You need to know how to lower the voltage in the circuit so as not to damage the electrical appliances. Everyone knows that two wires are suitable for houses - zero and phase. This is called a single phase network. Three-phase is extremely rarely used in the private sector and apartment buildings. There is simply no need for it, since all household appliances are powered by a single-phase alternating current network. But in the technique itself, it is required to make transformations - to lower the alternating voltage, convert it to constant, change the amplitude and other characteristics. It is these points that need to be considered.
Voltage reduction with transformers
The easiest way is to use a low voltage transformer that performs the conversion. The primary winding contains a greater number of turns than the secondary. If there is a need to reduce the voltage by half or three, the secondary winding may not be used. The primary winding of the transformer is used as an inductive divider (if there are taps from it). In household appliances, transformers are used, from the secondary windings of which a voltage of 5, 12 or 24 Volts is removed.
These are the most commonly used values in modern household appliances. 20-30 years ago, most of the equipment was powered by a voltage of 9 volts. And tube TVs and amplifiers required a constant voltage of 150-250 V and an alternating voltage of 6.3 for incandescent filaments (some lamps were powered from 12.6 V). Therefore, the secondary winding of the transformers contained the same number of turns as the primary. In modern technology, inverter power supplies (like on computer PSUs) are increasingly used, their design includes a step-up transformer, it has very small dimensions.
Inductor Voltage Divider
Inductance is a coil wound with a copper (usually) wire on a metal or ferromagnetic core. A transformer is a type of inductance. If a tap is made from the middle of the primary winding, then there will be equal voltage between it and the terminal leads. And it will be equal to half the supply voltage. But this is the case if the transformer itself is designed to work with just such a supply voltage.
But you can use several coils (for example, you can take two), connect them in series and connect to the AC network. Knowing the values of inductances, it is easy to calculate the drop on each of them:
- U (L1) = U1 * (L1 / (L1 + L2)).
- U (L2) = U1 * (L2 / (L1 + L2)).
In these formulas, L1 and L2 are the inductances of the first and second coils, U1 is the supply voltage in Volts, U (L1) and U (L2) are the voltage drop at the first and second inductances, respectively. The circuit of such a divider is widely used in circuits of measuring devices.
Capacitor divider
A very popular circuit used to reduce the value of the AC mains. It cannot be used in DC circuits, since the capacitor, according to Kirchhoff’s theorem, in a DC circuit is a gap. In other words, no current will flow through it. But on the other hand, when working in an alternating current circuit, the capacitor has a reactance that is able to suppress the voltage. The divider circuit is similar to the one described above, but capacitors are used instead of inductances. The calculation is carried out according to the following formulas:
- Capacitor reactance: X (C) = 1 / (2 * 3.14 * f * C).
- Voltage drop on C1: U (C1) = (C2 * U) / (C1 + C2).
- Voltage drop on C2: U (C1) = (C1 * U) / (C1 + C2).
Here C1 and C2 are capacitors, U is the voltage in the supply network, f is the current frequency.
Resistor Divider
The circuit is much similar to the previous ones, but constant resistors are used. The calculation method of such a divider is slightly different from the above. The circuit can be used both in AC and DC circuits. We can say that it is universal. With its help, you can assemble a step-down voltage converter. The calculation of the fall on each resistor is made according to the following formulas:
- U (R1) = (R1 * U) / (R1 + R2).
- U (R2) = (R2 * U) / (R1 + R2).
One nuance should be noted: the value of the load resistance should be 1-2 orders of magnitude smaller than that of dividing resistors. Otherwise, the accuracy of the calculation will be very rough.
Power Supply Practical Circuit: Transformer
To select a power transformer, you need to know a few basic data:
- The power of consumers to be connected.
- Supply voltage value.
- The value of the required voltage in the secondary winding.
To calculate the number of turns in the primary winding, you need 50 divided by the cross-sectional area of the core. The cross section is calculated by the formula:
S = 1.2 * √P1.
And power P1 = P2 / efficiency. The efficiency of the transformer will never be more than 0.8 (or 80%). Therefore, when calculating the maximum value is taken - 0.8.
Power in the secondary winding:
P2 = U2 * I2.
These data are known by default, so it will not be difficult to calculate. Here's how to lower the voltage to 12 volts using a transformer. But this is not all: household appliances are powered by direct current, and at the output of the secondary winding - alternating. It will take a few more transformations.
Power supply circuit: rectifier and filter
Next is the conversion of AC to DC. For this, semiconductor diodes or assemblies are used. The simplest type of rectifier consists of a single diode. It is called half-wave. But the bridge circuit, which allows not only to rectify alternating current, but also to get rid of ripples as much as possible, has gained maximum popularity. But such a converter circuit is still incomplete, since the semiconductor diodes alone cannot get rid of the variable component. And 220 V step-down transformers are capable of converting an alternating voltage into the same in frequency, but with a lower value.
Electrolytic capacitors are used in power supplies as filters. By Kirchhoff’s theorem, such a capacitor in an alternating current circuit is a conductor, and when working with a constant circuit, it is a gap. Therefore, the constant component will flow unhindered, and the variable will close itself, therefore, will not go beyond this filter. Simplicity and reliability are exactly what characterizes such filters. Resistors and inductances can also be used to smooth out ripples. Similar designs are used even in automotive alternators.
Voltage stabilization
You have learned how to lower the voltage to the desired level. Now it needs to be stabilized. For this, special devices are used - zener diodes, which are made of semiconductor components. They are installed at the output of the DC power supply. The principle of operation is that the semiconductor is able to pass a certain voltage, the excess is converted into heat and given through the radiator to the atmosphere. In other words, if the output of the PSU is 15 volts, and the stabilizer is installed at 12 V, then it will miss as much as you need. And the difference of 3 V will be used to heat the element (the law of conservation of energy is valid).
Conclusion
A completely different design is a step-down voltage regulator, it makes several transformations. First, the mains voltage is converted to constant with a high frequency (up to 50,000 Hz). It is stabilized and fed to a pulse transformer. Next, the inverse conversion occurs to the operating voltage (mains or lower in value). Thanks to the use of electronic keys (thyristors), the constant voltage is converted into alternating voltage with the required frequency (in the networks of our country - 50 Hz).