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Protection device diagram for any power supply. Regulated power supply with overload protection

Protection device diagram for any power supply. Regulated power supply with overload protection

You have already had to build homemade products with a variety of supply voltages: 4.5, 9, 12 V. And each time you had to purchase the appropriate number of batteries or cells. But the necessary power sources are not always available, and their service life is limited. That is why the home laboratory needs a universal source suitable for almost all cases of amateur radio practice. This can be the AC power supply described below, providing any DC voltage from 0.5 to 12 V. While the amount of current drawn from the unit can reach 0.5 A, the output voltage remains stable. And one more advantage of the block is that it is not afraid of short circuits, which are often encountered in practice during the verification and adjustment of structures, which is especially important for a novice radio amateur.

The power supply diagram is shown on rice. one. Mains voltage is supplied through plug XI, fuse FX and switch S1 to the primary winding of step-down transformer T1. The alternating voltage from the secondary winding is supplied to the rectifier, assembled on diodes VI - V4. The output of the rectifier will already have a constant voltage, it is smoothed out by capacitor C1.

This is followed by a voltage regulator, which includes resistors R2-R5, transistors V8, V9 and a zener diode V7. Variable resistor R3 can be set at the output of the unit (in sockets X2 and X3) any voltage from 0.5 to 12 V.

Short circuit protection is implemented on the transistor V6. As soon as the short in the load disappears, the voltage set earlier will appear at the output again without any restarts.

On the secondary winding of the step-down transformer 13 - 17 volts.

Diodes can be any of the D226 series (for example, D226V, D226D, etc.) - Capacitor C1 of type K50-16. Fixed resistors - MLT, variable - SP-1. Instead of the Zener diode D814D, you can use D813. Transistors V6, V8 can be taken as MP39B, MP41, MP41A, MP42B with the highest possible current transfer coefficient. Transistor V9 - P213, P216, P217 with any letter index. Suitable and P201 - P203. The transistor must be installed on the radiator.

The remaining parts - switch, fuse, plug and sockets - of any design.

As usual, after completing the installation, first check the correctness of all connections, and then arm yourself with a voltmeter and proceed to check the power supply. After inserting the plug of the block into the mains socket and applying power to the switch S1, immediately check the voltage on the capacitor C1 - it should be 15-19 V. Then set the variable resistor R3 slider to the upper position according to the diagram and measure the voltage at the X2 and XZ sockets - it should be about 12 V. If the voltage is much less, check the operation of the zener diode - connect a voltmeter to its terminals and measure the voltage. At these points, the voltage should be about 12 V. Its value can be significantly less due to the use of a zener diode with a different letter index (for example, D814A), as well as if the outputs of the transistor V6 are not turned on correctly or if it malfunctions. To exclude the influence of this transistor, unsolder the output of its collector from the anode of the zener diode and again measure the voltage at the zener diode. If in this case the voltage is low, check the resistor R2 for compliance with its nominal value (360 ohms). When you achieve the desired voltage at the output of the power supply (about 12 V), try moving the resistor slider down the circuit. The output voltage of the unit should gradually decrease to almost zero.
Now check the operation of the unit under load. Connect a resistor with a resistance of 40-50 ohms and a power of at least 5 watts to the sockets. It can be composed, for example, of four parallel-connected MLT-2.0 resistors (power 2 W) with a resistance of 160-200 ohms. In parallel with the resistor, turn on the voltmeter and set the slider of the variable resistor R3 to the upper position according to the diagram. The voltmeter needle should show a voltage of at least 11 V. If the voltage drops more, try reducing the resistance of resistor R2 (install a 330 or 300 ohm resistor instead).

The time has come to check the operation of the circuit breaker. You will need an ammeter for 1-2 A, but it is quite possible to use a tester such as Ts20, included in the measurement of direct current up to 750 mA. First, set the output voltage to 5-6 V with a variable resistor of the power supply, and then connect the ammeter probes to the output sockets of the unit: the negative probe to the X2 socket, the positive probe to the X3 socket. At the first moment, the ammeter needle should jump to the final division of the scale, and then return to zero. If so, the machine is working properly.

The maximum output voltage of the block is determined only by the stabilization voltage of the zener diode. And it can be from 11.5 to 14 V for the D814D (D813) indicated on the diagram. Therefore, if necessary, slightly increase the maximum voltage, select a zener diode with the desired stabilization voltage or replace it with another one, for example D815E (with a stabilization voltage of 15 V). But in this case, you will have to change the resistor R2 (reduce its resistance) and use a transformer with which the rectified voltage will be at least 17 V at a load of 0.5 A (measured at the capacitor terminals).

The final stage is the graduation of the scale of the variable resistor, which you must paste on the front panel of the case in advance. You will need, of course, a DC voltmeter. Controlling the output voltage of the unit, set the variable resistor slider to different positions and mark the voltage value for each of them on the scale.

Adjustable power supply with short circuit protection on the KT805 transistor.

