Rectifiers. How and why? Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The power supply consists of several of the most important parts.

Mains transformer - on the diagram it is indicated similar to the figure,

Rectifier - its designation may be different. The rectifier consists of one, two or four diodes, depending on which rectifier. Now we will understand.

a) is a simple diode.

b) - diode bridge. Consists of four diodes included as in the figure.

c) - the same diode bridge, only for brevity it is drawn simpler. The pin assignments are the same as for the bridge under the letter b).

filter capacitor. This thing is invariable both in time and in space, it is denoted as follows:

There are many designations for the capacitor, as many as in the world of notation systems. But in general they are all the same. Let's not get confused. And for clarity, let's draw the load, denote it as Rl - load resistance. This is our scheme. We will also outline the contacts of the power source to which we will connect this load.

Next - a couple of postulates

- The output voltage is defined as Ucont = U*1.41. That is, if we have 10 volts of alternating voltage on the winding, then on the capacitor and on the load we will get 14,1V. Like that.

- Under load, the voltage sags a little, and how much depends on the design of the transformer, its power and the capacitance of the capacitor.

- Rectifier diodes should be 1,5-2 times more current than necessary. For stock. If the diode is intended for installation on a radiator (with a nut or a bolt hole), then at a current of more than 2-3A it must be placed on a radiator.

Let me also remind you what a bipolar voltage is. If someone forgot. We take two batteries and connect them in series. The middle point, that is, the connection point of the batteries, we will call the common point. Among the people, it is also known as mass, earth, body, common wire. Bourgeois call it GND (ground - earth), often referred to as 0V (zero volt). Voltmeters and oscilloscopes are connected to this wire, with respect to it, input signals are applied to the circuits and output signals are taken. That is why its name is a common wire. So, if we connect the tester with a black wire to this point and measure the voltage on the batteries, then the tester will show plus 1,5 volts on one battery, and minus 1,5 volts on the other. This voltage +/-1,5V is called bipolar. Both polarities, that is, plus and minus, must be equal. That is, +/-12, +/-36V, +/-50, etc. A sign of bipolar voltage is if three wires (plus, common, minus) go from the circuit to the power supply. But this is not always the case - if we see that the circuit is powered by +12 and -5, then such power is called two-level, but there will still be three wires to the power supply. Well, if as many as four voltages go to the circuit, for example +/-15 and +/-36, then we will simply call this power - bipolar two-level.

Well, now to the point

1. Bridge rectifier circuit

The most common scheme. Allows you to get a unipolar voltage from one winding of the transformer. The circuit has minimal voltage ripple and is simple in design.

2. Half-wave circuit

Just like the bridge, it prepares us a unipolar voltage from one winding of the transformer. The only difference is that this circuit has twice the ripple compared to the bridge, but one diode instead of four greatly simplifies the circuit. It is used at low load currents, and only with a transformer, a lot of load power, because. such a rectifier causes a one-sided remagnetization of the transformer.

3. Full-wave mid-point

Two diodes and two windings (or one winding with a mid-point) will supply us with a low ripple voltage, plus we will get less losses compared to a bridge circuit, because we have 2 diodes instead of four.

4. Bridge circuit of a bipolar rectifier

For many, this is a sore subject. We have two windings (or one with a midpoint), we remove two identical voltages from them. They will be equal, the ripples will be small, since the circuit is bridged, the voltage on each capacitor is calculated as the voltage on each winding multiplied by the root of two - everything is as usual. The wire from the midpoint of the windings equalizes the voltages on the capacitors if the positive and negative loads are different.

5. Voltage doubling circuit

These are two half-wave circuits, but with diodes connected in different ways. It is used if we need to get double the voltage. The voltage on each capacitor will be determined by our formula, and the total voltage on them will be doubled. Like the half-wave circuit, this one also has large ripples. You can see a bipolar output in it - if the middle point of the capacitors is called the ground, then it turns out as in the case of batteries, take a closer look. But a lot of power cannot be removed from such a scheme.

6. Getting a different polarity voltage from two rectifiers

It is not at all necessary that these are the same power supplies - they can be either different in voltage or different in power. For example, if our circuit consumes 12A at +1 volts, and 5A at -0,5 volts, then we need two power supplies - + 12V 1A and -5V 0,5A. You can also connect two identical rectifiers to get a bipolar voltage, for example, to power an amplifier.

7. Parallel connection of identical rectifiers

It gives us the same voltage, only with twice the current. If we connect two rectifiers, then we will have a double increase in current, three - a triple, and so on.

Well, if everything is clear to you, my dears, then I will probably give you homework. The formula for calculating the capacitance of the filter capacitor for a full-wave rectifier:

For a half-wave rectifier, the formula is slightly different:

Two in the denominator - the number of "cycles" of rectification. For a three-phase rectifier, the denominator will be three.

In all formulas, variables are called like this:

Cf - capacitance of the filter capacitor, uF

Ro - output power, W

U - output rectified voltage, V

f - AC voltage frequency, Hz

dU - pulsation range, V

For reference - allowable pulsations:

Microphone amplifiers - 0,001 ... 0,01%

Digital technology - ripple 0,1...1%

Power amplifiers - the ripple of the loaded power supply is 1 ... 10%, depending on the quality of the amplifier.

These two formulas are valid for voltage rectifiers with frequencies up to 30 kHz. At higher frequencies, electrolytic capacitors lose their effectiveness, and the rectifier is calculated a little differently. But that is another topic.

Publication: radiokot.ru

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