Transformerless power supply with adjustable output voltage, 220/16-26 volts 2 watts. Scheme, description

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Transformerless power supply with adjustable output voltage, 220/16-26 volts 2 watts. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The proposed power supply allows you to smoothly change the output voltage over a wide range (Fig. 6.11). Its feature is the use of adjustable negative feedback from the output of the block to the transistor stage VT1, connected in parallel with the output of the diode bridge. This stage is a parallel control element and is controlled by a signal from the output of a single-stage amplifier to VT2. The output signal VT2 depends on the voltage difference supplied from the variable resistor R7, connected in parallel to the output of the power supply, and the reference voltage source on the diodes VD3, VD4.

Transformerless power supply with adjustable output voltage, 220/16-26 volts 2 watts

Essentially, the circuit is an adjustable shunt regulator. The role of the ballast resistor is played by the quenching capacitor C1, the role of the parallel controlled element is played by the transistor VT1. This power supply works as follows. When connected to the network, the transistors VT1 and VT2 are locked, the storage capacitor C2 is charged through the diode VD2.

When the base of the transistor VT2 reaches a voltage equal to the reference voltage on the diodes VD3, VD4, the transistors VT2, VT1 begin to unlock. Transistor VT1 shunts the output of the diode bridge, and its output voltage begins to drop, which leads to a decrease in the voltage on the storage capacitor C2 and to blocking of transistors VT2 and VT1. This, in turn, causes a decrease in shunting of the output of the diode bridge, an increase in the voltage on C2 and unlocking of VT2, VT1, etc.

Due to the negative feedback acting in this way, the output voltage remains constant (stabilized) with and without the load R9 on, at idle. Its value depends on the position of the R7 potentiometer slider. The upper (according to the diagram) position of the slider corresponds to a larger output voltage.

The maximum output power of the above device is 2 watts.

The output voltage adjustment limits are from 16 to 26 V, and with a shorted VD4 diode, the adjustment limits are from 15 to 19,5 V. In these ranges, when R9 is turned off (load shed), the increase in output voltage does not exceed one percent. VT1 works in variable mode: when working on load R9 - in linear mode; at idle - in pulse-width modulation (PWM) mode with a voltage ripple frequency across capacitor C2 - 100 Hz. In this case, the voltage pulses on the collector of the transistor VT1 have gentle fronts. The linear mode is lightweight, the VT1 transistor heats up a little and can work with little or no heatsink.

A slight heating takes place in the lower position of the R7 potentiometer slider at the minimum output voltage. At idle, with the load R9 disconnected, the thermal regime of the transistor VT1 worsens in the upper position of the engine R7 In this case, the transistor VT1 must be installed on a small radiator, for example, in the form of a square-shaped aluminum plate with a side of 3 cm, 1 ... 2 thick mm.

Regulating transistor VT1 - medium power, with a large transfer coefficient (composite). Its collector current should be 2-3 times the maximum load current. The collector voltage VT1 must not be less than the maximum output voltage of the power supply. As VT1, npn transistors KT972A, KT829A, KT827A, etc. can be used. Transistor VT2 operates in low current mode, so any low-power pnp transistor is suitable - KT203A ... V, KT361A ... G, KT313A / B, KT209A / B.

According to the principle of the above scheme, similar power supplies can be built for other required power values.

Author: Semyan A.P.

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