSU, it simply will not allow the computer to turn on, immediately disconnecting the PSU, since the voltages will obviously below par. Therefore, this circuit only turns on with a Power Good signal.

This signal is supplied by a monitoring circuit or a PWM controller (pulse-width modulation, used in all modern switching power supplies, which is why they got their name, the English abbreviation - PWM, familiar from modern coolers - to control their rotational speed supplied to them the current is modulated in a similar manner.)

Power Good signal flow diagram according to ATX12V specification.
VAC is the incoming AC voltage, PS_ON # is the "power on" signal, which is given when the power button on the system unit is pressed. "O / P" is an abbreviation for "operating point", i.e. working value. And PWR_OK is the Power Good signal. T1 is less than 500ms, T2 is between 0.1ms and 20ms, T3 is between 100ms and 500ms, T4 is less than or equal to 10ms, T5 is greater than or equal to 16ms, and T6 is greater than or equal to 1ms.

Undervoltage and overvoltage protection (UVP / OVP)

Protection in both cases is implemented using the same circuit that monitors the output voltages of + 12V, + 5V and 3.3V and turns off the power supply if one of them is higher (OVP - Over Voltage Protection) or lower (UVP - Under Voltage Protection ) a certain value, which is also called the "trip point". These are the main types of protection that are currently present in virtually all, moreover, the ATX12V standard requires OVP.

Some problem is that both OVP and UVP are usually configured so that the trip points are too far from the nominal voltage value and in the case of OVP this is direct compliance with the ATX12V standard:

Those. you can make a PSU with an OVP trip point of + 12V at 15.6V, or + 5V at 7V and it will still be ATX12V compatible.

This will give out for a long time, say, 15V instead of 12V without triggering protection, which can lead to failure of PC components.

On the other hand, the ATX12V standard clearly stipulates that the output voltages should not deviate by more than 5% from the nominal value, but at the same time, the OVP can be configured by the PSU manufacturer to operate at a deviation of 30% along the + 12V and + 3.3V lines and in 40% - on the + 5V line.

Manufacturers choose trip point values \u200b\u200busing one or another monitoring microcircuit or PWM controller, because the values \u200b\u200bof these points are rigidly set by the specifications of a particular microcircuit.

Take the popular PS223 monitoring IC as an example, which is used in some that are still on the market today. This IC has the following trigger points for OVP and UVP modes:

Other ICs provide a different set of trigger points.

Once again, we remind you how far from the normal voltage values \u200b\u200bthe OVP and UVP are usually configured. In order for them to work, the power supply must be in a very difficult situation. In practice, cheap power supplies that have no other types of protection besides OVP / UVP fail before OVP / UVP is triggered.

Overcurrent protection (OCP)

In the case of this technology (English abbreviation OCP - Over Current Protection), there is one issue that should be considered in more detail. According to the international standard IEC 60950-1, computer equipment should not transmit more than 240 Volt-amperes over any conductor, which in the case of direct current gives 240 watts. The ATX12V specification includes a requirement for overcurrent protection on all circuits. In the case of the most loaded 12 volt circuit, we get the maximum allowable current of 20 amperes. Naturally, such a limitation does not allow the manufacture of a power supply unit with a power of more than 300 watts, and in order to bypass it, the + 12V output circuit was divided into two or more lines, each of which had its own overcurrent protection circuit. Accordingly, all power supply terminals with + 12V contacts are divided into several groups according to the number of lines, in some cases they are even color coded in order to adequately distribute the load along the lines.

However, in many cheap power supply units with the declared two + 12V lines, in practice, only one current protection circuit is used, and all + 12V wires inside are connected to one output. In order to implement the adequate operation of such a circuit, protection against the current load is triggered not at 20A, but at, for example, 40A, and limiting the maximum current along one wire is achieved by the fact that in a real system the + 12V load is always distributed among several consumers and even more wires.

Moreover, sometimes it is possible to figure out whether a particular power supply unit uses a separate current protection for each + 12V line, only by disassembling it and looking at the number and connection of shunts used to measure the current strength (in some cases, the number of shunts may exceed the number of lines, since multiple shunts can be used to measure current on the same line).

Another interesting moment is that, unlike protection against overvoltage / undervoltage, the permissible current level is regulated by the power supply manufacturer, by soldering resistors of one rating or another to the outputs of the control microcircuit. And on cheap power supplies, despite the requirements of the ATX12V standard, this protection can only be installed on the + 3.3V and + 5V lines, or absent altogether.

Overheating protection (OTP)

As its name suggests (OTP - Over Temperature Protection), overheating protection turns off the power supply if the temperature inside its case reaches a certain value. Not all power supplies are equipped with it.