The figure below shows a diagram of a simple stabilized power supply. It contains a step-down transformer (T1), a bridge rectifier (VD1 - VD4), a capacitor filter (C1) and a semiconductor voltage regulator. The voltage regulator circuit allows you to smoothly adjust the output voltage in the range from 0 to 12 volts and is protected from short circuits at the output (VT1). An additional transformer winding is provided for powering a low-voltage soldering iron, as well as for experiments with alternating electric current. There is an indication of constant voltage (LED HL2) and variable voltage (LED HL1). To turn on the entire device, the SA1 toggle switch is used, and the soldering iron - SA2. The load is disconnected by SA3. To protect AC circuits from overloads, fuses FU1 and FU2 are provided. The output voltage values are marked on the output voltage regulator knob (potentiometer R4). If desired, you can install a pointer voltmeter at the output of the stabilizer or assemble a voltmeter with a digital display.

The figure below shows a fragment of a modified stabilizer circuit with an indication of a short circuit in the load. In normal mode, the green LED is lit, when the load is closed, it is red.

Implementing a protection circuit is not difficult, especially since it is very important to protect all your devices from short circuits and overloads. If for some reason a short circuit occurs in the device, 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 revision, according to the scheme 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. A complementary pair is called transistors with the same parameters, but different directions of p-n junctions.

Those. all parameters of voltage, current, power and others for transistors are exactly the same. The difference only manifests itself in the type of transistor p-n-p or n-p-n. We will also give examples of complementary pairs to make it easier for you to buy. From the Russian nomenclature: KT361/KT315, KT3107/KT3102, KT814/KT815, KT816/KT817, KT818/KT819. BD139 / BD140 are perfect as imports. 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, empirically), the keys of a complementary pair of transistors are closed. The voltage at the output of the device disappears and the LED lights up, indicating the operation of the protective system of the device.

The button between the transistor allows you to reset the protection (in the stationary state it is closed, i.e. it works to open). You can reset the protection in another way, just turn the unit off and on. Protection is relevant for power supplies or battery chargers.

Every radio amateur who regularly designs electronic devices, I think, has a regulated power supply at home. The thing is really convenient and useful, without which, having tried it in action, it becomes difficult to manage. Indeed, if we need to check, for example, an LED, then 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 we need and go ahead.

Many novice radio amateurs first assemble a simple adjustable power supply, without adjusting the output current and short circuit protection. So it was with me, about 5 years ago I assembled a simple power supply unit with only the output voltage adjustable from 0.6 to 11 volts. Its scheme is shown in the figure below:

But a few months ago I decided to upgrade this power supply and supplement its circuit with a small short circuit protection circuit. I found this scheme 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 schematic diagram of the power supply I assembled earlier. In the event of a short circuit in the powered circuit, the short circuit indication LED goes out to indicate this, and the output current becomes 30 milliamps. It was decided by taking part of this scheme to supplement his own, which he did. The original diagram from the Radio magazine, which includes the add-on, is shown in the figure below:

The following figure shows part of this circuit that will need to be assembled.

The value of some parts, in particular resistors R1 and R2, must be recalculated upwards. If someone still has questions about where to connect the outgoing wires from this circuit, I will give the following figure:

I will also add that in the assembled circuit, regardless of whether it will be the first circuit, or the circuit from the Radio magazine, you must put a 1 kΩ resistor at the output, between plus and minus. In the diagram from Radio magazine, this is resistor R6. Then it remains to pickle the board and assemble everything together in the power supply case. Mirror boards in the program Sprint layout no need. Short Circuit Protection PCB Drawing:

About a month ago, I came across a circuit for an output current regulator attachment that could be used in conjunction with this power supply. taken from this site. Then I assembled this prefix in a separate case and decided to connect it as needed to charge batteries and similar actions, where output current control is important. I give a diagram of the set-top box, the kt3107 transistor in it was replaced by kt361.

But later the idea came to me to combine, for convenience, all this in one building. I opened the case of the power supply 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 body of the power supply, 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 by a six-pin toggle switch for 2 positions. I give a drawing of the printed circuit board of the current regulator:

In the PCB drawing, R3.1 and R3.3 are pins 1 and 3 of the variable resistor, counting from the left. If someone wants to repeat, I give the connection diagram of the toggle switch for switching:

I attached the printed circuit boards of the power supply, protection circuits and current regulation circuits in the archive. Material prepared by AKV.

The connection diagram of the transistor to the power supply is shown in Fig. 1, and the current-voltage characteristics of the transistor for various resistances of the resistor R1 are shown in Fig. 2. This is how protection works. If the resistance of the resistor is zero (i.e., the source is connected to the gate), and the load draws a current of about 0.25 A, then the voltage drop across the field-effect transistor does not exceed 1.5 V, and practically all the rectified voltage will be on the load. When a short circuit appears in the load circuit, the current through the rectifier increases sharply and, in the absence of a transistor, can reach several amperes. The transistor limits the short circuit current to 0.45...0.5 A, regardless of the voltage drop across it. In this case, the output voltage will become zero, and the entire voltage will drop across the FET. Thus, in the event of a short circuit, the power consumed from the power source will not more than double in this example, which in most cases is quite acceptable and will not affect the "health" of the power supply parts.

