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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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In amateur radio practice, it is often necessary to obtain two or more voltages to power different stages of one device. A simple example of this is the power supply of specialized microcircuits (voltage 5 ... 9 and 12 ... 15 V). In order not to "produce" power supplies and use simple transformers with one secondary winding (provided that the powered device does not require a lot of current), you can go the easy way and get several voltages from one source. This approach will also save space in the device case and the cost of the transformer, which, as a rule, is proportional to its power and the number of windings.

For example, it is easy to get double the voltage from a transformer power supply if the rectifier is made according to a half-wave circuit, or the mains transformer has a secondary winding with a tap from the middle. Such cases are repeatedly described in the literature. But when the rectifier is made according to the bridge circuit (which is most often found in practice), then you can get twice the voltage by using the circuit shown in Fig. 1.

Bridge rectifier VD1...VD4 and smoothing capacitor C1 form a "classic" power supply with output voltage Un. A feature of the circuit is an additional voltage doubling channel assembled on elements C2, VD5, VD6, C3.

A positive half-wave of voltage from the secondary winding of transformer T1 charges capacitor C5 through diode VD2. During the negative half-wave, the VD5 diode is closed, and the capacitor C2 is connected in series with the secondary winding T1, and the voltages across the capacitor and winding T1 add up. From this voltage, the capacitor C6 is charged through the diode VD3, so that it gets close to twice the voltage.

When a load is connected, the voltage decreases (the greater the load current, the lower the voltage). The load current of the doubling channel flows through the diode VD1, the common wire and the capacitor C2. As a result, the total current flows through the VD1 diode (the main channel and the additional one). This must be taken into account when choosing diodes and a transformer for a future power source.

The proposed voltage doubling source circuit is suitable as a proven option for powering relatively simple devices with low current consumption (up to 1 A through the main channel). The channels in this circuit depend on each other, and with an increase in the load current in the main channel, the voltage in the additional channel decreases even with its minimum load. Therefore, at high currents, it is better to use the classic circuit with two windings of a step-down transformer and separate rectifiers.

The second option for increasing the voltage is the use of electronic multipliers. A good example of a multiplier in household appliances is a high voltage multiplier for powering a kinescope in television receivers. All multipliers operate on the same principle, the input of which receives voltage pulses.

A simple example of a multiplier is the circuit shown in Fig. 2.

Pulses of any shape with repetition frequency f=10...12 kHz and duty cycle 0=2...3 are fed to the input. Such pulses are generated by almost any generator built according to the classical scheme on TTL or CMOS microcircuits. However, given the low load capacity of these microcircuits, it is necessary to turn on a buffer amplifier at the output of the generator (an emitter follower or several elements of the microcircuit connected in parallel). The amplitude of the input signal UBX must be at least 5 V. Since such a multiplier is obviously designed for a small output current, diodes VD1 ... VD6 are used types KD521, KD522, D220, D310 and similar. Oxide capacitors - type K50-24 and similar.

The output current of this node will not exceed the output current of the generator, therefore, such a voltage multiplier is used to power only individual microcircuits or low-current stages of the device that require increased voltage. The dependence of the output voltage (Un) on the output current is inversely proportional (the higher Un. the lower the output current). The maximum output current for the doubling output (2Un) in this circuit is 40 mA at Un=6 V, for the 3Un output at the same voltage U„ - 48 mA, 4Un - 55 mA. The maximum output current is at Un=15 V for the output 2Un - 10 mA, 3Un - 5 mA, 4Un - 2,5 mA.

Similarly, based on this circuit, a negative voltage multiplier is obtained. The difference is that all the diodes are turned on the other way around and the polarity of the oxide capacitors changes (Fig. 3).

In practice, it has been established that the negative voltage of the multiplier relative to the base voltage will not exceed -3Un. The other two (lower) output voltages will be -2Un and -Un. In this case, it is not possible to obtain a voltage of -4Un without changing the circuit.

Author: A.Kashkarov, St. Petersburg

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