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Transistor Key Transformerless Power Supply on IRF840 (English version)

Types of transformerless power supplies

In industrial production equipment. And radio amateurs very often use transformerless low-power power supplies in their designs. Usually, two types predominate.

The first is pulsed, in which the mains voltage is rectified and then powers a pulse generator of a higher frequency, from the secondary voltage of which the output voltage is obtained by rectification.

Or the second, simplest option, “on a quenching capacitor,” which is a kind of symbiosis of a parametric stabilizer and a rectifier.

бестрансформаторный блок питания

The mains voltage in such circuits is used completely (the entire amplitude or the entire half-wave), and the excess voltage is extinguished by the reactance of the high-voltage capacitor.

But there is a third option, for some reason almost forgotten. This is a key circuit of a transformerless source , in which not the entire amplitude of the network voltage is used, but only a small section of it, from zero to a certain specified value.

Operating principle of the power supply

Such a stabilizer works something like this: when the AC sinusoid of the electrical network passes through zero, the switch turns on and remains on until the half-wave of the mains voltage reaches a certain value. At this threshold value, the switch closes, and then the rest of the half-wave is not supplied to the output of the power source.

Thus, in fact, the key power supply cuts off half-waves of the mains voltage at some level. Then this pulsating voltage is smoothed out by a capacitor and, if required, stabilized by a stabilizer made according to any circuit.

Such a source does not contain a pulse generator, RF transformer, chokes, damping resistors or capacitors. It simply uses only small pieces of half-waves, from zero to some value. A schematic diagram of a key power source operating on this principle is shown in the figure here.

Voltage from the mains is supplied to the rectifier bridge using diodes VD1-VD4. There is no smoothing capacitor at the output of this diode bridge, so there will be a pure ripple voltage. Representing two half-waves, positive and negative, with the negative one inverted towards the positive.

In general, the voltage varies from zero to 300V.

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Rice. . Schematic diagram of a key transformerless power supply based on the IRF840 transistor.

Transistor VT1 contains a comparator that measures the voltage at the output of the rectifier, and transistor VT2 is a switch. Resistors R1 and R2 form a voltage divider based on VT1. By adjusting resistor R2, you can set the opening threshold of VT1. for example, equal to 18V. Until the voltage at the output of the bridge VD1-VD4 reaches this value, VT1 is closed.

In this case, an unlocking voltage is supplied to gate VT2, and it is open. The voltage through it and the diode passes to capacitor C1 and charges it. Then, as soon as the voltage at the rectifier output exceeds the set threshold, transistor VT1 opens and bypasses the gate of VT2.

The VT2 key will close. And it will open only upon a decline in the pulsating voltage, when its value is below the opening threshold VT1. Thus, a voltage of about 18V will accumulate at C1.

The article is taken as a basis: Karnaukhov V. G. RK-07-2020.

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