Latr (laboratory autotransformer). Schemes and step-by-step instructions on how to make an autotransformer with your own hands Latr from an old autotransformer diagram

A transformer that has an electrical connection between the windings is called a laboratory autotransformer, or LATR. The load circuit voltage is directly proportional to the secondary circuit winding. Depending on the design, obtaining the desired output voltage is done by connecting to the appropriate terminals or rotating a manual regulator (Fig. 1). This article describes how to make LATR at home.

Preparation of material

To assemble the LATR you will need the following materials and devices:

• Copper winding;
• Toroidal or rod magnetic circuit. Can be purchased at a specialized store or removed from damaged equipment;
• Heat-resistant varnish;
• Rag tape;
• Housing with fixed connectors for connecting load and power.

For a laboratory LATR with a variable transformation ratio, you may additionally need:

1. Digital or analog voltmeter.
2. Rotary mechanism, including a handle and a slider with a carbon brush. It will regulate the voltage.

Wire calculation

It is not advisable to use an autotransformer for large transformations for the following reasons:

• There is a high risk of receiving currents close to a short circuit. This is compensated by special electronic circuits or additional resistance. For small loads it is more profitable to use an electronic LATR.
• The advantages over transformers are lost: high efficiency, saving on conductor and steel, small dimensions and weight, cost.

We are determining within what limits the LATR will operate. We select 220 V for the network supply. We select 127, 180 and 250 V as secondary voltages. We limit the power to 300 W. You can choose your own values ​​and make similar calculations using the example of this article.

The winding is calculated based on the larger current. The highest current will be when converting a voltage of 220 to 127 V. The autotransformer in this case is a step-down one, and circuit 1 is suitable for it. Based on the provided circuit, we calculate the maximum current I passing in the winding of both circuits:

I = I2 – I1 = P / U2 – P / U1 = 300 / 127 – 300 / 220 = 1 A

• where I, I2, I3 – currents in the corresponding sections of the circuit, A;
• P – power, W;
• U1, U2 – primary and secondary circuit voltages, V.

The wire diameter is calculated using the formula:

d = 0.8 * √I = 1 mm.

From Table 1, select the wire type and cross-section. We make the choice taking into account the calculated current and the average current density for transformers - 2 A/mm².

The LATR transformation coefficient n is calculated using the formula:

n = U1 / U2 = 220 / 127 = 1.73

For further calculation, we calculate the design power Pр:

Pр = P * k * (1 – 1/n) = 300 * 1.2 * (1 – 1/1.73) = 151.92 W

where k is a coefficient that takes into account the efficiency of the autotransformer.

To determine the number of turns per 1 volt, it is necessary to calculate the cross-sectional area of ​​the core S and determine the type of magnetic circuit:

S = √ Pр = √ 151.92 = 12.325 cm²

W0 = m / S = 35 / 12.325 = 2.839

• where W0 is the number of turns per 1 volt;
• m – 50 for rod and 35 for toroidal magnetic cores.

If the steel is not of very high quality, it is worth increasing the W0 value by 20-30%. Also, when calculating turns, their number should be increased by 5-10% to avoid voltage drop. We calculate the number of turns for selected voltages 127, 180, 220 and 250 V:

w = W0 * U

We get 360, 511, 624 and 710 turns.

To calculate the length of the wire, we wrap one turn around the magnetic circuit and measure its length. Then we multiply by the maximum number of turns and add 25-30 centimeters for each terminal to the terminal.

Build process

To assemble an adjustable LATR, we select a toroidal magnetic core (Fig. 2). We insulate the place where the winding is applied with rag tape. We bring out the wire for the first power terminal. We bring out all subsequent wires without breaking them. We fix the first turn on the magnetic core and begin to wind the calculated amount. When a turn corresponding to one of the selected voltages is reached, we remove the loop and continue winding the wire. Figure 3 shows the winding process on a wooden frame.

After applying the winding, we varnish LATR. We fill the container with the selected varnish and dip the autotransformer into it. Leave to dry for a long time.

After drying, place the autotransformer in the housing. We connect the first output wire to the power connector. This connector must be electrically connected to the common load terminal, so we connect them together with some kind of conductor. We connect the loop output for 220 V to the second power terminal. We connect the remaining wires to the corresponding terminals of the secondary circuit. “Scheme” 2 shows the wire terminals.

