12 volt power supply circuits. power unit

A rectifier is a device for converting alternating voltage to direct voltage. This is one of the most common parts in electrical appliances, from hair dryers to all types of power supplies with output voltage DC. Eat different schemes rectifiers and each of them copes with its task to a certain extent. In this article we will talk about how to make a single-phase rectifier and why it is needed.

Definition

A rectifier is a device designed to convert alternating current into direct current. The word “constant” is not entirely correct; the fact is that at the output of the rectifier, in the sinusoidal alternating voltage circuit, in any case there will be an unstabilized pulsating voltage. In simple words: constant in sign, but varying in magnitude.

There are two types of rectifiers:

Half-wave. It rectifies only one half-wave of the input voltage. Characterized by strong ripples and low voltage relative to the input.

Full wave. Accordingly, two half-waves are rectified. The ripple is lower, the voltage is higher than at the rectifier input - these are two main characteristics.

What does stabilized and unstabilized voltage mean?

Stabilized is a voltage that does not change in value regardless of the load or input voltage surges. For transformer power supplies this is especially important because output voltage depends on the input and differs from it by K transformation times.

Unstabilized voltage - changes depending on surges in the supply network and load characteristics. With such a power supply, due to drawdowns, the connected devices may malfunction or become completely inoperable and fail.

Output voltage

The main quantities of alternating voltage are amplitude and effective value. When they say “in a 220V network,” they mean the effective voltage.

If we talk about the amplitude value, then we mean how many volts from zero to the top point of the half-wave of a sine wave.

Omitting the theory and a number of formulas, we can say that it is 1.41 times less than the amplitude. Or:

The amplitude voltage in a 220V network is equal to:

The first scheme is more common. It consists of a diode bridge - connected to each other by a “square”, and a load is connected to its shoulders. The bridge type rectifier is assembled according to the diagram below:

It can be connected directly to a 220V network, as done in, or to the secondary windings of a network (50 Hz) transformer. Diode bridges according to this scheme can be assembled from discrete (individual) diodes or use a ready-made diode bridge assembly in a single housing.

The second circuit - a midpoint rectifier cannot be connected directly to the network. Its meaning is to use a transformer with a tap from the middle.

At its core, these are two half-wave rectifiers connected to the ends of the secondary winding; the load is connected with one contact to the diode connection point, and the second to the tap from the middle of the windings.

Its advantage over the first circuit is the smaller number of semiconductor diodes. The disadvantage is the use of a transformer with a midpoint or, as they also call it, a tap from the middle. They are less common than conventional transformers with a secondary winding without taps.

Ripple Smoothing

Power supply with pulsating voltage is unacceptable for a number of consumers, for example, light sources and audio equipment. Moreover, permissible light pulsations are regulated in state and industry regulations.

To smooth out ripples, they use a parallel-installed capacitor, an LC filter, various P- and G-filters...

But the most common and simplest option is a capacitor installed in parallel with the load. Its disadvantage is that to reduce ripple on a very powerful load, you will have to install very large capacitors - tens of thousands of microfarads.

Its operating principle is that the capacitor is charged, its voltage reaches amplitude, the supply voltage after the point of maximum amplitude begins to decrease, from this moment the load is powered by the capacitor. The capacitor discharges depending on the resistance of the load (or its equivalent resistance if it is not resistive). The larger the capacitance of the capacitor, the smaller the ripple will be when compared with a capacitor with a lower capacitance connected to the same load.

In simple words: the slower the capacitor discharges, the less ripple.

The discharge rate of the capacitor depends on the current consumed by the load. It can be determined using the time constant formula:

where R is the load resistance, and C is the capacitance of the smoothing capacitor.

Thus, from a fully charged state to a completely discharged state, the capacitor will be discharged in 3-5 t. It charges at the same speed if the charge occurs through a resistor, so in our case it does not matter.

It follows that in order to achieve an acceptable level of ripple (it is determined by the load requirements for the power source), you need a capacitance that will be discharged in a time several times greater than t. Since the resistance of most loads is relatively small, a large capacitance is needed, therefore, in order to smooth out ripples at the output of the rectifier, they are used, they are also called polar or polarized.

