How to get additional voltage from a bridge rectifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters
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Many single-output power supplies consist of a transformer, a rectifier, and a filter capacitor (Figure 1).
Rice. 1. A typical single voltage power supply circuit contains a transformer, a rectifier and a filter capacitor
Such a circuit is relatively inexpensive, easy to manufacture, but has only one supply voltage at its output. Circuits that use op-amps, data converters, and other analog circuits often require additional voltages to operate. These voltages can be greater than the main source voltage, or have a negative polarity. In this case, additional transformer windings and additional rectifiers are usually used. This approach is appropriate if the power consumption from each winding is approximately the same, but usually analog circuit power supplies consume much less current, and then the use of additional windings and rectifiers is unjustified. Note that if the voltage of the additional power source should be lower than the voltage of the main one, then it is enough to use a series integrated voltage regulator (for example, to obtain voltages of +5 V from a source of +7 ... + 15 volts, you can use the KR142EN5A or IC7805 microcircuits).
Since the input and output of a bridge rectifier do not have a common point, it is impossible to use standard peak detectors and voltage multiplier stages here. However, the AC inputs of a rectifier bridge are capable of supplying and receiving current relative to their outputs. By slightly modifying a conventional bridge, you can get a voltage doubler (Fig. 2).
Rice. 2. Connecting to the main circuit of the voltage doubler
Using a similar structure and connecting it to a common wire, you can get a negative voltage at the output. On fig. 3 shows a modified version of the circuit with additional sources of both positive and negative voltage.
Rice. 3. Modification of the circuit, allowing you to get positive and negative voltages at the output
In the above power supply circuits, a transformer with an output voltage of 12 V was used only as an example, but this method can be applied to any other voltages. It is necessary to pay attention to the fact that the operating voltage of the series-connected capacitors and filter capacitors at the output of the increased voltage source will be higher than that of the filter capacitor of the main source. The voltage on the filter capacitor only reaches the peaks of the rectified AC voltage, and on the remaining capacitors this voltage will be almost twice as much (minus the drop on the diodes), which should be taken into account in the calculations. The ratings of all capacitors are determined by the required output power and they do not have to be the same.
Theoretically, the negative and doubled output voltages of the power supply are capable of delivering power close to that of the main rectifier. Significant losses occur, mainly due to the series-connected Cseries capacitors. The use of these larger capacitors makes it possible to reduce energy losses, while not forgetting the appropriate surge currents, and therefore selecting these capacitors for the appropriate operating voltage. If, nevertheless, it is required to deliver increased power from a double voltage source, then you will have to consider the option with an additional transformer or use additional secondary windings.
Author: Horst Koelzo, Canada
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