For a laboratory autotransformer with a variable transformation ratio, we add a housing and make a mount for the regulator handle. We attach a slider with a carbon brush to the handle. The brush should touch the top of the winding tightly. We mark the area over which the brush will move, and in this place we get rid of the insulation. This way the brush will have direct electrical contact with the secondary winding. We replace the secondary voltage terminals, in addition to the common one, with one connected to a carbon brush (diagram 3). When connecting, secure the voltmeter.

If you follow the written article, you can easily make LATR with your own hands.

Examination

To ensure the smooth and reliable operation of the device, we perform the following points:

1. We connect the autotransformer to a 220 V network;
2. We check for the absence of smoke, burning smell, strong noise;
3. We use a voltmeter to check the compliance of the output values;
4. After 10 - 20 minutes of operation, turn off the LATR. Check to see if the winding is overheated.
5. We turn the LATR back into the network and connect the load for a long time.

If there are no problems, the autotransformer is ready for operation.

Transformer devices ensure the normal functioning of various electrical equipment. A laboratory autotransformer (LATR) performs the functions of a kind of power supply for AC mains voltage. What is LATR, what are its features and the basic principle of operation will be discussed further.

Peculiarities

Considering what LATR is, it should be noted that this is a type of autotransformer. It is characterized by low power and does not require a state register. The operating principle of the laboratory regulating autotransformer is to adjust the AC voltage single-phase(on the left in the photo) or three-phase networks (right).

The LATR circuit includes a toroidal steel core. There is only one contour on it. This device does not have two separate windings. The contours are combined. One part can be classified as coils of the primary type, and the other - as coils of the secondary type. The LATR regulating autotransformer has enough simple diagram. The user can independently adjust the number of turns of the secondary winding. This distinguishes the presented type of units from other transformers. We wrote about how to assemble an LATR with your own hands.

Design

It is possible to regulate the presented unit through the presence of a rotary knob in the design. With its help, the number of turns of the secondary circuit is set. The handle is connected to the carbon brush. Adjustable autotransformers allow you to control the windings after turning on the equipment. In this case, the brush, according to the instructions, slides along the contour, setting the transformation rate.

One of the outputs of the secondary winding is connected to the carbon brush. Its other end is connected to the input side of the network. Consumers are connected to the output terminals, and they, in turn, are connected to the mains. This makes the use of the equipment efficient and convenient.

A voltmeter is installed on the front panel of the device. He takes readings from the secondary circuit. This allows you to quickly respond to overloads. The voltmeter allows you to make precise adjustments.

There is a ventilation grill on the body. This ensures natural cooling of the magnetic drive.

Varieties

There is equipment designed to regulate the voltage of three-phase or single-phase network. In the second version, the electronic LATR has one winding and one core. A three-phase unit includes three cores in its design. Each of them has one winding.

LATRs can both lower and increase voltage. This is their main feature. Single-phase varieties create a network voltage from 0 to 250 V. Three-phase LATR (380 V in the network) can regulate the range from 0 to 450 V.

It should be noted that the efficiency of both types of devices is high. It reaches 99%. This creates a sinusoidal output voltage.

Application

LATRs are used in research centers and laboratories for testing AC equipment. Sometimes such devices are necessary to stabilize the mains voltage. For example, when its level in the network is insufficient at the moment.

However, its scope of application is limited. If there are constant fluctuations and surges in the network, the use of an autotransformer will be pointless. In this case, you will need to install a stabilizer. The main purpose of LATR is to fine-tune the voltage to perform various research tasks and tests.

Such equipment may be required in the process of setting up industrial devices, highly sensitive equipment, and radio electronics. They provide proper nutrition equipment operating at low voltage. They are also used when charging batteries.

Having considered the main features of laboratory autotransformers, you can correctly use the unit for various purposes, increasing the efficiency and ease of setting up various equipment.

Half a century ago, the laboratory autotransformer was very common. Today, the electronic LATR, the circuit of which every radio amateur should have, has many modifications. Old models had a current-collecting contact located on the secondary winding, which made it possible to smoothly change the value of the output voltage, made it possible to quickly change the voltage when connecting various laboratory instruments, changing the heating intensity of the soldering iron tip, adjusting electric lighting, changing the speed of the electric motor and much more. LATR is of particular importance as a voltage stabilization device, which is very important when setting up various devices.

Modern LATR is used in almost every home to stabilize voltage.

Today, when electronic consumer goods have flooded store shelves, purchasing a reliable voltage regulator has become a problem for a simple radio amateur. Of course, you can also find an industrial design. But they are often too expensive and bulky, and this is not always suitable for home use. So many radio amateurs have to “reinvent the wheel” by creating an electronic LATR with their own hands.