Please note that it is highly not recommended to confuse the polarity of an electrolytic capacitor, because this can lead to its failure and even explosion. Modern capacitors are protected from explosion - they have a cross-shaped stamping on the top cover, along which the case will simply crack. But a stream of smoke will come out of the condenser; it will be bad if it gets into your eyes.

The capacitance is calculated based on the ripple factor that needs to be ensured. In simple terms, the ripple coefficient shows by what percentage the voltage sags (pulsates).

C=3200*In/Un*Kp,

Where In is the load current, Un is the load voltage, Kn is the ripple factor.

For most types of equipment, the ripple coefficient is taken to be 0.01-0.001. Additionally, it is advisable to install as large a capacity as possible to filter out high-frequency interference.

How to make a power supply with your own hands?

The simplest DC power supply consists of three elements:

1. Transformer;

3. Capacitor.

This is an unregulated DC power supply with a smoothing capacitor. The voltage at its output is greater than the alternating voltage on the secondary winding. This means that if you have a 220/12 transformer (the primary is 220V and the secondary is 12V), then at the output you will get 15-17V constant. This value depends on the capacitance of the smoothing capacitor. This circuit can be used to power any load, if it does not matter to it that the voltage can “float” when the supply voltage changes.

A capacitor has two main characteristics - capacitance and voltage. We figured out how to select the capacitance, but not how to select the voltage. The capacitor voltage must exceed the amplitude voltage at the rectifier output by at least half. If the actual voltage on the capacitor plates exceeds the nominal voltage, there is a high probability of its failure.

Old Soviet capacitors were made with a good voltage reserve, but now everyone uses cheap electrolytes from China, where at best there is a small reserve, and at worst it will not withstand the specified rated voltage. Therefore, do not skimp on reliability.

The stabilized power supply differs from the previous one only by the presence of a voltage (or current) stabilizer. The simplest option- use L78xx or others, such as the domestic KREN.

This way you can get any voltage, the only condition when using such stabilizers is that the voltage to the stabilizer must exceed the stabilized (output) value by at least 1.5V. Let's look at what is written in the datasheet of the 12V stabilizer L7812:

The input voltage should not exceed 35V, for stabilizers from 5 to 12V, and 40V for stabilizers 20-24V.

The input voltage must exceed the output voltage by 2-2.5V.

Those. for a stabilized 12V power supply with a stabilizer of the L7812 series, it is necessary that the rectified voltage be in the range of 14.5-35V, in order to avoid sags, it would be an ideal solution to use a transformer with a 12V secondary winding.

But the output current is quite modest - only 1.5A, it can be amplified using a pass transistor. If you have , you can use this scheme:

It shows only the connection of a linear stabilizer; the “left” part of the circuit with the transformer and rectifier is omitted.

If you have NPN transistors like KT803/KT805/KT808, then this one will do:

It is worth noting that in the second circuit, the output voltage will be 0.6V less than the stabilization voltage - this is a drop at the emitter-base transition, we wrote more about this. To compensate for this drop, diode D1 was introduced into the circuit.

It is possible to install two linear stabilizers in parallel, but this is not necessary! Due to possible deviations during manufacturing, the load will be distributed unevenly and one of them may burn out because of this.

Install both the transistor and the linear stabilizer on the radiator, preferably on different radiators. They get very hot.

Regulated Power Supplies

The simplest adjustable power supply can be made with an adjustable linear stabilizer LM317, its current is also up to 1.5 A, you can amplify the circuit with a pass transistor, as described above.

Here is a more visual diagram for assembling an adjustable power supply.

With a thyristor regulator in the primary winding, essentially the same regulated power supply.

By the way, a similar scheme is used to regulate the welding current:

Conclusion

A rectifier is used in power supplies to produce direct current from alternating current. Without its participation, it will not be possible to power a DC load, for example LED strip or radio.

Also used in a variety of chargers for car batteries, there are a number of circuits using a transformer with a group of taps from the primary winding, which are switched by a flip switch, and only a diode bridge is installed in the secondary winding. The switch is installed on the side high voltage, since the current there is several times lower and its contacts will not burn from this.

Using the diagrams from the article, you can assemble a simple power supply both for constant operation of some device and for testing your electronic homemade products.

The schemes are no different high efficiency, but they produce a stabilized voltage without much ripple, you should check the capacitance of the capacitors and calculate it for a specific load. They are perfect for low-power audio amplifiers and will not create additional background noise. An adjustable power supply will be useful for car enthusiasts and auto electricians to test the generator voltage regulator relay.

