Power supply in a table lamp 220/12 volts 0,5 amperes. Scheme, description

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Power supply in a table lamp 220/12 volts 0,5 amperes. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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In modern table lamps, low-voltage 12 V halogen lamps are widely used. For their power supply from a 220 V network, step-down (220/12 V) transformers are often used, which at the same time serve as a load that increases the structure's resistance to tipping over. The power of halogen lamps in table lamps usually does not go beyond 20 ... 50 W; step-down transformers are also designed for the same power. It is very important that their primary and secondary windings are wound, as a rule, in separate sections, which increases electrical safety.

About power supplies based on such a transformer, made in the form of an independent design, described in the author's articles [1, 2]. But if the dimensions of the base (stand) of the table lamp allow, then the power supply can be built into it. Such a source is useful for powering players (3 V), portable radios (6 and 9 V), charging cell phones (5 V). It can also be used as a multi-fixed voltage stabilized laboratory source. Refusal of smooth adjustment is due to the desire to simplify the use of the device as much as possible.

Power supply in a table lamp
Fig. 1

A diagram of a power supply built into a table lamp with a set of the above output voltage values is shown in fig. 1. It contains a bridge rectifier based on diodes VD1-VD4, capacitor C1, which smooths out the ripple of the rectified voltage, and a voltage regulator based on the DA1 chip. The desired voltage is selected by the SA2 switch, which connects resistors R3-R4 shunting it in parallel with the resistor R6. LEDs HL1-HL4 of different colors of glow - indicators of the source on and the selected value of the output voltage. The SB1 button is a regular lamp switch, the SA1 switch is used to turn it off (for example, if there is no need for lighting).

Power supply in a table lamp
Fig. 2

Most parts of the source are mounted on a printed circuit board (Fig. 2) made of one-sided foil fiberglass with a thickness of 1,5 ... 2 mm. The board is designed for the use of fixed resistors P1-4, C2-23 and imported oxide capacitors (for example, the TK series from Jamicon). Switch SA1 - P1T or other small-sized switch SA2 - any small-sized biscuit, for example PG2-9-6P2N, socket XS1 - small-sized, used to connect headphones in monophonic equipment. LED HL1 - super-bright white glow, the rest - ordinary (HL2 - yellow glow, HL3 - green, HL4 - red). Chip DA1 must be provided with a heat sink with a cooling surface of 15.. .20 cm2 in the form of a plate 2...3 mm thick made of aluminum alloy.

Power supply in a table lamp

The author's version of the source is mounted in a Camelion KD117N luminaire (Fig. 3). The board is glued to the transformer, the LEDs are fixed with glue in the holes drilled in the left side of the base, the switch is installed in its rear part, the switch is in the top, the XS1 socket is in the right. The luminaire is designed for the use of a halogen lamp with a power of up to 35 W. If it is replaced with a 20-watt one, then there will be a headroom in power that can be used for a power source. If the maximum output current is required (up to 1,5 A), then the lamp will have to be turned off to avoid overloading the transformer. Two-wire cords with appropriate connectors are used to connect source-powered devices.

The LM337T chip can be replaced with an integrated LM317T stabilizer, but it has a different pinout (input - pin 3, output - pin 2), so you will have to change the configuration of the corresponding printed conductors on the board. In addition, in this case, it is necessary to reverse the polarity of the inclusion of diodes, LEDs and capacitors. The polarity of the output voltage will be positive with respect to the common wire.

In order to obtain a stabilized output voltage of 12 V, the 1N4004 diodes must be replaced with Schottky diodes (for example, 1N5819) and a resistor shunting R3 should be selected. It should be noted that the maximum output current at this voltage should not exceed 0,5 A (with a larger value, large ripples may occur)

In conclusion, about one more useful function of the described power source in a table lamp. The use of a super-bright white LED (HL1) allows you to use it as a night light. To do this, without connecting the load, set the SA2 switch to the "9 V" position.

Literature

1. Nechaev I. Power supply based on a step-down transformer for halogen lamps. - Radio, 2008, No. 12, p. 28, 29.
2. Nechaev I. Small laboratory IP. - Radio, 2009, No. 5, p. 21-23.

Author: I. Nechaev, Moscow; Publication: radioradar.net

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