Simple voltage regulation device

One of the simplest LATR models, the diagram of which is shown in Fig. 1, is also accessible to beginners. The voltage regulated by the device is from 0 to 220 volts. The power of this model is from 25 to 500 W. The regulator power can be increased to 1.5 kW; for this, thyristors VD1 and VD2 should be installed on radiators.

These thyristors (VD1 and VD2) are connected in parallel with the load R1. They pass current in opposite directions. When the device is connected to the network, these thyristors are closed, and capacitors C1 and C2 are charged through resistor R5. The magnitude of the voltage received at the load is changed as necessary using a variable resistor R5. It, together with capacitors (C1 and C2), creates a phase-shifting circuit.

Rice. 2. Scheme of LATR, which provides sinusoidal voltage without interference in the system.

A feature of this technical solution is the use of both half-cycles of alternating current, so the load uses not half power, but full power.

The disadvantage of this circuit (the price to pay for simplicity) is that the shape of the alternating voltage at the load is not strictly sinusoidal, which is due to the specific operation of thyristors. This may cause interference on the network. To eliminate the problem, in addition to the circuit, you can install filters in series with the load (chokes), for example, take them from a faulty TV.

The main reason for creating an electronic LATR with your own hands is the excess of unreliable regulators on the electrical goods market. A way out of the situation may be an industrial-type sample, but such specimens are expensive and have impressive dimensions, which makes it difficult to use at home.

What is the device

It is worth mentioning that laboratory autotransformers (LATR) were widely used half a century ago. Previous versions of the device had a current-collecting contact, which was located on the secondary winding. This made it possible to smoothly change the output voltage (its value).

If all kinds of laboratory instruments were connected, there was an option to quickly change the voltage. For example, if necessary, it was easy to influence the degree of heating of the soldering iron, adjust the brightness of the lighting, the speed of the electric motor, and much more. This is a kind of regulating power supply.

Figure 1. Scheme of a simple version of LATR.

The current version of LATR has various modifications. In general, it can be considered a transformer in which an alternating voltage of one value is transformed into an alternating voltage of another. The device is widely used as a voltage stabilizer. The main feature is the ability to change the voltage at the output of the device. LATRs come in several versions:

• single-phase;
• three-phase.

The three-phase version consists of three single-phase laboratory autotransformers mounted in a single housing. By the way, there are significantly fewer people who want to become the owner of a three-phase option.

A simple device for regulation

There is a very simple version of LATR, which is available even for beginners; its diagram is shown in Fig. 1. The voltage range regulated by such a device is within 0-220 volts. This homemade regulator has a power of 25-500 W. The power of the device can be increased by installing thyristors VD1 and VD2 on radiators.

Semiconductor devices (we are talking about thyristors VD1 and VD2) should be connected in parallel with the load R1. The current they pass has opposite directions. When the device is turned on, the thyristors remain closed, unlike capacitors C1 and C2, which are charged by resistor R5. If there is a need, using resistor R5 you can change the voltage that is obtained during load. The resistor and capacitors create a phase-shifting circuit.

Figure 2. LATR with a bipolar transistor.

A phase-shifting circuit is an electrical four-port network, the harmonic signal at the output of which is shifted in phase relative to the input signal. They are common in self-propelled guns as adjustment devices that provide stability and the necessary quality of control. Special cases are differentiating and integrating chains.

This technical solution allows the load to be used not at half power, but at full power. This is achieved due to the fact that both half-cycles of alternating current are used.

The disadvantages include the form of alternating voltage at the load. In this version it is not strictly sinusoidal. The specific operation of semiconductor devices is the main reason. The presence of such a feature can cause interference in the network. But they can be eliminated by additionally installing chokes (series load filters) on the circuit. Such filters can be found even in a faulty TV.

Voltage regulator: version with transformer

A laboratory autotransformer, which will not cause interference in the network and is capable of producing a sinusoidal voltage at the output, is a little more complicated than the previous one.

Its circuit (Fig. 2) contains a bipolar transistor VT1. It acts as a regulatory element in such a device. The power of this transistor is determined depending on the required load. In the circuit, it is connected in series with the load and functions as a rheostat. This option provides the ability to adjust the operating voltage during both active and reactive loads.

Unfortunately, there is a drawback here too. It lies in the fact that the activated control transistor generates too much heat. To eliminate it, you will need a heat sink that will have sufficient power. In this case, the area of ​​such a radiator must be at least 250 cm².

