Such transformer calculation method Of course, it’s very approximate, but it’s quite suitable for amateur radio practice.
In addition, all of the following calculations are relevant only for transformers with an Ш-shaped core and for operation with an industrial frequency current of 50 Hz.
So, let's begin....
Problem: you need a transformer with an output voltage of 12V and a current on the secondary winding of at least 1A. (if there are several windings, the currents add up).
The power of the secondary winding is 12V* 1A =12W.
Since the efficiency of transformers is approximately 85%, the power taken by the primary during operation will be approximately 1.2 times higher and the result will be 12W * 1.2 = 14.4W.
Where S - core area, P1 - primary winding power.
it will be 4.93 sq. cm. (well, in general, round up to 5....)
This is the required minimum core area. If you can use more, that's even better.
Here:
W is the number of turns,
Ktr - transformation coefficient,
Sc - cross-sectional area of the core.
Since we decided to take Ktr = 50, we calculate:
W1= 50/5 * 220 = 2200
W2= 50/5 * 12 = 120
where I is the current flowing through the winding.
Oh, yes.... we don’t yet know the current that the primary will consume....
Well, that’s not a problem either: we know the voltage, the power too, it turns out:
I1= P1/U1 = 14.4/220 = 0.065A.
So:
The diameter of the wire for the primary will be:
D1 = 0.7 * root of 0.065 = 0.18 mm.
For the secondary winding:
D2 = 0.7 * root of 1 = 0.7 mm.
That's the whole calculation!
The most common magnetic cores are the following types:
The appearance and main dimensions of the cores are shown in the figure:
The toroidal magnetic core transformer is the most compact and efficient, can be used at powers from 30 to 1000 W, and especially when minimal magnetic flux dissipation is important or when the minimum volume requirement is paramount.
Having advantages in volume, weight and characteristics over other types of transformer designs, toroidal transformers are also the least technologically advanced to manufacture.
Secondary power
P2 = I2 * U2 = Pout
Before moving on to the formula, we will make a few reservations:
P = 1.9 * Sc * So
P = 40 * U / Sc
Where:
Sc is the cross-sectional area of the core [sq. cm];
U - primary winding voltage [V];
The number of turns in the secondary winding can be calculated using the same formula, increasing the number of turns by about 5% (transformer efficiency), but you can do it simpler: after the primary is wound, we wind 10 turns on top of it and measure the voltage. Knowing what voltage is required to be obtained at the output of the transformer and knowing what voltage is per 10 turns, we determine the required number of turns.
I’ll immediately make a reservation that I will consider single-phase transformers for powering ground-based stationary radio equipment with a power of tens to hundreds of watts, which has the most common application.
Before you start calculating a transformer, of which there can be a great variety, it is necessary to agree on the criteria for its quality, which will certainly affect the construction of calculation formulas. I believe that the main quality indicator of a power transformer for radio equipment is its reliability. A consequence of reliability is the minimum heating of the transformer during operation (in other words, it must always be cold!) and the minimum drop in output voltages under load (in other words, the transformer must be “hard”).
Other optimization criteria besides reliability, such as copper savings, minimal dimensions or weight, high power density, ease of winding, cost minimization, limited service life (so that new ones are bought more often to replace burnt ones) I do not consider acceptable in engineering practice. I also consider methods of “knocking out” the maximum power core from an existing standard size unacceptable. - Such transformers do not work for a long time and get hot as hell.
If you want to save money, buy cheap Chinese goods or Soviet consumer goods. But remember: “The miser always pays twice!”
The transformer should work and not create problems. This is its main function.
Based on this, we will calculate it!
First of all, it is necessary to understand some minimal theory.
So: power transformer. Not ideal. Therefore, these imperfections need to be understood and taken into account correctly. There are two main imperfections in a power transformer.
1. Losses on the active resistance of the wire of the windings (depend on the material of the wire and on the density of the current flowing through it).
2. Magnetization reversal losses in the core - on a certain “magnetic resistance” (depending on the core material and the value of magnetic induction).
It is these two imperfections that must be reasonably minimal in order for the transformer to satisfy reliability requirements.