In power supplies, you can see a thermistor attached to a heatsink (although in some power supplies it can be soldered directly to the PCB). This thermistor is connected to the fan speed control circuit and is not used for overheating protection. In PSUs equipped with overheating protection, two thermistors are usually used - one for controlling the fan, the other for overheating protection itself.

Short circuit protection (SCP)

Short Circuit Protection (SCP) is probably the oldest of its kind, because it is very easy to implement with a pair of transistors without using a monitoring chip. This protection is necessarily present in any power supply unit and turns it off in the event of a short circuit in any of the output circuits, in order to avoid a possible fire.

Many homemade blocks have such a drawback as the lack of protection against power reversal. Even an experienced person can inadvertently confuse the polarity of the power supply. And there is a high probability that after that charger will become unusable.

This article will cover 3 variants of protection against polarity reversalthat work flawlessly and do not require any setup.

Option 1

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

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

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

Lights up green at the same time led indicator, indicating that the connection is correct.

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

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

In this case, the red LED will light up, which is deliberately connected in the wrong way. It will indicate that the polarity of the battery connection is reversed.

The VD1 diode protects the circuit from self-induction, which occurs when the relay is turned off.

If such protection is implemented in , it is worth taking a 12 V relay. The permissible relay current 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 protects both from power reversal and short circuit at the same time.

The principle of operation of this circuit is as follows. In normal operation, plus from the power supply through the LED and resistor R9 opens the field-effect transistor, and the minus through the open junction of the "field" goes to the output of the circuit to the battery.

In the event of a polarity reversal or short circuit, the current in the circuit rises sharply, as a result of which a voltage drop occurs on the "field" and on the shunt. This voltage drop is sufficient to trigger the low-power transistor VT2. Opening, the latter locks the field-effect transistor, closing the gate with mass. At the same time, the LED lights up, since the power for it is provided by the open junction of the transistor VT2.

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

The circuit is very reliable in operation and is able to remain in a protected state for an infinitely long time.

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

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

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

And the fuse is the weak link in this circuit, which will burn out instantly. After that it will have to be changed.

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

Such a scheme does not provide one hundred percent protection, since there have been cases when the charger burned out faster than the fuse.

Outcome

In terms of efficiency, 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 for the last one, have a self-healing function, that is, work 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: a homemade power bank scheme

Short circuits occur in any electrical installation, regardless of their complexity. Even if the wiring is new, the lamps and sockets are in good order, and the electrical equipment is produced by world-famous manufacturers, no one is insured against short circuits. And you need to protect yourself from them.

Protection devices against emergency modes in the network

Fuses are the simplest protection devices. Previously, only them were used to eliminate emergency modes in household electrical wiring. Some devices still use fuses. The reason is that they have high performance and are indispensable for protecting semiconductor devices.

After blowing, the fuse is either replaced with a new one, or the fuse inside it changes. Inserts for the same fuse body are available for different current ratings. But the need to keep a supply of fuses for quick replacement at the facility or in the apartment is a disadvantage of fuses.

The most common fuse in Soviet times was the "plug".

They were replaced by automatic plugs of the type Steam, produced for currents of 10, 16 and 25 A. They were screwed into place of plugs, were reusable and had two protective elements, called releases. One protected from short circuits and worked instantly, the second - from overloads and worked with a time delay.

The same releases have all circuit breakers that replaced the fuses. The instantaneous release is called electromagnetic, because its operation is based on the principle of retraction of the coil stem when the rated current is exceeded. The rod hits the latch and the spring opens the contact system of the switch.

A time-delayed release is called a thermal release. It works on the principle of a thermostat in an iron or electric heater. When a current passes through it, a bimetallic plate heats up and slowly bends to the side. The more current through it, the faster the bending occurs. Then it acts on the same latch, and the machine turns off. If the action of the current has stopped, the plate cools down, returns to its original position, and no shutdown occurs.

In the old electrical switchboards, the automatic switches in the carbolite case of the 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 part is either rusted or has lost speed. And not all of them have instantaneous short circuit protection. In some devices, only a thermal release was installed. And the speed of operation of the electromagnetic release in these series machines is lower than in modular ones.

Therefore, such protective devices must be replaced with modern ones, until they have done things by their inaction.

Defense building principles

In apartment buildings, automatic machines are installed in a dashboard on the landing. This is enough to protect the apartments. But if you installed a personal shield when replacing the electrical wiring, then it is better to install a personal automatic machine for each group of consumers in it. There are several reasons for this.