This model uses transformer T1, which must have a power of 12 to 15 W and a secondary voltage of 6 to 10 V. The current is rectified using a VD6 diode bridge. The rectified current to transistor VT1 in any half-cycle version passes through the bridge of diodes VD2 and VD5. To adjust the base current of transistor VT1, you must use the variable resistor R1. Thus, the load current parameters change.

Using the PV1 voltmeter, the voltage value at the output of the device is monitored. The voltmeter is taken with the expectation of a voltage from 250 to 300 V. If there is a need to increase the load power, you should replace the transistor VD1 and diodes VD2-VD5 with more powerful ones. This, of course, will be followed by an increase in the radiator area.

As you can see, self-assembly LATRA is possible, you just need to have knowledge in this area and acquire the necessary materials.

LATR - laboratory adjustable autotransformer - one of the types of autotransformers, which is an autotransformer of relatively low power, and is designed to regulate alternating voltage (alternating current) supplied to the load from a single-phase or three-phase alternating current network.

The LATR, like any other network transformer, is based on an electrical steel core. But on the toroidal core of the LATR, unlike other types of network transformers, there is only one winding (primary), part of which can act as a secondary winding, and the number of turns of the secondary winding can be quickly adjusted by the user, this is what distinctive feature LATRA from simple autotransformers.

To regulate the number of turns per secondary winding, the design of the autotransformer includes a rotary knob, to which a sliding carbon brush is connected. When you turn the handle, the brush slides from turn to turn along the winding, this is how it is adjusted.

One of the secondary terminals of the laboratory autotransformer is directly connected to the sliding brush. The second secondary pin is common to the network input side. Consumers are connected to the output terminals of the LATR, and its input terminals are connected to a single-phase or three-phase electrical network. In a single-phase LATR there is one core and one winding, and in a three-phase one there are three cores, and each has one winding.

The voltage at the output of the LATR can be either greater than the input or less, for example, for a single-phase network, the adjustable range is from 0 to 250 volts, and for a three-phase network, from 0 to 450 volts. It is noteworthy that the efficiency of LATR is higher, the closer the output voltage is to the input voltage, and can reach 99%. Output voltage form - .

On the front panel of the LATR there is a secondary circuit voltmeter for the ability to quickly monitor overload and more accurately set the output voltage. The LATR housing has ventilation holes through which natural air cooling of the magnetic core and winding occurs.

Laboratory autotransformers are used in laboratories for research purposes, for testing AC equipment, and simply for manual stabilization of the network voltage if it is currently below the required nominal value.

Of course, if the voltage in the network constantly fluctuates, then an autotransformer will not save you; you will need a full-fledged stabilizer. In other cases, LATR is just what you need to fine-tune the voltage for the task at hand. Such tasks may be: setting up industrial equipment, testing highly sensitive equipment, setting up radio-electronic devices, powering low-voltage equipment, charging batteries, etc.

Since LATR has only one winding, common to the primary and secondary circuits, the current of the secondary winding turns out to be common to the primary and secondary circuits. From this point of view, it is obvious that the secondary winding current and the primary current in the common turns are oppositely directed, therefore the total current is equal to the difference between the currents I1 and I2, that is, I2 – I1 = I12 is the current in the common turns. So it turns out that when the value of the secondary voltage is close to the input voltage, the common turns can be wound with a wire of a smaller cross-section than in the case of manufacturing a two-winding transformer.

The design feature of the LATR forces us to separate the concepts of “throughput power” and “design power”. The rated power is that which is transferred from the primary winding to the secondary circuit by means of electromagnetic induction through the core, like a conventional two-winding transformer, and the throughput power is the sum of the throughput power and the power that is transmitted only through the electrical component, that is, without the participation of magnetic induction in the core.

It turns out that in addition to the calculated power, purely electrical power equal to U2*I1 is also transferred to the secondary circuit. This is why autotransformers require a smaller magnetic core to transmit the same power compared to conventional two-winding transformers. This is the reason for the higher efficiency of autotransformers. In addition, less copper is required for the wire.

So, with a small transformation ratio, LATR can boast the following advantages: efficiency up to 99.8%, smaller magnetic core size, lower material consumption. And all this is due to the presence of an electrical connection between the primary and secondary circuits. On the other hand, the absence between the circuits leads to the danger of phase current from the output terminals of the LATR and even from one of the terminals, so you need to be extremely careful when working with a laboratory autotransformer.