A regulated power supply is used in all areas of electronics, and if you improve it with short-circuit protection or a current stabilizer on two transistors, you will get an almost full-fledged laboratory power supply.

Hello to all DIYers. Many radio amateurs know that the power supply is an expensive part of all electronics and it is often not possible to purchase a good power supply, but everyone who is starting to understand the radio business has an old computer unit that has been lying around for a long time and is not used. In this article I will tell you how to make a laboratory power supply for various devices, such as an amplifier.

First you need to decide what you need for assembly, this is:
* The computer unit itself, the power of mine was 350 watts, which is enough for everything with a reserve.
* Plywood, I found 4 pieces of it.
* Jigsaw.
* Screwdrivers.
* Soldering iron and soldering accessories.
* Drill.
* Sandpaper, coarser grit.
* Nails, I preferred nails with small heads.
* Rubber stoppers obtained from chemical test tubes.

First, unscrew the top bolts that hold the cover.

With the current level of development of the element base of radio-electronic components, a simple and reliable power supply with your own hands can be made very quickly and easily. This does not require high-level knowledge of electronics and electrical engineering. You will soon see this.

Making your first power source is quite an interesting and memorable event. Therefore, an important criterion here is the simplicity of the circuit, so that after assembly it immediately works without any additional settings and adjustments.

It should be noted that almost every electronic, electrical device or appliance needs power. The only difference is in the basic parameters - the magnitude of voltage and current, the product of which gives power.

Making a power supply with your own hands is a very good first experience for novice electronics engineers, since it allows you to feel (not on yourself) the different magnitudes of currents flowing in devices.

The modern power supply market is divided into two categories: transformer-based and transformerless. The first ones are quite easy to manufacture for beginner radio amateurs. The second indisputable advantage is the relatively low level of electromagnetic radiation, and therefore interference. A significant drawback by modern standards is the significant weight and dimensions caused by the presence of a transformer - the heaviest and most bulky element in the circuit.

Transformerless power supplies do not have the last drawback due to the absence of a transformer. Or rather, it is there, but not in the classical presentation, but works with high-frequency voltage, which makes it possible to reduce the number of turns and the size of the magnetic circuit. As a result, the overall dimensions of the transformer are reduced. High frequency is formed by semiconductor switches, in the process of switching on and off according to a given algorithm. As a result, strong electromagnetic interference occurs, so such sources must be shielded.

We will be assembling a transformer power supply that will never lose its relevance, since it is still used in high-end audio equipment, thanks to the minimal level of noise generated, which is very important for obtaining high-quality sound.

Design and principle of operation of the power supply

The desire to obtain a finished device as compact as possible led to the emergence of various microcircuits, inside of which there are hundreds, thousands and millions of individual electronic elements. Therefore, almost any electronic device contains a chip whose standard power supply is 3.3 V or 5 V. Auxiliary elements can be powered from 9 V to 12 V DC. However, we know well that the outlet has an alternating voltage of 220 V with a frequency of 50 Hz. If it is applied directly to a microcircuit or any other low-voltage element, they will instantly fail.

From this it becomes clear that the main task network block power supply (BP) consists of reducing the voltage to an acceptable level, as well as converting (rectifying) it from alternating to direct. In addition, its level must remain constant regardless of fluctuations in the input (in the socket). Otherwise, the device will be unstable. Therefore, another important function of the power supply is voltage level stabilization.

In general, the structure of the power supply consists of a transformer, rectifier, filter and stabilizer.

In addition to the main components, a number of auxiliary components are also used, for example, indicator LEDs that signal the presence of supplied voltage. And if the power supply provides for its adjustment, then naturally there will be a voltmeter, and possibly also an ammeter.

Transformer

In this circuit, a transformer is used to reduce the voltage in a 220 V socket to the required level, most often 5 V, 9 V, 12 V or 15 V. At the same time, galvanic isolation of high-voltage and low-voltage circuits is also carried out. Therefore, in any emergency situations, the voltage on the electronic device will not exceed the value of the secondary winding. Galvanic isolation also increases the safety of operating personnel. In case of touching the device, a person will not fall under the high potential of 220 V.