The active resistance of the windings and, as a consequence, their heating, is determined by the current density in the wire included in the calculation. Therefore, its value should be optimal. Based on extensive practical experience, I recommend using a current density value in a copper wire of no more than 3.2 amperes per square millimeter of cross-section. When using silver wire, the current density can be increased to 3.5 amperes per square millimeter. But for an aluminum wire it should not exceed 2 amperes per square millimeter. The specified current density values must absolutely not be exceeded! And from these values we will derive formulas for determining the diameter of the winding wire, which we will use in the calculation.
It is possible to wind the windings with a thicker wire (at a lower current density). More subtle - absolutely not! However, it is not worth winding the windings with thicker wire, since then we risk not fitting the required number of turns into the core window. And a good transformer should have many turns in order to minimize magnetic losses and so that its core does not heat up.
Most cold-rolled electrical steels retain their linearity up to a magnetic induction value of 1.35 Tesla or 13,500 Gauss. But we must not forget that the voltage in an electrical outlet can vary from 198 to 242 volts, which corresponds to a normalized 10 percent deviation from the nominal value, both positive and negative. That is, if we want our transformer to operate reliably over the entire range of supply voltages, we must design it so that the core does not approach nonlinearity at any permissible supply voltage. Including at 242 volts. And therefore, at a nominal voltage of 220 volts, the magnetic induction should be selected no more than 1.2 Tesla or 12,000 Gauss.
Compliance with these two specified requirements will ensure high efficiency of the transformer and high stability of output voltages when the load current changes from zero to the maximum value. In other words, we will get a very “hard” transformer. That's what you need! But an increase in the calculated induction value of more than 1.2 Tesla will lead not only to heating of the core, but also to a decrease in the “rigidity” of the transformer. If we calculate the transformer for an induction value of more than 1.3 Tesla, then we will get a “soft” transformer, the output voltages of which smoothly drop as the load current increases from zero to its rated value. Such transformers are not suitable for all radio devices. However, in transistor circuits you can successfully use a rectified voltage stabilizer. But this is additional circuitry, additional dimensions, additional power dissipation, additional money and additional unreliability. Isn't it better to make a good transformer right away?
In a soft supply transformer, the voltage on some secondary windings depends on the current consumed in others - due to drawdown in the common circuits - on the active resistance of the primary winding and on the magnetic resistance. For example, if we power a push-pull tube amplifier operating in class B or AB mode from a soft transformer, then a change in consumption along the anode circuit will lead to additional fluctuations in the filament voltage of the lamps. And, since the filament voltage of the lamps also has a permissible spread of 10% of the nominal value, a soft transformer will introduce additional instability into this voltage of another 10, or even 15 percent. And this is inevitable, first it will reduce the output power of the amplifier at high volumes (inertial volume drops), and over time it will lead to an earlier loss of emission from the lamps.
Savings on a power transformer are reflected in more expensive losses in radio tubes and in the parameters of radio devices. This is truly true: “Saving is the path to ruin and poverty!”
Currently, the most common magnetic cores are of the following configurations:
We will carry out further calculations of the transformer using strict classical formulas from an electrical engineering textbook:
1. Subject to the agreements reached, the efficiency of the transformer (at the most common powers of 80 - 200 W) will be no lower than 95 percent, or even higher. Therefore, in the formulas we will use the efficiency value = 0.95.
2. The fill factor of the core window with copper for toroidal transformers is 0.35. For conventional frame armor or rod armor - 0.45. With wide frames and a large winding length of one layer (h), the value of Km can reach 0.5 ... 0.55, as, for example, in magnetic cores of type B69 and B35, the parameters of which are shown in the figure. With frameless industrial winding, Km can have values of up to 0.6 ... 0.65. For reference: the theoretical limit of the Km value for layer placement of a round wire without insulation in a square window is 0.87.
The given practical values of Km are achievable only with even laying of the wire strictly turn to turn, thin interlayer and inter-winding insulation and termination of the terminals outside the core window (on the side extensions of the winding). When making frame windings under amateur conditions, in laboratory or pilot production conditions, it is better to take the value Km = 0.45 ... 0.5.
Of course, all this applies to conventional power transformers for lamp or transistor equipment, with output and supply voltages up to 1000 volts, where increased insulation requirements are not imposed on the windings and the termination of their terminals.
