Key transformerless power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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In their designs, radio amateurs very often use transformerless low-power power supplies. Usually, they are a kind of symbiosis of a parametric stabilizer and a rectifier. The mains voltage in such circuits is used completely (the entire amplitude), and the excess voltage is quenched by a constant resistor, on which power is released or by the reactance of a high-voltage capacitor. Both schemes can hardly be called the optimal solution, except from the point of view of extreme simplicity.

But there is also a key circuit of a transformerless source, in which not the entire amplitude of the mains voltage is used, but only its small section - from zero to a certain specified value. Such a stabilizer works like this; when the sinusoid of the alternating current of the mains passes through zero, the key turns on and remains on until the half-wave of the mains voltage reaches a certain value. Then the key is closed.

Thus, it cuts off the half-waves of the mains voltage at some level. Then this pulsating voltage is smoothed out by a capacitor and stabilized by a stabilizer. There are no quenching resistors or capacitors in such a source. It just uses only small pieces of half-waves. A schematic diagram of a source operating according to this principle is shown in Figure 1.

The mains voltage is rectified by a diode bridge VD1-VD4.

There is no capacitor at the output of this bridge, so there will be a pulsating voltage here, varying from zero to 300 V. Transistor VT1 is a comparator, and transistor VT2 is a key. Resistors R1 and R2 form a voltage divider based on VT1. By adjusting the resistor R2, you can set the opening threshold VT1, for example, equal to 18 V.

Until the voltage at the output of the bridge VD1-VD4 reaches this value, the transistor VT1 is closed. The gate of the transistor VT2 is supplied with an unlocking voltage and it is open. The voltage through it and the VD5 diode passes to the capacitor C1 and charges it. Then, as soon as the voltage at the output of the rectifier exceeds the set threshold, the transistor VT1 opens and shunts the gate VT2.

The VT2 key will close. And it will open only on the decline of the pulsating voltage, when its value is below the opening threshold VT1. Thus, a voltage of about 1 ... 15 V will be accumulated on C18, which is supplied to the integral stabilizer. A1 and to the output of the source.

The source according to the circuit in Figure 1 gives a stable voltage of 5 V at a current of up to 100 mA.

Figure 2 shows a diagram of a key source on a specialized SR037 chip. The circuit gives two voltages of 18V and 5V, both with a maximum current of 30mA.

When working with these sources, one must not forget that their secondary circuits have a galvanic connection with the mains, and take all appropriate electrical safety measures.

Author: Karnaukhov V.T.

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