Radio and electrical power supplies almost always use rectifiers designed to convert AC to DC. This is due to the fact that almost all electronic circuits and many other devices must be powered from DC sources. A rectifier can be any element with a nonlinear current-voltage characteristic, in other words, differently flowing current in opposite directions. In modern devices, planar semiconductor diodes are usually used as such elements.
Circuit semiconductor diode.
Flat semiconductor diodes
Along with good conductors and insulators, there are many substances that are intermediate in conduction between these two classes. They call such substances semiconductors. The resistance of a pure semiconductor decreases with increasing temperature, unlike metals whose resistance increases under these conditions.
By adding a small amount of impurity to a pure semiconductor, one can substantially change its conductivity. There are two classes of such impurities:
Figure 1. Planar diode: a. device diode; b. diode designation in electrical circuits; at. the appearance of planar diodes of different power.
- Donor - converting pure material into n-type semiconductor, containing an excess of free electrons. This type of conductivity is called electronic.
- Acceptor - converting the same material into a p-type semiconductor, having an artificially created lack of free electrons. The conductivity of such a semiconductor is called hole. "Hole" - a place that left the electron, behaves like a positive charge.
A layer on the border of p- and n-type semiconductors (pn junction) has one-way conductivity - it conducts current well in one (forward) direction and very poorly in the opposite (reverse) direction. The device of the planar diode is shown in Figure 1a. The base is a semiconductor plate (germanium) with a small amount of a donor impurity (n-type) on which a piece of indium is placed, which is an acceptor impurity.
After heating, indium diffuses into the adjacent areas of the semiconductor, turning them into p-type semiconductors. At the border of regions with two types of conductivity, a p – n junction occurs. The output connected to the p-type semiconductor is called the anode of the resulting diode, the opposite - its cathode. The image of the semiconductor diode on the circuit diagrams is shown in Fig. 1b, the appearance of planar diodes of different power - in Fig. 1c.Back to table of contents
Figure 2. Current characteristics in various circuits.
The current flowing in a conventional lighting network is variable. Its magnitude and direction change 50 times within one second. The graph of its voltage versus time is shown in Fig. 2a Positive half periods are shown in red, negative ones in blue.
Since the magnitude of the current varies from zero to the maximum (amplitude) value, the concept of the effective value of current and voltage is introduced. For example, in a lighting network, the effective value of a voltage of 220 V - in the heater included in this network, the same heat is generated for the same periods of time as in the same device in a 220 V DC circuit.
But in fact, the voltage in the network varies in 0.02 with the following:
- first quarter of this time (period) - increases from 0 to 311 V;
- second quarter of the period - decreases from 311 V to 0;
- third quarter of the period - decreases from 0 to 311 V;
- the last quarter of the period increases from 311 V to 0.
In this case, 311 V is the voltage amplitude Uabout. Amplitude and effective (U) voltages are interconnected by the formula:
Uo = √2 * U.
Figure 3. Diode bridge.
When an alternating current of a series-connected diode (VD) and load is connected to the circuit (Fig. 2b), current flows through it only during positive half-periods (Fig. 2c). This happens due to the one-sided conduction of the diode. Such a rectifier is called half-wave - one half of the period the current in the circuit is, during the second - is absent.
The current flowing through the load in such a rectifier is not constant, but pulsating. It can be turned into almost constant by switching on the capacitor of the filter C parallel to the loadf large enough capacity. During the first quarter of the period, the capacitor is charged to an amplitude value, and in the intervals between pulsations it is discharged to the load. The voltage becomes almost constant. The effect of smoothing is stronger, the greater the capacitance of the capacitor.Back to table of contents
Diode bridge circuit
A more perfect is the full-wave straightening scheme, when both the positive and negative half-periods are used. There are several varieties of such schemes, but most often used pavement. Diagram of the diode bridge is shown in Fig. 3c. On it, the red line shows how the current flows through the load during positive, and the blue - negative half-periods.
Figure 4. A 12 volt rectifier circuit using a diode bridge.
Both the first and the second half of the period, the current through the load flows in the same direction (Fig. 3b). The number of pulsations for one second is not 50, as with half-wave straightening, but 100. Accordingly, with the same capacitance of the filter capacitor, the smoothing effect will be more pronounced.
As you can see, to build a diode bridge, 4 diodes are needed - VD1-VD4. Previously, diode bridges were depicted in principle diagrams in exactly the same way as in fig. 3c. Nowadays, the image shown in Fig. 2 is generally accepted. 3g. Although there is only one image of a diode on it, one should not forget that the bridge consists of four diodes.
The bridge circuit is most often assembled from individual diodes, but monolithic diode assemblies are sometimes used. They are easier to mount on the board, but when one arm of the bridge fails, the entire assembly is replaced. Select the diodes from which the bridge is mounted, based on the magnitude of the current flowing through them and the magnitude of the permissible reverse voltage. This data allows you to get instructions to the diodes or reference books.
The complete diagram of a 12 volt rectifier using a diode bridge is shown in fig. 4. T1 is a step-down transformer, the secondary winding of which provides a voltage of 10-12 V. The FU1 fuse is a significant detail from the point of view of safety and should not be neglected. The brand of diodes VD1-VD4, as already mentioned, is determined by the amount of current that will be consumed from the rectifier. Capacitor C1 - electrolytic, with a capacity of 1000.0 microfarads or higher, for a voltage not lower than 16 V.
The output voltage is fixed, its value depends on the load. The greater the current, the smaller the magnitude of this voltage. To obtain an adjustable and stable output voltage, a more complex circuit is required. Receive adjustable voltage from the circuit shown in fig. 4 in two ways:
- By applying to the primary winding of the transformer T1 an adjustable voltage, for example, from LATR.
- Having made several taps from the secondary winding of the transformer and, respectively, put a switch.
It is hoped that the descriptions and diagrams above will provide practical assistance in assembling a simple rectifier for practical needs.