3. The overall power of the transformer, in watts, on a specifically selected core is determined by the formula:
Where:
η
= 0.95 - transformer efficiency;
Sc And So- cross-sectional area of the core and window, respectively [sq. cm];
f- lower operating frequency of the transformer [Hz];
B= 1.2 - magnetic induction [T];
j- current density in the winding wire;
Km- coefficient of filling of the core window with copper;
Kc= 0.96 - coefficient of filling of the core section with steel;
4. Having specified the winding voltages, the number of required turns can be calculated using the following formula:
Where:
U 1, U 2, U 3, ... - winding voltages in volts, and n 1, n 2, n 3, ... - number of turns of windings.
If we have strictly followed the initial agreements, and we are making a rigid transformer, then the number of turns of both the primary and secondary windings is determined by the same formula. If we use a transformer at the maximum power value for the existing core size, calculated using this formula, or we design low-power transformers (less than 50 W), with a large number of turns and thin wire windings, then the number of turns of the secondary windings should be increased by 1/√η once. Taking into account our agreement, this will be 1.026 or more than calculated by 2.6%.
As for the voltages of the filament windings, here it is worth recalling the instructions of the most important book on radio tubes: , issued for radio development engineers by the State Committee on Electronic Technology of the USSR in 1964.
You need to open this manual on page 13, carefully examine the graph in Figure 1, and understand from it that the optimal filament voltage of radio tubes to maintain their maximum reliability and, accordingly, durability is 95% of the nominal value. Which for lamps with a filament voltage of 6.3 volts will be exactly 6 volts. Therefore, there is no need to increase the number of turns of filament windings by 2.6%. Let it be as it is.
5. Determine the winding currents:
Primary current: I 1 = P / U 1
When using a full-wave rectifier, the average current of each half of the winding will be 1.41 times (root of two) less than the required rectified load current. If a bridge semiconductor rectifier is used, the winding current will be 1.41 times greater than the rectified load current. Therefore, we must not forget to substitute direct current consumption into the formulas for determining the diameters of wires, in the first case divided, and in the second, multiplied by 1.41.
6. We calculate the diameters of the winding wires based on the currents flowing in them using the following formulas (for copper, silver or aluminum):
We round the resulting values upward to the nearest standard wire diameter.
7. We check the calculation. The power of the primary winding - the product of the supply voltage and the consumed current - must be equal to the sum of the powers of all secondary windings. That is: U 1 x I 1 = U 2 x I 2 + U 3 x I 3 + U 4 x I 4 + ...
Having wound the transformer, to carry out further calculations of the rectifier it is necessary to measure some of its parameters.
Active resistance of the primary winding.
Active resistance of secondary windings.
The exact values of the voltages of the secondary windings, of course, check that the voltage in the network is 220 volts. If it differs from the nominal value (but is within the range of 198 - 242), then recalculate the measured values proportionally.
No-load current of the primary winding (what current the transformer consumes from the network when there is no load on its secondary windings).
Eg,
A toroidal two-winding power transformer with a power of 530 Watts, which I myself manually wound in 1982 on a core from a burned-out household transitional 400-watt 127/220 volt autotransformer, called in the Yug-400 retail chain, had the following parameters:
Magnetic induction at a voltage of 220 volts - 1.2 Tesla,
The number of turns of the primary winding (220 volts) is 1100.
The diameter of the primary winding wire is 0.96 mm.
The number of turns of the secondary winding (127 volts) is 635.
The diameter of the secondary winding wire is 1.35 mm.
At the same time, the no-load current turned out to be 7 (seven!) milliamps.
For eighteen years, without turning off, my “bachelor” refrigerator “Saratov-II” (the same one during operation with which the autotransformer “Yug” burned out) was powered through this transformer after our area was transferred to a network voltage of 220 volts.
For comparison.
The “native”, industrial, winding of that same 220-volt “South” transformer contained 880 turns. Not surprisingly, it heated itself like a bastard, even being only an autotransformer, and eventually burned out. Yes, this is understandable, because the Soviet household industry was interested in increasing consumer demand. Well, this was achieved not by a wide range of products, but by a limited period of their work!
There is no need to save money - this is the same as spoiling yourself.
Good luck!
Every car enthusiast dreams of having a battery charging rectifier at his disposal. Without a doubt, this is a very necessary and convenient thing. Let's try to calculate and make a rectifier for charging a 12-volt battery.
A typical car battery has the following parameters:
Accordingly, the charge current is 0.1 of the battery capacity, or 3.5 - 6 amperes.
The rectifier circuit for charging the battery is shown in the figure.
First of all, you need to determine the parameters of the rectifier device.