The design of the transformer is quite simple. It consists of a core that performs the function of a magnetic circuit, which is made of thin plates that conduct magnetic flux well, separated by a dielectric, which is a non-conductive varnish.

At least two windings are wound on the core rod. One is primary (also called network) - 220 V is supplied to it, and the second is secondary - reduced voltage is removed from it.

The operating principle of the transformer is as follows. If voltage is applied to the mains winding, then, since it is closed, alternating current will begin to flow through it. Around this current, an alternating magnetic field arises, which collects in the core and flows through it in the form of a magnetic flux. Since there is another winding on the core - the secondary one, under the influence of an alternating magnetic flux an electromotive force (EMF) is generated in it. When this winding is shorted to a load, alternating current will flow through it.

Radio amateurs in their practice most often use two types of transformers, which mainly differ in the type of core - armored and toroidal. The latter is more convenient to use in that it is quite easy to wind the required number of turns onto it, thereby obtaining the required secondary voltage, which is directly proportional to the number of turns.

The main parameters for us are two parameters of the transformer - voltage and current of the secondary winding. We will take the current value to be 1 A, since we will use zener diodes for the same value. About that a little further.

We continue to assemble the power supply with our own hands. And the next order element in the circuit is a diode bridge, also known as a semiconductor or diode rectifier. It is designed to convert the alternating voltage of the secondary winding of the transformer into direct voltage, or more precisely, into rectified pulsating voltage. This is where the name “rectifier” comes from.

There are various rectification circuits, but the bridge circuit is the most widely used. The principle of its operation is as follows. In the first half-cycle of the alternating voltage, current flows along the path through the diode VD1, resistor R1 and LED VD5. Next, the current returns to the winding through open VD2.

A reverse voltage is applied to the diodes VD3 and VD4 at this moment, so they are locked and no current flows through them (in fact, it flows only at the moment of switching, but this can be neglected).

In the next half-cycle, when the current in the secondary winding changes its direction, the opposite will happen: VD1 and VD2 will close, and VD3 and VD4 will open. In this case, the direction of current flow through resistor R1 and LED VD5 will remain the same.

A diode bridge can be soldered from four diodes connected according to the diagram above. Or you can buy it ready-made. They come in horizontal and vertical versions in different housings. But in any case, they have four conclusions. The two terminals are supplied with alternating voltage, they are designated by the sign “~”, both are the same length and are the shortest.

The rectified voltage is removed from the other two terminals. They are designated “+” and “-”. The “+” pin has the longest length among the others. And on some buildings there is a bevel near it.

Capacitor filter

After the diode bridge, the voltage has a pulsating nature and is still unsuitable for powering microcircuits, and especially microcontrollers, which are very sensitive to various kinds of voltage drops. Therefore it needs to be smoothed out. To do this, you can use a choke or a capacitor. In the circuit under consideration, it is enough to use a capacitor. However, it must have a large capacitance, so an electrolytic capacitor should be used. Such capacitors often have polarity, so it must be observed when connecting to the circuit.

The negative terminal is shorter than the positive one and a “-” sign is applied to the body near the first one.

Voltage stabilizer L.M. 7805, L.M. 7809, L.M. 7812

You probably noticed that the voltage in the outlet is not equal to 220 V, but varies within certain limits. This is especially noticeable when connecting a powerful load. If you do not apply special measures, then it will change in a proportional range at the output of the power supply. However, such vibrations are extremely undesirable and sometimes unacceptable for many electronic elements. Therefore, the voltage after the capacitor filter must be stabilized. Depending on the parameters of the powered device, two stabilization options are used. In the first case, a zener diode is used, and in the second, an integrated voltage stabilizer is used. Let's consider the application of the latter.

In amateur radio practice, voltage stabilizers of the LM78xx and LM79xx series are widely used. Two letters indicate the manufacturer. Therefore, instead of LM there may be other letters, for example CM. The marking consists of four numbers. The first two - 78 or 79 - mean positive or negative voltage, respectively. The last two digits in in this case instead of them, two X's: xx, indicate the value of the output U. For example, if at the position of two X's there is 12, then this stabilizer produces 12 V; 08 – 8 V, etc.

For example, let's decipher the following markings:

LM7805 → 5V positive voltage

LM7912 → 12 V negative U

Integrated stabilizers have three outputs: input, common and output; designed for current 1A.

