Theory: rectifiers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Semiconductor diode rectifiers are used to convert alternating current to direct current. The simplest half-wave rectifier on a single diode (Fig. 62) is used only for low currents, high load resistances and low requirements for output voltage ripple. Its main drawback is that the direct rectified current, flowing through the secondary winding of the transformer, creates a permanent magnetization of the magnetic circuit and thereby worsens its operation.

At the output of the rectifier, a high-capacity oxide capacitor is included to smooth out ripples. Without it, the voltage at the output of the rectifier would have the form of sinusoidal half-waves, as shown in Fig. 63 with a dashed line, - after all, the diode passes only a positive half-wave of voltage on the secondary winding of the transformer. In the presence of a smoothing capacitor C1, the output voltage will look like. shown in the figure as a solid line. It consists of a constant voltage U and an alternating ripple voltage U. The ripple frequency is equal to the mains frequency - 50 Hz, and the period T is 0,02 s.

The amplitude of the ripple can be estimated by noting that the discharge of the capacitor C1 to the load (powered device) occurs according to an exponential law: U = Ume-1 / RC, where R is the load resistance equal to the rectified voltage (V) divided by the consumed current (A) , and C is the capacitance of the capacitor C1 (F). If the time constant is equal to the product of RC. more than T., the relation U = T·In/C is approximately satisfied. where In is the load current. We see that the ripples decrease with an increase in the load resistance of the rectifier (i.e., with a decrease in the consumed current) and the capacitance of the smoothing capacitor. In special cases, more complex smoothing filters are used, containing a series-connected resistor or inductor and an additional smoothing capacitor.

More perfect full-wave rectifier (Fig. 64). The secondary winding of the transformer is designed for twice the voltage, but has a tap from the middle, i.e. consists, as it were, of two identical windings (their beginnings are shown by dots). During one half-wave of voltage, the diode VD1 is open, and during the other, VD2. There is no permanent magnetization of the transformer magnetic circuit now, since the currents in the halves of the secondary winding are directed in different directions, and the output ripple frequency is 100 Hz, which makes it possible to approximately halve the capacitance of the smoothing capacitor C1. The halves of the secondary winding are also calculated for half the current.

Another type of full-wave rectifier is a bridge (Fig. 65). containing four diodes and only one secondary winding. At each half-wave of alternating voltage, two diodes are included in the opposite diagonals of the bridge. This circuit is most widely used; for such rectifiers, special assemblies of four diodes connected in a bridge circuit are produced.

When calculating the transformer, they operate with the effective values ​​of the voltage on the windings. The DC voltage on the smoothing capacitor of the rectifier without load is equal to the amplitude value Um. i.e. 1.41 times more. For example, if the secondary winding is designed for a voltage of 12 V (2x12 V for the circuit in Fig. 64), the open-circuit voltage of the rectifier will be about 17 V. Under load, it decreases somewhat due to the drop in the direct resistance of the diodes and on the resistance of the windings

Rectifier diodes are characterized by permissible forward current and reverse voltage (for D7A diodes, for example, this will be 0.3 A and 50 V). For a half-wave rectifier (Fig. 62), the forward current of the diode must be no less than the current consumed by the load, and the reverse voltage must be no less than 2UW. In a full-wave rectifier, two diodes work alternately, so the allowable forward current of the diodes can be half as much, and the reverse voltage is the same. In the bridge circuit (Fig. 65), the direct current is equal to half the rectified one, and the reverse voltage is Um. Diodes are usually chosen with some forward current and reverse voltage margin.

Of the other interesting rectifier circuits, the voltage doubling circuit should be mentioned (Fig. 66). It contains, as it were, two half-wave rectifiers connected in parallel at the input and in series at the output (the same principle is used in multipliers of a higher multiplicity). The rectified voltage here reaches 2Um, but two smoothing capacitors of considerable capacity are needed.

Sometimes a rectifier is required that creates two voltages of the same magnitude, but of opposite polarity, relative to the common wire (to power operational amplifiers, some UMZCH, etc.). In this case, it is convenient to combine two rectifiers according to the circuit in Fig. 64, but with the diodes and capacitor reversed in the second rectifier. What happens in this case is shown in Fig. 67.

The diodes form a conventional bridge, and the midpoints of the secondary winding and capacitors are connected to a common wire.

Author: Author: V.Polyakov

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