The secondary winding of the rectifier for charging the battery must be designed for voltage:
U2 = Uak + Uo + Ud where:
— U2 — voltage on the secondary winding in volts;
— Uak — battery voltage is 12 volts;
— Uo — the voltage drop across the windings under load is about 1.5 volts;
— Ud — the voltage drop across the diodes under load is about 2 volts.
Total voltage: U2 = 12.0 + 1.5 + 2.0 = 15.5 volts.
Let us accept with a margin for voltage fluctuations in the network: U2 = 17 volts.
Let's take the battery charge current I2 = 5 amperes.
The maximum power in the secondary circuit will be:
P2 = I2 x U2 = 5 amps x 17 volts = 85 watts.
The power of the transformer in the primary circuit (the power that will be consumed from the network), taking into account the efficiency of the transformer, will be:
P1 = P2 / η = 85 / 0.9 = 94 watts. Where:
— P1 — power in the primary circuit;
— P2 — power in the secondary circuit;
-η = 0.9 - transformer efficiency, efficiency.
Let's take P1 = 100 watts.
Let's calculate the steel core of the Ш-shaped magnetic circuit, the transmitted power depends on the cross-sectional area.
S = 1.2√ P where:
— S cross-sectional area of the core in cm2;
— P = 100 watts power of the primary circuit of the transformer.
S = 1.2√ P = 1.2 x √100 = 1.2 x 10 = 12 cm2
The cross section of the central rod on which the frame with winding will be located S = 12 cm2.
Let us determine the number of turns per 1 volt in the primary and secondary windings using the formula:
n = 50 / S = 50 / 12 = 4.17 turns.
Let's take n = 4.2 turns per 1 volt.
Then the number of turns in the primary winding will be:
n1 = U1 · n = 220 volts · 4.2 = 924 turns.
Number of turns in the secondary winding:
n2 = U2 · n = 17 volts · 4.2 = 71.4 turns.
Let's take 72 turns.
Let's determine the current in the primary winding:
I1 = P1 / U1 = 100 watts / 220 volts = 0.45 amperes.
Current in the secondary winding:
I2 = P2 / U2 = 85 / 17 = 5 amperes.
The diameter of the wire is determined by the formula:
d = 0.8 √I.
Wire diameter in the primary winding:
d1=0.8 √I1 = 0.8 √ 0.45 = 0.8 · 0.67 = 0.54 mm.
Wire diameter in the secondary winding:
d2 = 0.8√ I2 = 0.8 5 = 0.8 2.25 = 1.8 mm.
The secondary winding is wound with taps.
The first withdrawal is made from 52 turns, then from 56 turns, from 61, from 66 and the last 72 turns.
The conclusion is made in a loop without cutting the wires. then the insulation is peeled off the loop and the outlet wire is soldered to it.
The rectifier charging current is adjusted in steps by switching taps from the secondary winding. A switch with powerful contacts is selected.
If there is no such switch, then you can use two toggle switches with three positions designed for a current of up to 10 amperes (sold in an auto store).
By switching them, you can sequentially output a voltage of 12 - 17 volts to the output of the rectifier.
Position of toggle switches for output voltages 12 - 13 - 14.5 - 16 - 17 volts.
The diodes must be designed, with a margin, for a current of 10 amperes and each must be placed on a separate radiator, and all radiators are isolated from each other. 
There can be one radiator, and the diodes are installed on it through insulated gaskets.
The area of the radiator for one diode is about 20 cm2, if there is one radiator, then its area is 80 - 100 cm2.
The charging current of the rectifier can be controlled with a built-in ammeter for a current of up to 5-8 amperes.
You can use this transformer as a step-down transformer to power a 12-volt emergency lamp from the 52-turn tap. (see diagram).
If you need to power a light bulb at 24 or 36 volts, then an additional winding is made, based on For every 1 volt there are 4.2 turns.
This additional winding is connected in series with the main one (see top diagram). It is only necessary to phase the main and additional windings (beginning - end) so that the total voltage is added up. Between points: (0 – 1) - 12 volts; (0 -2) - 24 volts; between (0 – 3) - 36 volts.
For example. For a total voltage of 24 volts, you need to add 28 turns to the main winding, and for a total voltage of 36 volts, another 48 turns of wire with a diameter of 1.0 millimeter.
A possible appearance of the rectifier housing for charging the battery is shown in the figure.