1. When replacing the socket, you do not need to turn off the lights in the apartment and use a flashlight.
2. To protect some consumers, you will reduce the rated current of the machine, which will make their protection more sensitive.
3. In case of 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 the event of an erroneous switching at a substation or line, 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 that is at zero may turn off, and the phase remains.

It is impractical to use a three-pole switch as the equivalent of three single-pole ones. Removing the bar connecting the three poles will not help. There are rods inside the switch that disconnect the remaining poles when one of them is triggered.

When using an RCD, it is imperative to protect the same line with a circuit breaker. RCD protects against leakage currents, but does not protect against short circuits and overloads. The functions of protection against leakage and emergency operating modes are combined in the differential 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 assumed based on the maximum permitted load.

The distribution network is designed in such a way that the rated currents of the protection devices increase as the power supply is approached. If your apartment is switched on through a 16 A single-phase circuit breaker, then all apartments in the entrance can be connected to a 40 A three-phase circuit breaker and are evenly distributed in phases. In the event that your machine does not turn off during a short circuit, after a while the overload protection will work at the driveway. Each subsequent protective device reserves the previous one. Therefore, do not overestimate the rated current of the circuit breaker. It may not work (there will not be enough current) or it will turn off together with a group of consumers.

Modern modular circuit breakers are available with characteristics "B", "C" and "D"... They differ in the multiplicity of cutoff actuation currents.

Be careful when using machines with characteristics "D" and "B".

And remember: if the short circuit is not disconnected, it will cause a fire. Take care of the health of the protection, and live in peace.

Almost every novice radio amateur strives at the beginning of his creativity to design a power supply unit in order to subsequently use it to power various experimental devices. And of course, I would like this power supply to "prompt" about the danger of failure of individual units in case of installation errors or faults.

Today there are many circuits, including those with a short circuit indication at the output. In most cases, a similar indicator is usually an incandescent lamp included in the load break. But with such an inclusion, we increase the input resistance of the power source or, more simply, we limit the current, which in most cases, of course, is acceptable, but not at all desirable.

The circuit shown in Fig. 1 not only signals a short circuit, absolutely not affecting the output resistance of the device, but also automatically disconnects the load when the output is short-circuited. In addition, the HL1 LED reminds that the device is plugged in, and the HL2 glows when the fuse FU1 blows, indicating that it needs to be replaced.

Electrical circuit diagram homemade block power supply with short circuit protection

Consider the work of a homemade power supply... The alternating voltage taken from the secondary winding of T1 is rectified by diodes VD1 ... VD4, assembled in a bridge circuit. Capacitors C1 and C2 prevent the penetration of high-frequency noise into the network, and the oxide capacitor C3 smoothes the ripple of the voltage supplied to the input of the compensation stabilizer, assembled at VD6, VT2, VT3 and providing at the output stable voltage 9 B.

The stabilization voltage can be changed by selecting a Zener diode VD6, for example, at KS156A it will be 5 V, at D814A - 6 V, with DV14B - V V, with DV14G -10 V, with DV14D -12 V. If desired, the output voltage can be made adjustable, for this, between the anode and cathode VD6, a variable resistor with a resistance of 3-5 kOhm is included, and the base VT2 is connected to the engine of this resistor.

Consider the operation of the power supply protective device... The load short circuit protection unit consists of germanium p-p-p transistor VT1, electromagnetic relay K1, resistor R3 and diode VD5. The latter in this case performs the function of a stabilizer that maintains a constant voltage of about 0.6 - 0.7 V relative to the total on the basis of VT1.

In the normal mode of operation of the stabilizer, the transistor of the protection unit is reliably closed, since the voltage at its base relative to the emitter is negative. When a short circuit occurs, the emitter VT1, like the emitter of the regulating VT3, turns out to be connected to the common negative wire of the rectifier.

In other words, the voltage at its base relative to the emitter becomes positive, as a result of which VT1 opens, K1 is triggered and disconnects the load with its contacts, the HL3 LED is on. After eliminating the short circuit, the bias voltage at the emitter junction VT1 again becomes negative and it closes, relay K1 is de-energized, connecting the load to the output of the stabilizer.

Details for the manufacture of the power supply. Any electromagnetic relay with the lowest possible actuation voltage. In any case, one indispensable condition must be observed: the secondary winding of T1 must produce a voltage equal to the sum of the stabilization voltages and relay operation, i.e. if the stabilization voltage, as in this case, is 9 V, and U is activated by the relay 6 V, then the secondary winding should be at least 15 V, but also not exceed the allowable one on the collector-emitter of the used transistor. The author used the TVK-110L2 as T1 on the prototype. Printed circuit board device is shown in Fig. 2.

Power supply PCB