If the output U significantly exceeds the input and the current limit is 1 A, then the stabilizer gets very hot, so it should be installed on a radiator. The design of the case provides for this possibility.

If the load current is much lower than the limit, then you don’t have to install a radiator.

The classic design of the power supply circuit includes: a network transformer, a diode bridge, a capacitor filter, a stabilizer and an LED. The latter acts as an indicator and is connected through a current-limiting resistor.

Since in this circuit the current-limiting element is the LM7805 stabilizer (allowable value 1 A), all other components must be rated for a current of at least 1 A. Therefore, the secondary winding of the transformer is selected for a current of one ampere. Its voltage should not be lower than the stabilized value. And for good reason, it should be chosen from such considerations that after rectification and smoothing, U should be 2 - 3 V higher than the stabilized one, i.e. A couple of volts more than its output value should be supplied to the input of the stabilizer. Otherwise it will not work correctly. For example, for LM7805 input U = 7 - 8 V; for LM7805 → 15 V. However, it should be taken into account that if the value of U is too high, the microcircuit will heat up very much, since the “extra” voltage is extinguished at its internal resistance.

The diode bridge can be made from 1N4007 type diodes, or take a ready-made one for a current of at least 1 A.

Smoothing capacitor C1 should have a large capacity of 100 - 1000 µF and U = 16 V.

Capacitors C2 and C3 are designed to smooth out high-frequency ripple that occurs when the LM7805 operates. They are installed for greater reliability and are recommendations from manufacturers of stabilizers of similar types. The circuit also works normally without such capacitors, but since they cost practically nothing, it is better to install them.

DIY power supply for 78 L 05, 78 L 12, 79 L 05, 79 L 08

Often it is necessary to power only one or a pair of microcircuits or low-power transistors. In this case, apply powerful block nutrition is not rational. Therefore, the best option would be to use stabilizers of the 78L05, 78L12, 79L05, 79L08, etc. series. They are designed for a maximum current of 100 mA = 0.1 A, but are very compact and no larger in size than a regular transistor, and also do not require installation on a radiator.

The markings and connection diagram are similar to the LM series discussed above, only the location of the pins differs.

For example, the connection diagram for the 78L05 stabilizer is shown. It is also suitable for LM7805.

The connection diagram for negative voltage stabilizers is shown below. The input is -8 V, and the output is -5 V.

As you can see, making a power supply with your own hands is very simple. Any voltage can be obtained by installing an appropriate stabilizer. You should also remember the transformer parameters. Next we will look at how to make a power supply with voltage regulation.

Hello to all radio amateurs, in this article I would like to introduce you to a power supply with voltage regulation from 0 to 12 volts. It is very easy to set the desired voltage, even in millivolts. The diagram does not contain any purchased parts - all this can be pulled out of old equipment, both imported and Soviet.

Schematic diagram of power supply unit (reduced)

The case is made of wood, in the middle there is a 12 volt transformer, a 1000 uF x 25 volt capacitor and a board that regulates the voltage.

Capacitor C2 must be taken with a large capacity, for example, to connect an amplifier to the power supply and so that the voltage does not drop to low frequencies.

It is better to install transistor VT2 on a small radiator. Because during prolonged operation it can heat up and burn out; I already burned out 2 of them until I installed a decent-sized radiator.

Resistor R1 can be set constant; it does not play a big role. On top of the case there is a variable resistor that regulates the voltage, and a red LED that shows whether there is voltage at the power supply output.

At the output of the device, in order not to constantly screw the wires to something, I soldered alligator clips - they are very convenient. The circuit does not require any settings and works reliably and stably; any radio amateur can really do it. Thank you for your attention, good luck everyone! .

At 1-2 amperes, but it is already problematic to obtain a higher current. Here we will describe a high-power power supply with a standard voltage of 13.8 (12) volts. The circuit is 10 amperes, but this value can be increased further. There is nothing special in the circuit of the proposed power supply, except that, as tests have shown, it is capable of delivering a current of up to 20 Amps for a short time or 10A continuously. To further increase power, use a larger transformer, diode bridge rectifier, higher capacitor capacity and number of transistors. For convenience, the power supply circuit is shown in several figures. The transistors do not have to be exactly the ones in the circuit. We used 2N3771 (50V, 20A, 200W) because there are many of them in stock.