In a household, it may be necessary to equip lighting in damp areas: basement or cellar, etc. These rooms have an increased risk of electric shock.
In these cases, you should use electrical equipment designed for reduced supply voltage, no more than 42 volts.
You can use a battery-powered flashlight or use a step-down transformer from 220 volts to 36 volts.
We will calculate and manufacture a single-phase power transformer 220/36 volts, with an output voltage of 36 volts powered by an AC electrical network of 220 volts.
To illuminate such premises An electric light bulb will do just fine at 36 Volts and a power of 25 - 60 Watts. Such light bulbs with a base for an ordinary electric socket are sold in electrical goods stores.
If you find a light bulb with a different power, for example 40 watts, there is nothing to worry about - that will do. It’s just that the transformer will be made with a power reserve.
Power in the secondary circuit: P_2 = U_2 I_2 = 60 watts
Where:
P_2 – power at the output of the transformer, we set 60 watts;
U _2 - voltage at the transformer output, we set 36 volts;
I _2 - current in the secondary circuit, in the load.
The efficiency of a transformer with a power of up to 100 watts is usually no more than η = 0.8.
Efficiency determines how much of the power consumed from the network goes to the load. The remainder goes to heating the wires and core. This power is irretrievably lost.
Let's determine the power consumed by the transformer from the network, taking into account losses:
P_1 = P_2 / η = 60 / 0.8 = 75 watts.
Power is transferred from the primary winding to the secondary winding through the magnetic flux in the magnetic core. 
Therefore, from the value P_1, power consumed from a 220 volt network, depends on the cross-sectional area of the magnetic circuit S.
The magnetic core is a W-shaped or O-shaped core made from sheets of transformer steel. The core will contain the primary and secondary windings of the wire.
The cross-sectional area of the magnetic circuit is calculated by the formula:
S = 1.2 · √P_1.
Where:
S is the area in square centimeters,
P_1 is the power of the primary network in watts.
S = 1.2 · √75 = 1.2 · 8.66 = 10.4 cm².
The value of S is used to determine the number of turns w per volt using the formula:
w = 50/S
In our case, the cross-sectional area of the core is S = 10.4 cm2.
w = 50/10.4 = 4.8 turns per 1 volt.
Let's calculate the number of turns in the primary and secondary windings.
Number of turns in the primary winding at 220 volts:
W1 = U_1 · w = 220 · 4.8 = 1056 turns.
Number of turns in the secondary winding at 36 volts:
W2 = U_2 w = 36 4.8 = 172.8 turns,
round up to 173 turns.
In load mode, there may be a noticeable loss of part of the voltage across the active resistance of the secondary winding wire. Therefore, for them it is recommended to take the number of turns 5-10% more than calculated. Let's take W2 = 180 turns.
The magnitude of the current in the primary winding of the transformer:
I_1 = P_1/U_1 = 75/220 = 0.34 amperes.
Current in the secondary winding of the transformer:
I_2 = P_2/U_2 = 60/36 = 1.67 amperes.
The diameters of the wires of the primary and secondary windings are determined by the values of the currents in them based on the permissible current density, the number of amperes per 1 square millimeter of conductor area. For transformers, current density, for copper wire, 2 A/mm² is accepted.
At this current density, the diameter of the wire without insulation in millimeters is determined by the formula: d = 0.8√I.
For the primary winding, the wire diameter will be:
d_1 = 0.8 · √1_1 = 0.8 · √0.34 = 0.8 · 0.58 = 0.46 mm. Let's take 0.5 mm.
Wire diameter for secondary winding:
d_2 = 0.8 · √1_2 = 0.8 · √1.67 = 0.8 · 1.3 = 1.04 mm. Let's take 1.1 mm.
IF THERE IS NO WIRE OF THE REQUIRED DIAMETER, then you can take several thinner wires connected in parallel. Their total cross-sectional area must be no less than that corresponding to the calculated one wire.
The cross-sectional area of the wire is determined by the formula:
s = 0.8 d².
where: d - wire diameter.
For example: we could not find a wire for the secondary winding with a diameter of 1.1 mm.
The cross-sectional area of the wire is 1.1 mm in diameter. is equal to:
s = 0.8 d² = 0.8 1.1² = 0.8 1.21 = 0.97 mm².
Let's round up to 1.0 mm².
Fromwe select the diameters of two wires, the sum of their cross-sectional areas is 1.0 mm².