Rice. 2

You can reduce the short circuit current by increasing the resistance of the resistor R1. It is necessary to choose a resistor such that the short-circuit current is approximately twice the maximum load current.
This protection method is especially convenient for power supplies with a smoothing RC filter - then the field effect transistor is turned on instead of the filter resistor (such an example is shown in Fig. 3).
Since almost all of the rectified voltage drops on the field effect transistor during a short circuit, it can be used for light or sound signaling. Here, for example, is a diagram for switching on a light signal - Fig. 7. When everything is in order with the load, the green LED HL2 is on. In this case, the voltage drop across the transistor is not enough to ignite the HL1 LED. But as soon as a short circuit appears in the load, the HL2 LED goes out, but HL1 flashes red.

Rice. 3

Resistor R2 is selected depending on the desired short-circuit current limitation according to the above recommendations.
The connection diagram of the sound signaling device is shown in fig. 4. It can be connected either between the drain and the source of the transistor, or between the drain and the gate, like the HL1 LED.
When sufficient voltage appears on the signaling device, the AF generator, made on a unijunction transistor VT2, comes into action, and a sound is heard in the BF1 headphone.
The unijunction transistor can be KT117A-KT117G, the phone is low-resistance (can be replaced with a low-power dynamic head).

Rice. four

It remains to be added that for low-current loads, a short-circuit current limiter on a KP302V field-effect transistor can be introduced into the power supply. When choosing a transistor for other blocks, its allowable power and drain-source voltage should be taken into account.
Of course, such automation can also be introduced into a stabilized power supply that does not have protection against short circuits in the load.

This is a small universal short circuit protection unit that is designed for use in network. It is specially designed to fit into most power supplies without rewiring their circuitry. The circuit, despite the presence of a microcircuit, is very easy to understand. Save it to your computer to see it in the best size.

To solder the circuit you will need:

1 - TL082 dual op amp
2 - 1n4148 diode
1 - tip122 NPN transistor
1 - BC558 PNP transistor BC557, BC556
1 - 2700 ohm resistor
1 - 1000 ohm resistor
1 - 10 kΩ resistor
1 - 22 kΩ resistor
1 - potentiometer 10 kΩ
1 - capacitor 470 microfarads
1 - capacitor 1 microfarad
1 - normally closed switch
1 - relay model T74 "G5LA-14"

Connecting the circuit to the PSU

Here, a low value resistor is connected in series with the output of the power supply. As soon as current starts flowing through it, there will be a small voltage drop and we will use this voltage drop to determine if the power is the result of an overload or a short circuit. At the heart of this circuit is an operational amplifier (op-amp) included as a comparator.

If the voltage at the non-inverting output is higher than the voltage at the inverting output, then the output is set to a "high" level.
If the voltage at the non-inverting output is lower than the voltage at the inverting output, then the output is set to a "low" level.

True, this has nothing to do with the logical 5 volt level of conventional microcircuits. When the op amp is "high", its output will be very close to the positive potential of the supply voltage, so if the supply is +12 V, the "high" will approach +12 V. When the op amp is "low", its output will be almost at the minus of the supply voltage, therefore, close to 0 V.

When using op amps as comparators, we usually have an input signal and a reference voltage to compare this input signal to. So we have a resistor with a variable voltage that is defined according to the current that flows through it and the reference voltage. This resistor is the most important part of the circuit. It is connected in series with the output power. You need to choose a resistor that has a voltage drop of about 0.5~0.7 volts when there is an overload current through it. An overload current occurs when the protection circuit operates and closes the power output to prevent damage to it.

You can choose a resistor using Ohm's law. The first thing to determine is the current overload of the power supply. To do this, you need to know the maximum allowable current of the power supply.

Let's say your power supply can deliver 3 amps (in this case, the voltage of the power supply does not matter). So, we got P \u003d 0.6 V / 3 A. P \u003d 0.2 Ohm. The next thing you should do is to calculate the power dissipation on this resistor using the formula: P=V*I. If we use our last example, we get: P = 0.6 V * 3 A. P = 1.8 W - 3 or 5 W resistor will be more than enough.

To make the circuit work, you will need to apply a voltage to it, which can be from 9 to 15 V. To calibrate, apply voltage to the inverting input of the op-amp and turn the potentiometer. This voltage will increase or decrease depending on which side you turn it. The value needs to be adjusted according to the input stage gain of 0.6 volts (something around 2.2 to 3 volts if your amplifier stage is similar to mine). This procedure takes some time, and the best way to calibrate is the scientific poke method. You may need to set the potentiometer to a higher voltage so that the protection does not trip at load peaks. Download the project file.

Among the many schemes of chargers for car batteries published on the network, automatic chargers deserve special attention. Such devices create a number of conveniences in the maintenance of batteries. Of the publications devoted to automatic chargers, works should be noted. These devices not only provide battery charging, but also carry out their training and recovery.