Transformerless uninterruptible power supply, 220/3 volts 300 milliamps. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Transformerless uninterruptible power supply, 220/3 volts 300 milliamps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Low-voltage powered portable electronic devices are often powered by batteries and accumulators.

Among such devices, the most popular are electromechanical and digital clocks, radios, flashlights, etc. wireless calls operating over a radio channel at a frequency of 303 MHz (there are options for a frequency of 933,25 MHz). The same devices include digital cameras with a 3,3 V external power connector (for example, Olimpus C-765), portable CD players (players), voice recorders, hair clippers, and even mobile phones.

All these devices (their list is not limited to those listed in the previous sentence, it is much wider) have in common that they are powered by cells (batteries and accumulators, external power supplies) with a nominal voltage of 3 V ± 10%. How to provide them with power when the "regular" adapter is lost, defective?

If the current consumption does not exceed 0,3 A, it is easiest to power the low-voltage device from the 220 V mains with a power source that has the "uninterrupted" function and provides power in the absence of mains voltage. On fig. 1.1 I offer readers a circuit diagram of the device.

Transformerless uninterruptible power supply, 220/3 volts 300 milliamps

The voltage taken from the divider C1, C2 is rectified by the diodes VD1, VD2 and smoothed by the oxide capacitor C3. Without load, the voltage across capacitor C3 does not exceed 14 V. Transistor VT3 is connected according to a common base circuit, and its collector-emitter junction is fully open (voltage drop does not exceed 0,5 V) - the voltage at the load (on the plates of oxide capacitor C4) is 3,3 V. In the absence of mains voltage, the current through the circuit (with a connected load) flows through the emitter-base junction VT1 (the total voltage drop across VD3 does not exceed 0,3 V).

And it can be further reduced if the VD3 diode is excluded from the circuit, which protects the transistor when mains voltage is applied to the input of the device. Thus, in autonomous power mode, at least 2,7 V is supplied to the load. This voltage can already be used to power an electromechanical alarm clock or wall clock.

The device does not contain a single resistor and generates practically no heat, even the VT1 transistor, since the current through its junction is very small. With the load disconnected, the current strength could not be fixed at all.

All diodes VD1, VD2 can be replaced with KD105V-KD105G, KD213, D226 with any letter index. It is desirable to use the VD3 diode from the D219, D220, D223 series. Oxide capacitors type K50-29 or similar. It is better to use capacitor C1 from an unnecessary ballast, a converter for a fluorescent (energy-saving) lamp - such capacitors are designed for high reactive power and practically do not emit heat when turned on.

Circuit elements are mounted on the board to fit the size of the battery compartment.

Phrasing the connection is not critical. When assembling and connecting the device, care should be taken, since its elements are under the voltage of the 220 V lighting network.

Author: Kashkarov A.P.

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