For example, these are two wires with a diameter of 0.8 mm. and an area of 0.5 mm².
Or two wires:
- the first with a diameter of 1.0 mm. and cross-sectional area 0.79 mm²,
- the second with a diameter of 0.5 mm. and a cross-sectional area of 0.196 mm².
which adds up to: 0.79 + 0.196 = 0.986 mm².
The coil is wound with two wires simultaneously; an equal number of turns of both wires is strictly maintained. The beginnings of these wires are connected to each other. The ends of these wires are also connected.
It turns out like one wire with the total cross-section of two wires.
See articles:Transformers are used in power supplies of various equipment to convert alternating voltage. Power supplies assembled using a transformer circuit are gradually reducing their prevalence due to the fact that modern circuitry makes it possible to lower the voltage without the most bulky and heaviest element of the power system. Transformers for power supplies are relevant in cases where dimensions and weight are not critical, but safety requirements are high. The windings (except for the autotransformer) provide galvanic separation and isolation of the primary (or mains) and secondary (output) voltage circuits.
The operation of the device is based on the well-known phenomenon of electromagnetic induction. The alternating current passing through the wire of the primary winding induces an alternating magnetic flux in the steel core, and this, in turn, causes the appearance of an induction voltage in the wire of the secondary windings.
Improvement of the transformer since its invention comes down to the choice of material and design of the core (magnetic core).
The metal for the magnetic core must have certain technical characteristics, so special iron-based alloys and special production technology have been developed.
For the manufacture of transformers, the following types of magnetic cores are most widely used:
A low-frequency power transformer, both step-down and step-up, has a core made of individual transformer iron plates. This design was chosen to minimize losses due to the formation of eddy currents in the core, which heat it and reduce the efficiency of the transformer.
Armor cores are most often made from W-shaped plates. Rod magnetic cores can be made of U-shaped, L-shaped or straight plates.
Ring magnetic cores are made of a thin strip of transformer steel, wound on a mandrel and secured with an adhesive.
Armor and rod cores can also be made from tape, and this technology is most often found in low-power devices.
Below is a method for calculating a transformer, which shows:
The calculation of a network transformer begins with determining its total power. Therefore, before calculating the transformer, you need to determine the power consumption of all, without exception, secondary windings. The core cross-section is selected according to the power. Again, efficiency also depends on power in a certain way. The higher the total power, the higher the efficiency. It is customary to focus on the following values in calculations:
The number of turns of the mains and secondary windings is calculated after selecting the magnetic core. The diameter or cross-section of the wires in each winding is determined based on the currents flowing through them.
The minimum cross-section of the core in cm2 is determined from the overall power. The overall power of a transformer is the total total power of all secondary windings, taking into account efficiency.
So, the power of the transformer can be determined, this is the total total power of all secondary windings:
By multiplying the resulting value by the efficiency, we complete the calculation of the overall power.
The area of the core rod is determined after the overall power of the transformer has been calculated from the following expression:
Knowing the cross-sectional area of the central core of the magnetic core, you can select the desired one from ready-made options.
Important! The core on which the windings will be located should, if possible, have a cross-section as close to square as possible. The cross-sectional area should be equal to or slightly larger than the calculated value.
The quality of work and manufacturability of the assembly also depends on the shape of the magnetic core. The best quality is achieved by designs made on a ring magnetic core (toroidal). They are distinguished by maximum efficiency for a given power, lowest no-load current and minimum weight. The main difficulty lies in making the windings, which at home have to be wound exclusively by hand using a shuttle.
The easiest way to make transformers is on split tape magnetic cores of the ShL (W-shaped) or PL (U-shaped) type. As an example, we can cite a powerful transformer for the power supply of an old color TV.
Old-time or modern cheap transformers are made using separate W- or U-shaped plates. The manufacturability of their windings is the same as that of split tape windings, but the difficulty lies in assembling the magnetic core. Such devices will almost always have an increased no-load current, especially if the iron used is of low quality.
The calculation of a transformer begins with determining the required number of winding turns per 1 V voltage. The found value will be the same for any windings. For your own purposes, you can use a simplified calculation method. You can calculate how many turns are needed per 1 V by substituting the cross-sectional area of the magnetic core rod in cm2 into the formula:
where k is a coefficient depending on the shape of the magnetic core and its material.
In practice, the following coefficient values are accepted with sufficient accuracy:
Large values are associated with the impossibility of densely filling the core with individual metal plates. As you can see, a toroidal transformer will have the least number of turns, hence the gain in product weight.
Knowing how many turns are needed for 1 V, you can easily find out the number of turns of each winding:
where U is the value of the open circuit voltage on the winding.
For low-power transformers (up to 50 W), the resulting number of turns of the primary winding must be increased by 5%. Thus, the voltage drop that occurs on the winding under load is compensated (in step-down transformers, the primary winding always has a greater number of turns than the secondary windings).
The diameter of the wire is calculated taking into account minimizing heating due to the flow of current. The approximate value is the current density in the windings of 3-7 A for each mm2 of wire. In practice, calculating the diameter of winding wires can be simplified using simple formulas, which gives acceptable values in most cases:
A smaller value is used to calculate the diameters of the wires of the secondary windings, since in a step-down transformer they are located closer to the surface and have better cooling.
Knowing the calculated value of the diameter of the winding wires, you need to select from those available those whose diameter is closest to the calculated one, but not less.
After determining the number of turns in all windings, it would not be superfluous to supplement the calculation of the transformer windings by checking whether the windings will fit into the window of the magnetic circuit. To do this, calculate the fill factor of the window:
For toroidal cores with internal diameter D, the formula is:
For W- and U-shaped magnetic cores, the coefficient should not exceed 0.3. If this value is greater, then it will not be possible to place the winding.
The way out of the situation would be to choose a core with a large cross-section, but this is only if the dimensions of the structure allow. As a last resort, you can reduce the number of turns in all windings simultaneously, but by no more than 5%. The no-load current will increase somewhat, and increased heating of the windings cannot be avoided, but in most cases this is not critical. You can also slightly reduce the cross-section of the wires, thereby increasing the current density in the windings.
Important! You cannot get carried away with increasing the current density, since this will cause a strong increase in heating and, as a result, a breakdown of the insulation and burnout of the windings.
The winding of the transformer winding wire is carried out on a frame made of thick cardboard or textolite, with the exception of toroidal cores, in which the winding is carried out directly onto the magnetic core, which must be carefully insulated before winding. You can use a ready-made plastic one, which is sold together with the magnetic core.
Inter-winding insulation must be laid between the individual windings. The most important thing is to well insulate the secondary winding from the primary. Transformer paper, varnished cloth, and fluoroplastic tape can be used as insulation. PTFE tape should be used with caution. Despite the highest electrical insulating qualities, a thin tape of fluoroplastic under the influence of tension or pressure (especially between the primary and secondary windings) is capable of “leaking” and exposing individual turns of the winding. Tape for sealing plumbing products suffers especially from this.
In some critical cases, during the winding process, you can impregnate the primary winding (if the transformer is a step-down transformer) with insulating varnish. Impregnating the finished device at home will have almost no effect, since the varnish will not penetrate into the depth of the winding. For these purposes, production facilities have vacuum impregnation equipment.
The winding terminals are made with pieces of flexible insulated wire for wires with a diameter of less than 0.5 mm. Thicker wire can be output directly. The soldering points of the flexible and winding wires must be additionally laid with several layers of insulation.
Note! When soldering leads, do not leave sharp ends of wires or frozen solder at the soldering site. Such places need to be carefully trimmed with side cutters.
When assembling, you need to consider the following nuances:
Note! The quality of the assembly will be better if the ends of the strip split core are varnished before assembly. Also, the finished assembled core can be varnished before final tightening.
In this case, you can achieve a significant reduction in extraneous sound.
Checking the finished transformer consists of measuring the no-load current and voltage of the windings under rated load and for heating at maximum load. All measurements of the calculated and assembled transformer should be carried out only after complete assembly, since with an loose core the no-load current can be several times higher than usual.
The no-load current varies greatly in different types of transformers and ranges from 10 mA for toroidal transformers, to 200 mA for those with an W-shaped core made of low-quality transformer iron.
A calculation of the transformer is given, which, if you have the skills, can be done in a couple of tens of minutes. For those who doubt their abilities or are afraid of making a mistake, the calculation of a power transformer can be performed using a calculation calculator that can work in both off-line and on-line modes. According to this technique, it is possible to rewind a burnt-out transformer. For a faulty transformer, the calculation is also based on the existing core and the voltage value of the secondary windings.
